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A    TREATISE   ON   CHEMISTRY 


OF  THE 

UNIVERSITY 

OF 

LIF( 


A 

TREATISE  ON  CHEMISTRI 


BT 

SIR  H.  E.  ROSCOE,  F.K.S.  AND  C.  SCHOKLEMMEE,  F.R.S. 


VOLUME  I 
THE  NON-METALLIC  ELEMENTS 


Chymia,  alias  Alchemia  et  Spagirica,  est  ars  corpora  vel  mixta,  vet  composita,  vel 
aggregata  etiam  in  principia  sua  resolvendi,  aut  ex  principiis  in  talia  com- 
binandi."— STAHL,  1723 


NEW  EDITION  COMPLETELY  KEVISED  BY  SIR  H.  E.  ROSCOE 
ASSISTED  BY  DRS.  H.  G.  COLMAN  AND  A.  HARDEN 


WITH  THREE  HUNDRED  AND  SEVENTY-FOUR  ILLUSTRATIONS  AND 
A   PORTRAIT  OF  DALTON  Eg&£A£ED  BY  C.   H.  JEENS 

' \  - 

THE 


OF 

. 


NEW  YORK 
D.    APPLETON    AND    COMPANY 

1903 


Authorized  Edition. 


PREFACE  TO  THE  FIRST  EDITION 

IT  has  been  the  aim  of  the  authors,  in  writing  the 
present  treatise,  to  place  before  the  reader  a  fairly 
complete,  and  yet  a  clear  and  succinct,  statement 
of  the  facts  of  Modern  Chemistry,  whilst  at  the  same 
time  entering  so  far  into  a  discussion  of  Chemical 
Theory  as  the  size  of  the  work  and  the  present 
transition  state  of  the  science  permit.  Special  atten- 
tion has  been  paid  to  the  accurate  description  of  the 
more  important  processes  in  technical  chemistry,  and 
to  the  careful  representation  of  the  most  approved 
forms  of  apparatus  employed.  As  an  instance  of  this, 
the  authors  may  refer  to  the  chapter  on  the  Manu- 
facture of  Sulphuric  Acid.  For  valuable  information 
on  these  points  they  are  indebted  to  many  friends 
both  in  this  country  and  on  the  Continent. 

The  volume  commences  with  a  short  historical 
sketch  of  the  rise  and  progress  of  chemical  science, 
and  a  few  words  relative  to  the  history  of  each 
element  and  its  more  important  compounds  prefaces 
the  systematic  discussion  of  their  chemical  properties. 
For  this  portion  of  their  work,  the  authors  wish  here 
to  acknowledge  their  indebtedness  to  Hermann  Kopp's 
classical  works  on  the  History  of  Chemistry. 

In  the  part  of  the  volume  devoted  to  the  description 
of  the  non-metallic  elements,  care  has  been  taken  to 


-••    O  O  f\  O 


vi  PREFACE 


select  the  most  recent  and  exact  experimental  data, 
and  to  give  references  in  all  important  instances,  as 
it  is  mainly  by  consulting  the  original  memoirs  that 
a  student  can  obtain  a  full  grasp  of  his  subject. 

Much  attention  has  likewise  been  given  to  the 
representation  of  apparatus  adapted  for  lecture-room 
experiment,  and  the  numerous  new  illustrations  re- 
quired for  this  purpose  have  all  been  taken  from 
photographs  o'f  apparatus  actually  in  use.  The  fine 
portrait  which  adorns  the  title-page  is  a  copy,  by  the 
skilful  hands  of  Mr.  Jeens,  of  a  daguerreotype  taken 
shortly  before  Dalton's  death. 

MANCHESTER,  July,  1877. 


PREFACE  TO  THE  THIRD  EDITION 

IN  this  new,  completely  revised  and  reprinted  edition  I 
have  endeavoured  to  carry  out  the  aims  which  were 
put  forward  in  the  preceding  preface  seventeen  years 
ago.  Deprived  of  the  aid  of  my  late  friend  and 
colleague,  I  have  been  fortunate  in  securing  the  help 
of  two  of  the  ablest  of  my  former  students,  and  to 
them  I  tender  my  thanks.  How  far  we  have  succeeded 
in  bringing  this  edition  up  to  the  level  of  the  science 
of  the  present  day,  it  will  be  for  the  public  to  judge. 
All  I  can  say  is  that  no  pains  have  been  spared  to 
do  so. 

H.  E.  EOSCOE. 

LONDON,  September  29th,  1894. 


CONTRACTIONS  EMPLOYED  IN  THIS  PABT 

Am.  Chem.  Journ.  =  American  Chemical  Journal. 

Annalen  =  Liebig's  Annalen  der  Chemie. 

Ann.  Chim.  Phys.  =  Annales  de  Chimie  et  de  Physique. 

Ber.  =  Berichte  der  deutschen  chemischen  Gesellschaft 

Bull.  Soc.  Chim.  =  Bulletin  de  la  Societe  Chimique  de  Paris. 

Chem.  Centr.  =  Chemisches  Centralblatt. 

Chem.  Zeit.  =  Chemiker  Zeitung. 

Compt.  Rend.  =  Comptes  Rendues  de  1' Academic  des  Sciences. 

Gazzetta  =  Gazzetta  Chimica  Italiana. 

Journ.  Chem.  Soc.  =  Journal  of  the  Chemical  Society. 

J.  Pr.  Chem.  =  Journal  fur  praktische  Chemie. 

Journ.  Soc.  Chem.  Ind.  =  Journal  of  the  Society  of  Chemical  Industry. 

Monatsh.  =  Monatshefte  fur  Chemie. 

Phil.  Mag.  =  Philosophical  Magazine. 

Phil.  Trans.  =  Philosophical  Transactions  of  the  Royal  Society. 

Pogg.  Ann.  =  Annalen  der  Physik  und  Chemie  (Poggendorf). 

Proc.  Chem.  Soc.  =  Proceedings  of  the  Chemical  Society. 

Proc.  Roy.  Soc.  =  Proceedings  of  the  Royal  Society. 

Rec.  Trav.  Chim.  =  Recueil  des  Travaux  Chimiques  des  Pays-Bas. 

Wied.  Ann.  =  Annalen  der  Physik  (Wiedemann). 

Zeit.  analyt.  Chem.  =  Zeitschrift  fiir  analytische  Chemie. 

Zeit.  angew.  Chem.  =  Zeitschrift  flir  angewandte  Chemie. 

Zeit.  anorg.  Chem.  —  Zeitschrift  fiir  anorganische  Chemie. 

Zeit.  phys.  Chem.  =  Zeitschrift  fiir  physikalische  Chemie. 

Zeit.  physiol.  Chem.  =  Zeitschrift  fur  physiologische  Chemie. 


CONTENTS 


PAGE 

HISTORICAL  INTRODUCTION 3 

GENERAL  PRINCIPLES  OF  THE  SCIENCE 41 

Properties  of  Matter    .         .         . 41 

Elementary  and  Compound  Bodies       ,.....,  51 

Properties  of  Gases      , ,59 

Relation  of  Volume  to  Pressure.     Boyle's  Law      ....  59 

Relation  of  Volume  to  Temperature.     Dalton's  Law    ...  60 

Kinetic  Theory  of  Gases 60 

Diffusion  of  Gases 63 

Continuity  of  Gaseous  and  Liquid  States  of  Matter      ...  72 

Liquefaction  of  Gases 74 

Boiling  Points  of  Liquids 85 

Laws  of  Chemical  Combination 86 

Combination  by  "Weight 87 

Combination  by  Volume 97 

Experimental  Methods  for  the  Determination  of  Molecular  Weights     .  104 

Aqueous  Solutions 116 

Chemical  Nomenclature 118 

THE  NON-METALLIC  ELEMENTS        ........    126 

Hydrogen 129 

Fluorine .142 

Fluorine  and  Hydrogen 147 

Chlorine 153 

Chlorine  and  Hydrogen        .         .        . 168 

Bromine 188 

Bromine  and  Hydrogen ,193 

Chlorine  and  Bromine          .         . 198 

Iodine 199 

Iodine  and  Hydrogen 206 

Iodine  Compounds  with  other  Halogens ,212 

Oxygen ,214 

The  Oxides          ,..,...,..,     234 

Ozone          .....        0 .235 

Oxygen  and  Hydrogen .         .         ,244 

Oxygen  and  Chlorine  .  313 

Oxygen  and  Bromine •        •         .        .        .     329 

Oxygen  and  Iodine 331 


CONTENTS 


Sulphur 336 

Sulphur  and  Hydrogen ,         .  349 

Sulphur  and  Chlorine 358 

Sulphur  and  Bromine  .....         .....  362 

Sulphur  and  Iodine 362 

Sulphur  and  Fluorine 363 

Sulphur  and  Oxygen 364 

Selenium 423 

Selenium  and  Hydrogen .        .         .  427 

Selenium  and  the  Halogens 428 

Selenium  and  Oxygen 431 

Selenium  and  Sulphur. 435 

Tellurium 436 

Tellurium  and  Hydrogen 439 

Tellurium  and  the  Halogens        .                  440 

Tellurium  and  Oxygen 442 

Tellurium  and  Sulphur 445 

Nitrogen      .                   446 

Nitrogen  and  Hydrogen 452 

Nitrogen  and  the  Halogens .         .         .477 

Nitrogen  and  Oxygen 481 

Nitrogen  and  Sulphur 516 

Nitrogen  and  Selenium .516 

Sulphonic  Acids  of  Ammonia  and  Hydroxylamine       ....  519 

The  Atmosphere 523 

Phosphorus 549 

Phosphorus  and  Hydrogen 566 

Phosphorus  and  the  Halogens 573 

Phosphorus  and  Oxygen       .         .         .         ,         .        .         .         .         .581 

Phosphorus  and  Nitrogen 609 

Arsenic 612 

Arsenic  and  Hydrogen          .         .         . 616 

Arsenic  and  the  Halogens 619 

Arsenic  and  Oxygen     .                  622 

Arsenic  and  Sulphur 629 

Arsenic  and  Selenium 632 

Arsenic  and  Phosphorus 633 

Boron 640 

Boron  and  Hydrogen 644 

Boron  and  the  Halogens 645 

Boron  and  Oxygen 650 

Boron  and  Sulphur 656 

Boron  and  Nitrogen 657 

Boron  and  Phosphorus 657 

Carbon        .                           658 

Carbon  and  Hydrogen 690 

Carbon  and  the  Halogens .705 

Carbon  and  Oxygen     . 705 

Carbon  and  Sulphur 732 

Carbon  and  Nitrogen 747 


CONTENTS  xi 


PAGE 

Coal-Gas 769 

Wood-Gas 787 

Oil-Gas       .                                           788 

Water-Gas o  790 

Nature  of  Flame .                 792 

Silicon  or  Silicium •        •  800 

Silicon  and  Hydrogen. 804 

Silicon  and  the  Halogens 806 

Silicon  and  Oxygen «        .         .814 

Silicon  and  Sulphur             .        .        •        •                 •        •        .        .  823 

Silicon  and  Nitrogen 824 

Silicon  and  Carbon 825 

CRYSTALLOGRAPHY 827 

COMPARISON  OF  METRICAL  WITH  ENGLISH  MEASURES    .       .       .       .874 

ADDENDUM « 876 

INDEX  877 


CHEMISTRY 


CHEMISTRY 


HISTOEICAL    INTEODUCTION 

IN  looking  back  at  the  history  of  our  Science,  we  find  that 
although  the  ancient  world  possessed  a  certain  empirical  know- 
ledge of  chemical  facts  derived  chiefly  from  an  acquaintance 
with  pharmaceutical  and  manufacturing  art,  the  power  of  con- 
necting or  systematizing  these  facts  was  altogether  wanting. 

The  idea  of  experimental  investigation  was  scarcely  understood, 
and  most  of  those  amongst  the  ancients  who  desired  to  promote 
a  knowledge  of  Nature  attempted  to  do  so  rather  by  pursuing 
the  treacherous  paths  of  speculation,  than  the  safe  though  tedious 
route  of  observation  and  experiment.  They  had  no  idea  of  the 
essential  differences  which  we  now  perceive  between  elements 
and  compound  substances,  nor  did  they  understand  the  meaning 
of  chemical  combination.  The  so-called  Aristotelian  doctrine 
of  the  four  elements,  Earth,  Water,  Air  or  Steam,  and  Fire, 
bore  no  analogy  to  our  present  views  as  to  the  nature  and  pro- 
perties of  the  chemical  elements,  for  with  Aristotle  these  names 
rather  implied  certain  characteristic  and  fundamental  properties 
of  matter  than  the  ideas  which  we  now  express  by  the  term 
chemical  composition.  Thus  "  Earth  "  implied  the  properties 
of  dryness  and  coldness ;  "  Water,"  those  of  coldness  and  wet- 
ness ;  "  Air  or  Steam,"  wetness  and  heat ;  "  Fire,"  dryness  and 
heat.  All  matter  was  supposed  to  be  of  one  kind,  the  variety 
which  we  observe  being  accounted  for  by  the  greater  or  less 
abundance  of  these  four  conditions  which  were  supposed  to  be 
essential  to  every  substance,  that  which  was  present  in  the 
greatest  degree  giving  to  the  substance  its  characteristic  pro- 
perties. To  men  holding  such  views,  a  change  of  one  kind  of 


HISTORICAL  INTRODUCTION 


matter  into  a  totally  different  kind  appeared  probable  and 
natural.  Thus,  the  formation  of  water  from  air  or  vice  versa 
is  described  by  Pliny  as  a  usual  phenomenon  seen  in  the 
formation  and  disappearance  of  clouds,  whilst  the  ordinary 
experience,  that  cold  acts  as  a  solidifying  and  hardening 
agent,  bears  out  Pliny's  view,  that  rock  crystal  is  produced 
from  moisture,  not  by  the  action  of  heat,  but  by  that  of  cold, 
so  that  it  is,  in  fact,  a  kind  of  ice.  A  transformation  of  one 
sort  of  substance  into  another  quite  different  thus  appears 
not  only  possible  but  probable,  and  we  are  not  surprised  to 
learn  that,  under  the  influence  of  the  Aristotelian  philosophy, 
which  throughout  the  middle  ages  was  acknowledged  to  be  the 
highest  expression  of  scientific  truth,  the  question  of  the  trans- 
mutability  of  the  base  into  the  noble  metals  was  considered 
to  be  perfectly  open. 

Much  light  of  a  very  interesting  and  remarkable  character 
has  been  thrown  upon  the  origin  of  alchemy,  the  artificial  pro- 
duction of  the  noble  metals,  by  the  discovery  of  an  Egyptian 
papyrus,  which  contains  more  than  a  hundred  metallurgical 
recipes  written  in  Greek,  many  of  which  consist  of  elaborate 
directions  for  the  falsification  of  the  precious  metals.  The  con- 
nection between  these  working  notes  of  a  fraudulent  Egyptian 
goldsmith  and  the  dreams  of  the  later  alchemists  is  attested 
by  the  reappearance  of  several  of  these  very  recipes  in  the 
writings  of  the  following  century  under  the  guise  of  formulae  of 
transmutation.1 

The  oldest  works  of  a  strictly  chemical  character  date  from 
about  the  beginning  of  the  third  century  of  our  era,  and  are  due 
to  Greek  authors  resident  in  Egypt,  where  our  science  appears 
to  have  had  its  birth.  Among  these  writers,  the  most  ancient 
and  distinguished  of  whom  is  known  as  Zosimus  of  Panopolis, 
the  possibility  of  the  transmutation  of  the  metals  is  fully 
accepted,  and  their  works  consist  mainly  of  directions  for  the 
achievement  of  this  object,  couched  however  in  language  so 
deeply  tinged  with  religious  mysticism,  symbolism  and  metaphor 
as  to  be  almost  unintelligible.2  They  allude  to  their  subject  as 
the  "  divine  art,"  and  it  is  not  until  the  fourth  century  that  we 
find  in  the  works  of  the  Byzantine  writers  the  term  Chemia, 
applied  to  the  art  which  treats  of  the  production  of  gold  and 

1  Berthelot,  Les  Origines  de  I'Alchimie,  p.    3.     Introduction  A  V Etude  de  la 
Chimie  des  Anciens  et  du  Moyen  Age,  pp.  21,  59. 

2  Berthelot,  Collection  des  Anciens  Alchimistes  Grecs.     Part  I. 


THE  ARABIAN  ALCHEMISTS 


silver.1  The  fact  that  all  these  authors  were  closely  connected 
with  the  celebrated  schools  of  Alexandria,  the  last  resting-place 
of  the  proscribed  secrets  of  the  Egyptian  priests,  adds  to  the 
probability  that  our  science  was  first  practised  in  Egypt. 
Plutarch,  indeed,  states  that  the  old  name  for  Egypt  was 
Chemia,  and  that  this  name  was  given  to  it  on  account  of  the 
black  colour  of  its  soil.  The  same  word  was  used  to  designate 
the  black  of  the  eye,  as  the  symbol  of  the  dark  and  mysterious. 
It  is  therefore  possible  that  chemistry  originally  meant  simply 
the  Egyptian — or  secret — knowledge,  whilst  others  identify  the 
name  with  the  Greek  %fyu-o9,  sap  or  liquid,  the  name  of  the 
agent  of  transmutation  being  applied  to  the  art. 

The  Aristotelian  philosophy  became  known  to  and  was  ex- 
tended by  the  Arabians,  who,  in  the  year  640,  overran  Egypt, 
and  thence,  through  Northern  Africa,  penetrated  into  Spain. 
They  first  became  acquainted  with  chemistry  in  Egypt,  and  pre- 
fixed the  Arabic  article  to  the  original  name,  so  that  the  word 
alchemy  has  from  that  time  been  used  to  signify  the  art  of 
making  gold  and  silver. 

The  works  of  Geber,  the  most  celebrated  of  Arabian  alchem- 
ists, are  handed  down  to  us  through  Latin  translations.  In 
these  books  we  learn  that  the  aim  of  the  science  of  which 
Geber  treats  is  the  transmutation  of  the  base  into  the  noble 
metals.  He  describes  many  chemical  operations,  such  as  filtra- 
tion, distillation,  crystallization,  and  sublimation;  and  by  these 
he  prepares  new  substances  or  purifies  the  old  ones.  Bodies 
such  as  alum,  green  vitriol,  saltpetre,  and  sal-ammoniac  are  em- 
ployed ;  and  we  find  that  he  was  able  to  prepare  nitric  acid,  or 
aquafortis,  and  from  it  the  valuable  solvent  for  gold,  aqua  regia. 
It  is  probable  that  even  sulphuric  acid  was  known  to  Geber,  and 
certainly  a  number  of  metallic  compounds,  amongst  which  were 
mercuric  oxide  and  corrosive  sublimate,  the  preparation  of 
which  he  describes,  were  known.  Geber  was  the  first  pro- 
pounder  of  a  chemical  theory.  He  asserts  that  the  essential 
differences  between  the  metals  are  due  to  the  preponderance  of 
one  of  two  principles,  mercury  and  sulphur — of  which  all  the 
metals  are  composed.  The  first  principle  is  characteristic  of 
the  truly  metallic  qualities,  whilst  the  latter  causes  the  peculiar 
changes  noticed  when  the  metals  are  exposed  to  heat.  The 
noble  metals  were  supposed  to  contain  a  very  pure  mercury,  and 
are,  therefore,  unalterable  by  heat,  whilst  the  base  metals  contain 

1  Kopp,  Beitrdge  zur  Gcschichte  der  Chemie.     1  Stuck,  p.  40. 


HISTORICAL  INTRODUCTION 


so  much  sulphur  that  they  lose  their  metallic  qualities  in  the  fire. 
These  constituents  may,  however,  not  only  be  present  in  different 
proportions,  but  also  in  different  degrees  of  purity  or  in  different 
states  of  division  ;  and  thus  it  might  naturally  be  supposed  that,  if 
not  by  a  variation  in  their  relative  quantity,  at  any  rate  by  a 
change  in  their  condition,  such  an  alteration  in  the  properties  of 
one  metal  may  be  brought  about  as  would  produce  from  it  some 
other  known  metal.  Thus  gold  and  silver  contain  a  very  pure 
mercury,  which  in  the  one  instance  is  combined  with  a  red  and 
in  the  other  with  a  white  sulphur ;  and  he  explains  that  the 
reason  why  these  two  metals  amalgamate  so  easily  is  because 
they  already  contain  a  large  quantity  of  mercury,  and  are 
therefore  quickly  attracted  by  the  liquid  metal. 

Whilst  Greece  and  Italy  sank  deeper  and  deeper  into  bar- 
barism, arts  and  science  flourished  under  Arabian  dominion, 
and  the  academies  of  Spain  were  thronged  with  students 
from  all  parts  of  the  Christian  world.  The  knowledge  of 
alchemy  spread  from  this  source  over  Western  Europe,  and  in 
the  thirteenth  century  we  find  alchemists  of  the  Arabian  school 
in  all  the  chief  countries  of  Europe.  In  Christian  Spain  lived 
the  celebrated  Raymund  Lully ;  in  France  we  hear  of  Arnold 
Villanovanus  ;  Albertus  Magnus  flourished  in  Germany.  Then 
Thomas  Aquinas,  pupil  of  Albertus,  was  also  an  alchemist, 
as  was  our  own  Roger  Bacon  (1214-1294),  who  was  tried  at 
Oxford  for  sorcery,  and  who,  to  disprove  these  charges,  wrote 
the  celebrated  treatise,1  in  which  he  shows  that  appearances 
then  attributed  to  supernatural  agency  were  due  to  common  and 
natural  causes.  It  was  Roger  Bacon,  from  his  rare  accomplish- 
ments and  learning  termed  Doctor  Mirabilis,  who  first  pointed 
out  the  possible  distinction  between  theoretical  alchemy,  or 
chemistry  studied  for  its  own  sake,  on  the  one  hand,  and  prac- 
tical alchemy,  or  the  striving  after  certain  immediately  useful 
ends,  on  the  other. 

Although  all  these  men  agreed  that  the  transmutation  of 
metals  was  not  only  possible  but  that  it  was  an  acknowledged 
fact,  and  that  for  the  preparation  of  gold  and  silver  the  philoso- 
pher's stone  was  needed,  it  is  difficult,  not  to  say  impossible,  now 
to  understand  their  methods  or  processes,  inasmuch  as  all  that 
they  have  written  on  this  subject  is  expressed  in  the  ambiguous 
and  inflated  diction  of  the  Byzantine  and  Arabian  authors. 

The  fourteenth  century  finds  the  study  of  alchemy  widely 
1  Epistola  de  secretis  operibus  artis  et  naturae,  et  nullitate  magice,  - 


TRANSMUTATION 


spread  over  the  civilized  world,  and  the  general  attention  which 
the  subject  attracted  gave  rise  to  the  discovery  of  a  large  number 
of  chemical  substances.  By  the  end  of  the  fifteenth  century, 
although  the  knowledge  of  chemical  facts  had  continued  to  in- 
crease, the  old  views  respecting  the  ultimate  composition  of 
matter  were  still  accepted.  In  addition,  however,  to  the  sulphur 
and  mercury,  supposed  by  Geber  to  be  the  universal  constitu- 
ents of  matter,  we  find  a  third  constituent,  viz.  salt,  introduced. 
We  must  bear  in  mind  however  that  these  three  principles,  like 
the  four  Aristotelian  elements,  were  not  supposed  to  be  identical 
with  the  common  substances  which  bear  their  names. 

That  men  of  such  wide  experience  and  great  powers,  as  the 
chemists  of  this  period  proved  themselves  to  be,  could  bring 
themselves  to  believe  in  the  possibility  of  the  discovery  of  the 
philosopher's  stone,  a  substance  of  such  potency  that  when 
thrown  on  the  base  metals  in  a  state  of  fusion  (moment  of  pro- 
jection) it  transmutes  them  into  gold  and  silver,  appears  to  us 
very  remarkable.  No  one  doubted  the  possibility  of  such  a 
transmutation,  and  the  explanation  may  be  found  in  the  fact, 
at  that  time  well  known,  that  the  colour  of  certain  metals  can 
be  altered  by  the  addition  of  other  bodies.  Thus  Geber  knew 
that  when  red  copper  is  melted  with  tutty  (an  impure  oxide  of 
zinc),  the  golden-yellow  brass  is  obtained  ;  and  also  that  other 
minerals  (those  which  we  now  know  to  contain  arsenic)  give  to 
copper  a  silver-white  colour.  Still  the  difference  between  these 
alloys  and  the  noble  metals  must  soon  have  been  discovered, 
and  the  possibility  of  the  transmutation  lay  rather  in  the  notion 
already  alluded  to,  that  the  different  metals  contained  the  same 
constituents  arranged  either  in  different  quantities  or  in  different 
states  of  purity.  Nor  were  experimental  proofs  of  this  view 
wanting.  Thus  Geber  believed  that  by  adding  mercury  to  lead 
the  metal  tin  was  formed,  and  the  solid  amalgam  does  closely 
resemble  tin  in  its  appearance.  Then  again  the  metallurgical 
processes  were  in  those  days  very  imperfect,  and  the  alchemists 
saw  proof  of  their  theory  in  the  formation  of  a  bead  of  pure 
silver  from  a  mass  of  galena,  or  in  the  extraction  of  a  few 
grains  of  gold  out  of  a  quantity  of  pyrites.  It  was  not  until 
the  beginning  of  the  seventeenth  century  that  it  was  proved 
that  galena  frequently  contains  silver,  and  that  traces  of  gold 
are  often  found  in  iron  pyrites.  Even  so  late  as  1709  we  find 
Homberg  stating  that  pure  silver  after  melting  with  pyrites  is 
found  to  contain  gold,  and  it  was  only  after  several  chemists  had 


HISTORICAL  INTRODUCTION 


performed  the  experiment  with  a  like  result  that  the  mineral 
itself  was  acknowledged  to  contain  traces  of  gold. 

Again,  it  was  not  at  this  time  recognised  that  some  salts  are 
metallic  compounds,  and  the  precipitation  of  copper  from  a  solu- 
tion of  blue-stone  by  metallic  iron  was  supposed  to  be  a  transmu- 
tation of  iron  into  copper.  These  apparent  experimental  proofs 
of  the  truth  of  the  alchemical  doctrine  were  accompanied  by  a 
mass  of  traditional  evidence ;  that  is,  of  stories  handed  down 
from  generation  to  generation,  in  which  cases  of  the  transmuta- 
tion of  metals  are  circumstantially  narrated.  Thus  the  belief  in 
the  fundamental  principle  of  alchemy  became  firmly  established.1 

A  satisfactory  explanation  of  the  belief  in  the  power  of  the 
philosopher's  stone  to  heal  disease  and  to  act  as  the  elixir  vitce, 
the  grand  panacea  for  human  ills,  is  more  difficult  to  find.  It 
may  possibly  have  at  first  arisen  from  a  too  literal  interpretation 
of  the  oriental  imagery  found  in  the  early  Arabian  writers,  where, 
although  the  peculiar  doctrine  of  elixir  vitse  is  unknown,  we 
find  such  passages  as  the  following  : — "  If  thoti  carriest  out  my 
prescription  with  due  care  thou  shalt  heal  the  bad  disease  of 
poverty."  The  Arabians  called  the  base  metals  "  diseased." 
Thus  Geber  says,  "  Bring  me  the  six  lepers,  so  that  I  may  heal 
them  ; " — that  is,  transmute  the  other  six  known  metals  into 
gold.  The  belief  in  the  healing  power  of  the  philosopher's  stone 
was  also  much  strengthened  by  the  discovery,  about  this  time, 
of  many  substances  which  produce  remarkable  effects  on  the 
human  frame,  and  of  these  the  alchemists  of  the  thirteenth 
century  write  in  the  most  exaggerated  and  exalted  terms. 

The  work  known  by  the  fantastic  title  of  the  Triumph-  Wagen 
des  Antimonii,  which  contains  a  large  amount  of  accurate 
information  concerning  the  preparation  and  the  medicinal  pro- 
perties of  many  of  the  compounds  of  antimony,  and  is  ascribed 
to  the  authorship  of  a  monk,  Basil  Valentine  by  name,  who  was 
supposed  to  have  lived  at  the  beginning  of  the  fifteenth  century, 
has  been  shown  by  the  late  Prof.  Schorlemmer  to  be  an  undoubted 
forgery  dating  from  about  1600,  the  information  being  culled 
from  the  works  of  other  writers  and  thrown  into  the  mystical, 
semi-religious  style  suitable  to  the  earlier  period.2  The  same 
appears  to  be  true  of  the  other  writings  attributed  to  this  author. 

The    man    who    effected    the    inestimable    union    between 

1  For  further  information  on  this  subject  Kopp's  classical  work,  Die  Geschichte 
der  Chemie,  or  Thomson's  History  of  Chemistry,  may  be  consulted. 

2  See  also  Kopp,  Beitrdge  III.  110. 


UNIV 
THE  MEDICAL  CHEMISTS 


chemistry  and  medicine  was  Paracelsus  (1493-1541).  He 
not  only  assumed  the  existence  of  three  components  of  all 
inorganic  substances,  but  he  was  the  first  who  included  animal 
and  vegetable  bodies  in  the  same  classification,  and  held  that 
the  health  of  the  organism  depends  on  the  continuance  of  the 
true  proportions  between  these  ingredients,  whilst  disease  is  due 
to  a  disturbance  of  this  proper  relation. 

The  era  thus  inaugurated  by  Paracelsus  continued  up  to  the 
end  of  the  seventeenth  century.  Chemistry  was  the  handmaid 
of  medicine,  and  questions  respecting  the  ultimate  composition 
of  matter  were  considered  of  secondary  importance  to  those 
relating  to  the  preparation  of  drugs.  Of  the  contemporaries 
of  Paracelsus,  Agricola  (1490-1555)  was  one  of  the  most  dis- 
tinguished, and  his  remarkable  work  De  Re  Metallica  contains 
a  complete  treatise  on  metallurgy  and  mining,  which  did  much 
to  advance  the  processes  of  technical  chemistry,  many  of  the 
methods  which  he  describes  being  in  use  even  at  the  present 
day.  Whilst  Agricola  devoted  himself  to  the  study  of  metallurgy, 
his  countryman,  Libavius,  greatly  assisted  the  general  progress 
of  science,  inasmuch  as  he  collected  together  in  writings  which 
are  characterised  by  a  clear  and  vigorous  style,  all  the  main  facts 
of  chemistry;  so  that  his  Alchemia,  published  in  1595,  may  be 
regarded  as  the  first  handbook  of  chemistry.  His  chief  object 
was  the  preparation  of  medicines,  but  he  still  maintained  the 
science  in  its  old  direction  and  distinctly  believed  in  the  trans- 
mutation of  metals. 

The  first  who  formally  declined  to  accept  the  Aristotelian 
doctrine  of  the  four  elements,  or  that  of  Paracelsus  of  the  three 
constituents  of  matter,  was  Van  Helmont  (1577-1644).  He 
denied  that  fire  had  any  material  existence,  or  that  earth  can  be 
considered  as  an  element,  for  it  can,  he  says,  be  produced  from 
water,  but  he  admitted  the  elementary  nature  of  air  and  water, 
and  gave  great  prominence  to  the  latter  in  its  general  dis- 
tribution throughout  animate  and  inanimate  nature.  Van 
Helmont' s  acknowledgment  of  air  as  an  element  is  the  more 
remarkable,  as  he  was  the  first  to  recognise  the  existence  of 
different  kinds  of  air  and  to  use  the  term  gas.  Thus,  his  "  gas 
sylvestre,"  which  he  clearly  distinguished  from  common  air,  is 
carbonic  acid  gas,  for  he  states  that  it  is  given  off  in  the  process 
of  fermentation,  and  also  formed  during  combustion,  and  that 
it  is  found  in  the  "  Grotto  del  Cane,"  near  Naples.  He  also 
mentions  a  "  gas  pingue  "  which  is  evolved  from  dung,  and  is 


10  HISTORICAL  INTRODUCTION 

inflammable.  It  was  Van  Helmont  who  first  showed  that  if  a 
metal  be  dissolved  in  an  acid  it  is  not  destroyed,  as  was  formerly 
believed,  but  can  again  be  obtained  from  solution  as  metal  by 
suitable  means ;  and  he  considered  the  highest  aim  of  the 
science  to  be  the  discovery  of  a  general  solvent  which  would  at 
the  same  time  serve  as  a  universal  medicine,  and  to  which  the 
name  of  "  alkahest "  was  given. 

Although  Van  Helmont  accomplished  much  towards  the 
overthrow  of  the  Paracelsian  doctrine,  his  discoveries  of  the 
different  gases  were  forgotten,  and  even  up  to  the  middle 
of  the  seventeenth  century  much  divergence  in  opinion  on 
fundamental  questions  prevailed.  Those  who  were  interested 
in  the  connection  of  chemistry  with  medicine  still  believed  in 
the  dreams  of  the  alchemist,  and  held  to  the  old  opinions ;  whilst 
those  who  advancing  with  the  times,  sought  to  further  the 
science  for  its  own  sake,  or  for  the  sake  of  its  important  technical 
applications,  often  upheld  views  more  in  accordance  with  those 
which  we  now  know  to  be  the  true  ones.  Among  the  names  of 
the  men  who,  during  this  period,  laboured  successfully  to  pro- 
mote the  knowledge  of  chemistry,  that  of  Glauber  (1603-1668) 
must  be  first  mentioned.  He  was  both  alchemist  and  medicinal 
chemist,  and  discovered  many  valuable  medicines.  Another  name 
of  importance  at  this  epoch  is  that  of  N.  Lemery  (1645-1715). 
He,  as  well  as  Lefebre  and  Willis,  believed  in  the  existence  of 
five  elements ;  mercury  or  spirit,  sulphur  or  oil,  and  salt  are  the 
active  principles;  water  or  phlegm,  and  earth  are  the  passive 
ones.  Lemery's  ideas  and  teachings  became  well  known  through 
the  publication  of  his  GOUTS  de  Chymie  (1675)  which  was  trans- 
lated into  Latin,  as  also  into  most  modern  languages,  and  exerted 
a  great  influence  on  the  progress  of  science.  In  this  work  the 
distinction  between  mineral  and  vegetable  bodies  was  first  clearly 
pointed  out,  and  thus  for  the  first  time  the  distinction  between 
Inorganic  and  Organic  chemistry  was  realized. 

Pre-eminent  amongst  the  far-seeing  philosophers  of  his  time 
stands  Robert  Boyle  (1627-1691).  It  is  to  Boyle  that  we  owe  the 
complete  overthrow  of  the  Aristotelian  as  well  as  the  Paracelsian 
doctrine  of  the  elements,  so  that,  with  him  we  begin  a  new 
chapter  in  the  history  of  our  science.  In  his  Sceptical  Chymist,1 

1  The  Sceptical  Chymist  or  Chemico- physical  Doubts  and  Paradoxes,  touching  the 
Experiments  whereby  vulgar  Spagyrists  are  wont  to  endeavour  to  evince  their  Salt, 
Sulphur,  Mercury,  to  be  the  true  Principles  of  Things.  First  published  in  1661 
(Boyle's  Works,  1772,  1,  458). 


"THE  SCEPTICAL  CHYMIST  "  11 


he  upholds  the  view  that  it  is  not  possible,  as  had  hitherto  been 
supposed,  to  state  at  once  the  exact  number  of  the  elements ;  that 
on  the  contrary  all  bodies  are  to  be  considered  as  elements  which 
are  themselves  not  capable  of  further  separation,  but  which  can 
be  obtained  from  a  combined  body,  and  out  of  which  the  com- 
pound can  be  again  prepared.  Thus  he  states,  "  That  it  may 
-as  yet  be  doubted  whether  or  no  there  be  any  determinate 
number  of  elements  ;  or,  if  you  please,  whether  or  no  all  com- 
pound bodies  do  consist  of  the  same  number  of  elementary  in- 
gredients or  material  principles."  1  Boyle,  it  is  clear,  was  the  first 
to  grasp  the  idea  of  the  distinction  between  an  elementary  and  a 
compound  body,  the  latter  being  a  more  complicated  substance 
produced  by  the  union  of  two  or  more  simple  bodies  and  differing 
altogether  from  these  in  its  properties.  He  also  held  that  chemical 
combination  consists  in  an  approximation  of  the  smallest  particles 
of  matter,  and  that  a  decomposition  takes  place  when  a  third 
body  is  present,  capable  of  exerting  on  the  particles  of  the  one 
element  a  greater  attraction  than  the  particles  of  the  other 
element  with  which  it  is  combined.  More,  however,  than  for  his 
views  on  the  nature  of  the  elements,  is  science  indebted  to  Boyle 
for  his  clear  statement  of  the  value  of  scientific  investigation  for 
its  own  sake,  altogether  independent  of  any  application  for 
the  purposes  either  of  the  alchemist  or  of  the  physician.  It 
was  Boyle  who  first  felt  and  taught  that  chemistry  was  not  to  be 
the  handmaid  of  any  art  or  profession,  but  that  it  formed  an  essen- 
tial part  of  the  great  study  of  Nature,  and  who  showed  that  from 
this  independent  point  of  view  alone  could  the  science  attain  to 
vigorous  growth.  He  was,  in  fact,  the  first  true  scientific  chemist, 
and  with  him  we  may  date  the  commencement  of  a  new  era  for 
our  science  when  the  highest  aim  of  chemical  research  was 
acknowledged  to  be  that  which  it  is  still  upheld  to  be,  viz.,  the 
simple  advancement  of  natural  knowledge. 

In  special  directions  Boyle  did  much  to  advance  chemical 
science  (his  published  writings  and  experiments  fill  six  thick 
quarto  volumes),  particularly  in  the  borderland  of  chemistry  and 
physics ;  thus  in  the  investigations  on  the  "  Spring  of  the  Air," 
he  discovered  the  great  law  of  the  relation  existing  between 
volumes  of  gases  and  the  pressures  to  which  they  are  subjected, 
which  still  bears  his  name. 

Although  Boyle  was  aware  of  the  fact  that  many  metals  when 
heated  in  the  air  form  calces  which  weigh  more  than  the  metals 
1  Boyle's  Works,  1772,  1,  560. 


12  HISTORICAL  INTRODUCTION 

themselves,  and  although  he  examined  the  subject  experimentally 
with  great  care,  his  mind  was  so  much  biassed  by  the  views  he  held 
respecting  the  material  nature  of  flame  and  fire  that  he  ignored 
the  true  explanation  of  the  increase  of  weight,  namely,  that  it  is 
due  to  the  absorption  of  a  ponderable  constituent  of  the  atmo- 
sphere, and  looked  upon  the  gain  as  a  proof  of  the  ponderable 
nature  of  fire  and  flame,  giving  many  experiments  having  for 
their  object  the  "  arresting  and  weighing  of  igneous  corpuscles." * 

Similar  views  are  found  expressed  in  his  essay  "  On  the 
mechanical  origin  and  production  of  fixedness/' 2  written  in 
1675,  where  Boyle,  speaking  of  the  formation  of  mercuric  oxide 
from  the  metal  by  exposure  to  the  air  at  a  high  temperature, 
says,  "  chemists  and  physicians  who  agree  in  supposing  this  pre- 
cipitate to  be  made  without  any  additament,  will,  perchance, 
scarce  be  able  to  give  a  more  likely  account  of  the  consistency 
and  degree  of  fixity,  that  is  obtained  in  the  mercury  ;  in  which, 
since  no  body  is  added  to  it,  there  appears  not  to  be  wrought 
any  but  a  mechanical  change,  and  though  I  confess  I  have 
not  been  without  suspicions  that  in  philosophical  strictness 
this  precipitate  may  not  be  made  per  se,  but  that  some 
penetrating  igneous  particles,  especially  saline,  may  have  asso- 
ciated themselves  with  the  mercurial  corpuscles." 

We  owe  the  next  advances  in  chemistry  to  the  remarkable 
views  and  experiments  of  Hooke  (Micrographia,  1665),  and  of 
John  Mayow  (Opera  Omnia  Medico-physica,  1681).  The  former 
announced  a-  theory  of  combustion,  which  although  it  attracted 
but  little  notice,  more  nearly  approached  the  true  explanation  than 
many  of  the  subsequent  attempts.  He  pointed  out  the  similarity 
of  the  actions  produced  by  air  and  by  nitre  or  saltpetre,  and  he 
concluded  that  combustion  is  affected  by  that  constituent  of  the 
air  which  is  fixed  or  combined  in  the  nitre.3  Hooke  did  not 
complete  his  theory  or  give  the  detail  of  his  experiments,  but 
similar  conclusions  seem  to  have  been  independently  arrived  at 
by  Mayow,  who  in  1669  published  a  paper,  De  Sal-Nitro  et  Spiritu 
Nitro-aereo,  in  which  he  points  out  that  combustion  is  carried  on 
by  means  of  this  "  spiritus  nitro-aereus  "  (another,  and  not  an  in- 
appropriate name  for  what  we  now  call  oxygen),  and  he  also 
distinctly  states  that  when  metals  are  calcined,  the  increase  of 
weight  observed  is  due  to  the  combination  of  the  metal  with  this 
"  spiritus."  Mayow  was  one  of  the  first  to  describe  experiments. 

1  Boyle's  Works,  3,  706—718.  2  Boyle's  Works,  4,  309. 

3  Micrographia,  pp.  103 — 5. 


THE  THEORY  OF  PHLOGISTON  13 

made  with  gases  collected  over  water,  in  which  he  showed  that 
air  is  diminished  in  bulk  by  combustion,  and  that  the  respiration 
of  animals  produces  the  same  effect.  He  proved  that  it  is  the 
nitre-air  which  is  absorbed  in  both  these  processes,  and  that  an 
inactive  gas  remains,  and  he  drew  the  conclusion  that  respiration 
and  combustion  are  strictly  analogous  phenomena.  There  is, 
therefore,  no  doubt  that  Mayow  clearly  demonstrated  the 
heterogeneous  nature  of  air,  although  his  conclusions  were  not 
admitted  by  his  contemporaries. 

Another  theory  which  was  destined  greatly  to  influence  and 
benefit  chemical  discovery,  was  advanced  about  this  time  by 
J.  J.  Becher  (1635 — 1682),  and  subsequently  much  developed 
and  altered  by  G.  E.  Stahl  (1660—1734).  It  made  special 
reference  to  the  alterability  of  bodies  by  fire,  and  to  the 
explanation  of  the  facts  of  combustion.  Becher  assumed  that 
all  combustible  bodies  are  compounds,  so  that  they  must 
contain  at  least  two  constituents,  one  of  which  escapes  during 
combustion,  whilst  the  other  remains  behind.  Thus  when 
metals  are  calcined,  an  earthy  residue  or  a  metallic  calx  remains ; 
metals  are  therefore  compounds  of  this  calx  with  a  combustible 
principle,  whilst  sulphur  or  phosphorus  are  compounds  contain- 
ing a  principle  which  causes  their  combustion.  Bodies  unalter- 
able by  fire  are  considered  to  have  already  undergone  combus- 
tion ;  to  this  class  of  bodies  quicklime  was  supposed  to  belong, 
and  it  was  assumed  that  if  the  substance  which  it  had  lost  in 
the  fire  were  again  added  a  metallic  body  would  result.  The 
question  as  to  whether  there  be  only  one  or  several  principles  of 
combustibility  was  freely  discussed,  and  Stahl  decided  in  favour 
of  the  former  of  these  alternatives,  and  gave  to  this  combustible 
principle  the  name  Phlogiston  ((frXoyio-ros,  burnt,  combustible). 

An  example  may  serve  to  illustrate  the  reasoning  of  the 
upholders  of  the  Phlogistic  theory.  Stahl  knew  that  oil 
of  vitriol  is  a  product  of  the  combustion  of  sulphur ;  hence 
sulphur  is  a  combination  of  oil  of  vitriol  and  phlogiston. 
But  this  latter  is  also  contained  in  charcoal,  so  that  if  we  can 
take  the  phlogiston  out  of  the  charcoal  and  add  it  to  the  oil  of 
vitriol,  sulphur  must  result.  In  order  that  this  change  may  be 
brought  about,  the  oil  of  vitriol  must  be  fixed  (i.e.  rendered 
non- volatile)  by  combining  it  with  potash  ;  if  then,  the  salt  thus 
obtained  is  heated  with  charcoal,  a  hepar  sulphuris  (a  compound 
also  produced  by  fusing  potash  with  sulphur)  is  obtained.  The 
argument  shows  that  when  charcoal  is  heated  with  oil  of  vitriol 


14  HISTORICAL  INTRODUCTION 

the  phlogiston  of  the  charcoal  combines  with  the  oil  of  vitriol, 
and  sulphur  is  the  result.  The  phlogiston  contained  in  sulphur 
is  not  only  identical  with  that  contained  in  charcoal,  but  also 
with  that  existing  in  the  metals,  and  in  all  organic  bodies,  for 
these  are  obtained  by  heating  their  calces  with  charcoal,  or  with 
oil  or  other  combustible  organic  bodies. 

The  amount  of  phlogiston  contained  in  bodies  was,  according 
to  Stahl,  very  small,  and  the  greatest  quantity  was  contained  in 
the  soot  deposited  from  burning  oil.  It  was  likewise  considered 
that  the  phlogiston  given  off  by  combustion  is  taken  up  again 
from  the  air  by  plants  ;  and  the  phenomena  of  fermentation  and 
decay  were  believed  to  depend  upon  a  loss  of  phlogiston  which, 
however,  in  this  case  only  escapes  slowly.  Stahl  explains 
why  combustion  can  only  occur  in  presence  of  a  good  supply 
of  air,  because  in  this  case  the  phlogiston  assumes  a  very 
rapid  whirling  motion,  and  this  cannot  take  place  in  a  closed 
space. 

However  false  from  our  present  position  we  see  the  phlogistic 
theory  in  certain  directions  to  be ;  and  although  we  may  now 
believe  that  the  extension  and  corroboration  of  the  positive  views 
enunciated  by  Hooke  and  Mayow  might  have  led  to  a  recognition 
of  a  true  theory  of  chemistry  more  speedily  than  the  adoption 
of  the  theory  of  phlogiston,  we  must  admit  that  its  rapid 
general  adoption  showed  that  it  supplied  a  real  want.  It  was 
this  theory  which  for  the  first  time  established  a  common 
point  of  view  from  which  all  chemical  changes  could  be  observed 
enabling  chemists  to  introduce  something  like  a  system  by  class- 
ing together  phenomena  which  are  analogous  and  are  probably 
produced  by  the  same  cause,  for  the  first  time  making  it  pos- 
sible for  them  to  obtain  a  general  view  of  the  whole  range  of 
chemical  science  as  then  known. 

It  may  appear  singular  that  the  meaning  of  the  fact  of  the 
increase  of  weight  which  the  metals  undergo  on  heating,  which 
had  been  proved  by  Boyle  and  others,  should  have  been  wholly 
ignored  by  Stahl,  but  we  must  remember  that  he  considered  their 
form  rather  than  their  weight  to  be  the  important  and  character- 
istic property  of  bodies. 

Stahl  also,  perhaps  independently,  arrived  at  the  same  con- 
clusion which  Boyle  had  reached,  concerning  the  truth  of  the 
existence  of  a  variety  of  elementary  bodies,  as  opposed  to  the 
Aristotelian  or  Paracelsian  doctrine,  and  the  influence  which  a 
clear  statement  of  this  great  fact  by  Stahl  and  his  pupils 


JOSEPH  BLACK  15 


— amongst  whom  must  be  mentioned  Pott  (1692 — 1777) 
and  Marggraf  (1709 — 1782) — exerted  on  the  progress  of  the 
science  was  immense.  It  is  only  after  Stahl's  labours  that  a 
scientific  chemistry  becomes,  for  the  first  time,  possible.  The 
essential  difference  between  the  teaching  of  the  science  then 
and  now  being  that  the  phenomena  of  combustion  were  then 
believed  to  be  due  to  a  chemical  decomposition,  phlogiston  being 
supposed  to  escape,  while  we  account  for  the  same  phenomena 
now  by  a  chemical  combination,  oxygen  or  some  element  being 
taken  up. 

Thus  Stahl  prepared  the  way  for  the  birth  of  modern  chemis- 
try. It  was  on  August  1st,  1774,  that  Joseph  Priestley  dis- 
covered oxygen  gas. 

Between  the  date  of  the  establishment  of  the  phlogistic  theory 
by  Stahl,  and  of  its  complete  overthrow  by  Lavoisier,  many 
distinguished  men  helped  to  build  up  the  new  science — Black, 
Priestley,  and  Cavendish  in  our  own  country,  Scheele  in  Sweden, 
and  Macquer  in  France.  The  classical  researches  of  Black  on  the 
fixed  alkalis  (1754) 1  not  only  did  much  to  shake  the  foundation 
of  the  phlogistic  theory,  but  they  may  be  described  with  truth 
as  the  first  beginnings  of  a  quantitative  chemistry,  for  it  was  by 
means  of  the  balance,  the  essential  instrument  of  all  chemical 
research,  that  Black  established  his  conclusions.  Up  to  this  time 
the  mild  (or  carbonated)  alkali  was  believed  to  be  a  more  simple 
compound  than  the  caustic  alkali.  When  mild  alkali  (potashes) 
was  brought  into  contact  with  burnt  (caustic)  lime,  the  mild  alkali 
took  up  the  principle  of  combustibility,  obtained  by  the  lime- 
stone in  the  fire,  and  it  became  caustic.  Black  showed  that  in 
the  cases  of  magnesia-alba  and  chalk  the  disappearance  of  the 
effervescence  on  treatment  with  an  acid  after  heating,  was 
accompanied  by  a  loss  of  weight.  Moreover,  as  Van  Helmont's 
older  observations  were  quite  forgotten,  he  was  the  first  clearly 
to  establish  the  existence  of  a  kind  of  air  or  gas,  termed  fixed 
air  (1752)  totally  distinct  both  from  common  atmospheric  air 
and  from  modifications  of  it,  by  impurity  or  otherwise,  such 
as  the  various  gases  hitherto  prepared  were  believed  to  be. 
This  fixed  air,  then,  is  given  off  when  mild  alkalis  become 
caustic,  and  is  taken  up  when  the  reverse  change  occurs. 

This  clear  statement  of  a  fact  which  of  itself  is  a  powerful 
argument  against  the  truth  of  the  theory  in  which  he  had  been 

1   "Experiments   upon   Magnesia-alba,    Quicklime,    and   other  Alkaline    sub- 
stances."— Edin.  Phys.  and  Literary  Essays,  1755. 


16  HISTORICAL  INTRODUCTION 

brought  up,  was  sufficient  to  make  the  name  of  Black  illustrious, 
but  he  became  immortal  by  his  discoveries  of  latent  and  specific 
heats,  the  principles  of  which  he  taught  in  his  classes  at  Glasgow 
and  Edinburgh  from  1763.  The  singularly  unbiassed  character 
of  Black's  mind  is  shown  in  the  fact  that  he  was  the  only 
chemist  of  his  age  who  completely  and  openly  avowed  his 
conversion  to  the  new  Lavoisierian  doctrine  of  combustion. 
From  an  interesting  correspondence  between  Black  and 
Lavoisier,  it  is  clear  that  the  great  French  chemist  looked  on 
Black  as  his  master  and  teacher,  speaking  of  Black's  having 
first  thrown  light  upon  the  doctrines  which  he  afterwards  more 
fully  carried  out.1 

This  period  of  the  history  of  our  science  has  been  called  that 
•of  pneumatic  chemistry,  because,  following  in  the  wake  of 
Black's  discovery  of  fixed  air,  chemists  were  now  chiefly  engaged 
in  the  examination  of  the  properties  and  modes  of  preparation 
of  the  different  kinds  of  airs  or  gases,  the  striking  and  very 
different  natures  of  which  naturally  attracted  interest  and  stimu- 
lated research. 

No  one  obtained  more  important  results  or  threw  more  light 
upon  the  existence  of  a  number  of  chemically  different  gases  than 
Joseph  Priestley.  In  1772  Priestley  was  engaged  in  the  examin- 
ation of  the  chemical  effect  produced  by  the  burning  of  com- 
bustible bodies  (candles)  and  the  respiration  of  animals  upon 
ordinary  air.  He  proved  that  both  these  deteriorated  the  air 
and  diminished  its  volume,  and  to  the  residual  air  he  gave  the 
name  of  phlogisticated  air.  Priestley  next  investigated  the 
action  of  living  plants  on  the  air  and  found  to  his  astonishment 
that  they  possess  the  power  of  rendering  the  air  deteriorated 
by  animals  again  capable  of  supporting  the  combustion  of  a 
candle. 

Fig.  1,  a  reduced  fascimile  of  the  frontispiece  to  Priestley's 
celebrated  Observations  on  Different  Kinds  of  Air,  shows  the 
primitive  kind  of  apparatus  with  which  this  father  of  pneumatic 
chemistry  obtained  his  results.  The  mode  adopted  for  generating 
and  collecting  gases  is  seen  ;  hydrogen  is  being  prepared  in  the 
phial  by  the  action  of  oil  of  vitriol  on  iron  filings,  and  the  gas  is 
being  collected  in  the  large  cylinder  standing  over  water  in  the 
pneumatic  trough ;  round  this  trough  are  arranged  various  other 
pieces  of  apparatus,  as,  for  instance,  the  bent  iron  rod  holding 
.  a  small  crucible  to  contain  the  substances  which  Priestley  desired 
1  Brit.  Assoc.  Reports,  1871,  p.  189. 


JOSEPH  PRIESTLEY 


17 


18  HISTORICAL  INTRODUCTION 

to  expose  to  the  action  of  the  gas.  In  the  front  is  seen  a  large 
cylinder  in  which  he  preserved  the  mice,  which  he  used  for 
ascertaining  how  far  an  air  was  impure  or  unfit  for  respiration, 
and  standing  in  a  smaller  trough  is  a  cylinder  containing  living 
plants,  the  action  of  which  on  air  had  to  be  ascertained. 

On  August  1st,  1774,  Priestley  obtained  oxygen  gas  by 
heating  red  precipitate  by  means  of  the  sun's  rays  concen- 
trated with  a  burning  glass,  and  termed  it  dephlogisticated  air,, 
because  he  found  it  to  be  so  pure,  or  so  free  from  phlogiston, 
that  in  comparison  with  it  common  air  appeared  to  be  impure. 
Priestley  also  first  prepared  nitric  oxide  (nitrous  air  or  gas), 
nitrous  oxide  (dephlogisticated  nitrous  air),  and  carbonic  oxide ; 
he  likewise  collected  many  gases  for  the  first  time  over  mercury, 
such  as  ammoniacal  gas  (alkaline  air),  hydrochloric  acid  gas. 
(marine  acid  air),  sulphurous  acid  gas  (vitriolic  acid  air),  and 
silicon  tetrafluoride  (fluor  acid  air).1  He  also  observed  that  when  a 
series  of  electric  sparks  is  allowed  to  pass  through  ammoniacal 
gas,  an  increase  of  volume  occurs,  and  a  combustible  gas  is  formed, 
whilst  on  heating  ammonia  with  calx  of  lead  phlogisticated  air 
(nitrogen  gas)  is  evolved. 

Priestley's  was  a  mind  of  rare  quickness  and  perceptive 
powers,  which  led  him  to  the  rapid  discovery  of  numerous  new 
chemical  substances,  but  it  was  not  of  a  philosophic  or  delibera- 
tive cast.  Hence,  although  he  had  first  prepared  oxygen,  and 
had  observed  (1781)  the  formation  of  water,  when  inflammable 
air  (hydrogen)  and  atmospheric  air  are  mixed  and  burnt  together 
in  a  copper  vessel,  he  was  unable  to  grasp  the  true  explanation 
of  the  phenomenon,  and  he  remained  to  the  end  of  his  days  a 
firm  believer  in  the  truth  of  the  phlogistic  theory,  which  he  had 
done  more  than  any  one  else  to  destroy. 

Priestley's  notion  of  original  research,  which  seems  quite 
foreign  to  our  present  ideas,  may  be  excused,  perhaps  justified,  by 
the  state  of  the  science  in  his  day.  He  believed  that  all  dis- 
coveries are  made  by  chance,  and  he  compares  the  investigation 
of  nature  to  a  hound,  wildly  running  after,  and  here  and  there 
chancing  on  game  (or  as  James  Watt  called  it,  "  his  random 
haphazarding"),  whilst  we  should  rather  be  disposed  to  compare 
the  man  of  science  to  the  sportsman,  who  having,  after  persistent 
effort,  laid  out  a  distinct  plan  of  operations,  makes  reasonably 
sure  of  his  quarry. 

In  some  respects  the  scientific  labours  of  Henry  Cavendish 
1  Priestley's  Observations  on  Different  Kinds  of  Air,  1}  328. 


HENRY  CAVENDISH  19 

(1731-1810)  present  a  strong  contrast  to  those  of  Priestley ;  the 
work  of  the  latter  was  quick  and  brilliant,  that  of  the  former  was 
slow  and  thorough.  Priestley  passed  too  rapidly  from  subject  to 
subject  even  to  notice  the  great  truths  which  lay  under  the 
surface  ;  Cavendish  made  but  few  discoveries,  but  his  researches 
were  exhaustive,  and  for  the  most  part  quantitative.  His 
investigation  on  the  inflammable  air  1  evolved  from  dilute  acid 
and  zinc,  tin,  or  iron,  is  a  most  remarkable  one.  In  this  memoir 
we  find  that  he  first  determined  the  specific  gravity  of  gases, 
and  used  materials  for  drying  gases,  taking  note  of 
alterations  of  volume  due  to  changes  of  pressure  and  tempera- 
ture. He  likewise  proved  that  by  the  use  of  a  given  weight  of 
each  one  of  these  metals,  the  same  volume  of  inflammable  gas 
can  always  be  obtained  no  matter  which  of  the  acids  be  employed, 
whilst  equal  weights  of  the  metals  gave  unequal  volumes  of  the 
gas.  Cavendish  also  found  that  when  the  above  metals  are 
dissolved  in  nitric  acid,  an  incombustible  air  is  evolved,  whilst  if 
they  are  heated  with  strong  sulphuric  acid  sulphurous  air  is 
formed.  He  concluded  that  when  these  metals  are  dissolved  in 
hydrochloric  or  in  dilute  sulphuric  acid  their  phlogiston  flies 
off,  whilst  when  heated  with  nitric  or  strong  sulphuric  acids, 
the  phlogiston  goes  off  in  combination  with  an  acid.  This  is 
the  first  occasion  in  which  we  find  the  view  expressed  that 
inflammable  air  is  phlogiston — a  view  which  was  generally  held, 
although  Cavendish  himself  subsequently  changed  his  opinion, 
regarding  inflammable  air  as  a  compound  of  phlogiston  and 
water. 

The  discovery  of  oxygen  by  Priestley,  and  of  nitrogen  by 
Rutherford,  naturally  directed  the  attention  of  chemists  to  the 
study  of  the  atmosphere,  and  to  the  various  methods  for  ascer- 
taining its  composition. 

Although  Priestley's  method  of  estimating  the  dephlogisti- 
cated  air  by  means  of  nitric  oxide  was  usually  employed, 
the  results  obtained  in  this  respect  by  different  observers 
were  very  different.  Hence  it  was  believed  that  the  composition 
of  the  air  varies  at  different  places,  and  in  different  seasons,  and 
this  opinion  was  so  generally  adopted,  that  the  instrument  used 
for  such  measurements  was  termed  a  eudiometer  (evSta,  fine 
weather,  and  /j,€rpov,  a  measure).  Cavendish  investigated  this 
subject  with  his  accustomed  skill  in  the  year  1781,  and  found 
that  when  every  possible  precaution  is  taken  in  the  analysis, 

1  On  Factitious  Air.     Hon.  Henry  Cavendish.     Phil.  Trans.  1766,  141. 


20  HISTORICAL  INTRODUCTION 


"the  quantity  of  pure  air  in  common  air  is  -jf,"  or  100  volumes 
of  air  always  contain  20'8  volumes  of  dephlogisticated,  and  79*2 
volumes  of  phlogisticated  air,  and  that,  therefore,  atmospheric  air 
had  an  unvarying  composition.  But  the  discovery  which  more 
than  any  other  is  for  ever  connected  with  the  name  of  Cavendish 
is  that  of  the  composition  of  water  (1781).1  In  making  this  dis- 
covery Cavendish  was  led  by  some  previous  observations  of 
Priestley,  and  his  friend  Warltire.  They  employed  a  detonating 
closed  glass  or  copper  globe  holding  about  three  pints,  so 
arranged  that  an  electric  spark  could  be  passed  through  a 
mixture  of  inflammable  air  (hydrogen)  and  common  air,2  but 
though  they  had  observed  the  production  of  water,  they  not 
only  overlooked  its  meaning,  but  believed  that  the  change  was 
accompanied  by  a  loss  of  weight.  Cavendish  saw  the  full 
importance  of  the  phenomenon  and  set  to  work  with  care  and 
deliberation  to  answer  the  question  as  to  the  cause  of  the 
formation  of  the  water.  Not  only  did  he  determine  the 
volumes  of  air  and  hydrogen,  and  of  dephlogisticated  air 
(oxygen)  and  inflammable  air  (hydrogen)  which  must  be  mixed 
to  form  the  maximum  quantity  of  water,  but  he  first  showed 
that  no  loss  of  weight  occurred  in  this  experiment  and  that  the 
formation  of  acid  was  not  an  invariable  accompaniment  of  the 
explosion. 

On  this  important  subject  it  is  interesting  to  hear  Caven- 
dish's own  words;  in  the  Philosophical  Transactions  for  1784, 
page  128,  we  read  : — 

"  From  the  fourth  experiment  it  appears  that  423  measures 
of  inflammable  air  are  nearly  sufficient  to  phlogisticate  1,000 
of  common  air ;  and  that  the  bulk  of  the  air  remaining  after 
the  explosion  is  then  very  little  more  than  four-fifths  of  the 
common  air  employed  ;  so  that,  as  common  air  cannot  be  reduced 
to  a  much  less  bulk  than  that  by  any  method  of  phlogistication, 
we  may  safely  conclude  when  they  are  mixed  in  this  proportion, 
and  exploded,  almost  all  the  inflammable  air  and  about  one-fifth 
part  of  the  common  air,  lose  their  elasticity  and  are  condensed 
into  a  dew  which  lines  the  glass."  Since  1,000  volumes  of  air 
contain  210  volumes  of  oxygen  and  these  require  420  volumes  of 
hydrogen  to  combine  with  them,  we  see  how  exact  Cavendish's 

1  Phil.  Trans.  1784,  119,  and  1785,  372,  Mr.  Cavendish's  experiments  on  air. 

2  A  similar  apparatus  (originally  due  to  Volta)  was  used  by  Cavendish.     The 
pear-shaped  glass  bottle  with  stopcock,  usually  called  Cavendish's  eudiometer, 
would  not  be  recognised  by  the  great  experimenter. 


HENRY  CAVENDISH  21 

experiments  were.  "  The  better,"  he  continues,  "  to  examine 
the  nature  of  the  dew,  500,000  grain  measures  of  inflammable 
air  were  burnt  with  about  24  times  that  quantity  of  common  air 
and  the  burnt  air  made  to  pass  through  a  glass  cylinder  eight 
feet  long  and  three-quarters  of  an  inch  in  diameter,  in  order  to 
deposit  the  dew  ....  By  this  means  upwards  of  135  grains 
of  water  were  condensed  in  the  cylinder,  which  had  no  taste  or 
smell,  and  which  left  no  sensible  sediment  when  evaporated 
to  dryness ;  neither  did  it  yield  any  pungent  smell  during  the 
evaporation  ;  in  short,  it  seemed  pure  water."  Cavendish  then 
sums  up  his  conclusions  from  these  two  sets  of  experiments 
as  follows  : — "  By  the  experiments  with  the  globe  it  appeared 
that  when  inflammable  and  common  air  are  exploded  in  a 
proper  proportion,  almost  all  the  inflammable  air,  and  near  one- 
fifth  of  the  common  air,  lose  their  elasticity  and  are  condensed 
into  dew.  And  by  this  experiment,  it  appears  that  this  dew  is 
plain  water,  and  consequently  that  almost  all  the  inflammable 
air  and  about  one-fifth  of  the  common  air  are  turned  into  pure 
water." 

Still  more  conclusive  was  the  experiment  in  which  Cavendish 
introduced  a  mixture  of  dephlogisticated  air  and  inflammable 
air  nearly  in  the  proportions  of  one  to  two  into  a  vacuous 
glass  globe,  furnished  with  a  stopcock  and  means  of  firing  by 
electricity.  "  The  stopcock  was  then  shut  and  the  included 
air  fired  by  electricity,  by  which  means  almost  all  of  it  lost 
'  its  elasticity.  By  repeating  the  operation  the  whole  of  the 
mixture  was  let  into  the  globe  and  exploded,  without  any  fresh 
exhaustion  of  the  globe." 

Priestley  had  previously  been  much  led  astray  by  the  fact  that 
he  found  nitric  acid  in  the  water  obtained  by  the  union  of  the 
gases.  Cavendish,  by  a  careful  series  of  experiments,  explained 
the  occurrence  of  this  acid,  for  he  showed  that  it  did  not  form 
unless  an  excess  of  dephlogisticated  air  was  used,  and  he  traced 
its  production  to  the  presence  in  the  globe  of  a  small  quantity  of 
phlogisticated  air  (nitrogen)  derived  from  admixture  of  common 
air.  He  likewise  proved  that  the  artificial  addition  of  phlogisti- 
cated air  increased  the  quantity  of  acid  formed  in  presence  of 
dephlogisticated  air  (oxygen),  whilst  if  the  latter  air  were  re- 
placed by  atmospheric  air  no  acid  was  formed,  in  spite  of  the 
large  amount  of  phlogisticated  air  (nitrogen)  present.  In  this 
way  he  showed  that  the  only  product  of  the  explosion  of  pure 
dephlogisticated  with  pure  inflammable  air  is  pure  water. 


22  HISTORICAL  INTRODUCTION 

Although  Cavendish  thus  distinctly  proved  the  fact  of  the  com- 
position of  water,  it  does  not  appear  from  his  writings  that  he 
held  clear  views  as  to  the  fact  that  water  is  a  chemical  compound 
of  its  two  elementary  constituents.  On  the  contrary,  he  seems 
to  have  rather  inclined  to  the  opinion  that  the  water  formed 
was  already  contained  in  the  inflammable  air,  notwithstanding 
the  fact  that  in  1783  the  celebrated  James  Watt  had  already 
expressed  the  opinion  that  "  water  is  composed  of  dephlogis- 
ticated  and  inflammable  air."  1  Cavendish's  general  conclusions 
in  this  matter  may  be  briefly  summed  up  in  his  own  words  as 
follows  : — "  From  what  has  been  said  there  seems  the  utmost 
reason  to  think  that  dephlogisticated  air  is  only  water  deprived 
of  its  phlogiston,  and  that  inflammable  air,  as  was  before  said, 
is  either  phlogisticated  water,  or  else  pure  phlogiston ;  but  in 
all  probability  the  former."  To  the  end  of  his  days  Cavendish 
remained  a  firm  supporter  of  the  phlogistic  view  of  chemical 
phenomena,  but  after  the  overthrow  of  this  theory  by  Lavoisier's 
experiments  the  English  philosopher  withdrew  from  any  active 
participation  in  scientific  research. 

Whilst  Priestley  and  Cavendish  were  pursuing  their  great 
discoveries  in  England,  a  poor  apothecary  in  Sweden  was 
actively  engaged  in  investigations  which  were  to  make  the  name 
of  Scheele  (1742-1786)  honoured  throughout  Europe.  These 
investigations,  whilst  they  did  not  bring  to  light  so  many  new 
chemical  substances  as  those  of  Priestley,  and  did  not  possess 
the  quantitative  exactitude  which  is  characteristic  of  the  labours 
of  Cavendish,  opened  out  ground  which  had  been  entirely 
neglected,  and  was  perhaps  unapproachable  by  the  English 
chemists.  Scheele's  discoveries  covered  the  whole  range  of 
chemical  science.  A  strong  supporter  of  the  phlogistic  theory, 
he  held  peculiar  views  (see  his  celebrated  treatise  Ueber  die 
Luft  und  das  Feuer)  as  to  the  material  nature  of  heat  and  light, 
and  their  power  of  combining  with  phlogiston,  and,  like  Stahl, 
he  considered  modification  in  the  forms  of  matter  to  be  of  much 
greater  importance  than  alteration  in  its  weight.  In  experiment- 
ing upon  the  nature  of  common  air  he  discovered  oxygen  gas 
independently  of,  but  probably  somewhat  later  than,  Priestley. 

The  investigations  which  led  Scheele  to  this  discovery  are  of 

interest  as  a  remarkable  example  of  exact  observations  leading 

to  erroneous  conclusions.     His  object  was  to  explain  the  part 

played  by  the  air  in  the  phenomenon  of  combustion  ;  and  for 

1  Letter  from  Watt  to  Black,  21st  April,  1783. 


KARL  WILHELM  SCHEELE  23 

this  purpose  he  examined  the  action  exerted  by  bodies  sup- 
posed to  contain  phlogiston  upon  a  confined  volume  of  air. 
Thus  he  found  that  when  a  solution  of  hepar  sulphuris  (an  alka- 
line sulphide)  was  brought  into  contact  with  a  given  volume 
of  air,  that  volume  gradually  diminished,  the  residual  air  being 
incapable  of  supporting  the  combustion  of  a  taper.  The  same 
result  was  observed  when  moist  iron  filings  or  the  precipitate 
formed  by  the  action  of  potash  on  a  solution  of  green  vitriol  was 
employed.  Scheele  argued  that  if  the  effect  of  the  combination 
of  phlogiston  with  air  is  simply  to  cause  a  contraction,  the 
remaining  air  must  be  heavier  than  common  air.  He  found, 
however,  that  it  was  in  fact  lighter,  and  hence  inferred  that  a 
portion  of  the  common  air  must  have  disappeared,  and  that 
common  air  must  consist  of  two  gases,  one  of  which  has  the 
power  of  uniting  with  phlogiston.  In  order  to  find  out  what 
had  become  of  the  portion  of  air  which  disappeared,  Scheele 
heated  phosphorus,  metals,  and  other  bodies  in  closed  volumes 
of  air,  and  found  that  these  act  just  as  the  former  kind  of  sub- 
stances had  done.  Hence  he  concluded  that  the  compound 
formed  by  the  union  of  the  phlogiston  with  one  of  the  constituents 
of  the  air  is  nothing  more  nor  less  than  heat  or  fire  which  escapes 
through  the  glass.  In  confirmation  of  the  truth  of  this 
hypothesis,  Scheele  believed  that  he  had  experimentally  realised 
the  decomposition  of  heat  into  phlogiston  and  fire-air.  Nitric 
acid  had,  in  his  belief,  a  great  power  of  combining  with 
phlogiston,  forming  with  it  red  fumes  5  he  found  that  when  he 
heated  nitre  in  a  retort,  over  a  charcoal  fire,  with  oil  of  vitriol, 
he  obtained,  in  addition  to  a  fuming  acid,  a  colourless  air, 
which  supported  combustion  much  better  than  common  air. 
This  he  explained  by  assuming  that  when  charcoal  burns,  the 
phlogiston  combines  with  the  fire-air  to  form  heat,  which  passes 
into  the  retort,  and  is  there  decomposed  into  phlogiston, 
which  by  combining  with  the  acid  gave  rise  to  the  red  nitrous 
fumes,  and  pure  fire-air.  He  conceived  that  he  had  brought 
about  the  same  chemical  decomposition  of  heat  by  warming 
black  oxide  of  manganese  with  sulphuric  acid,  or,  still  more 
simply,  by  heating  calx  of  mercury ;  for  here  it  was  clear  enough 
that  by  bringing  heat  and  calx  of  mercury  together,  the  phlo- 
giston combined  with  the  latter,  and  fire-air  was  liberated, 
thus : — 

Heat.  Mercury. 

Phlogiston  +  Fire-air  +  Calx  of  Mercury  =  Calx  of  Mercury  +  Phlogiston  +  Fire  air. 


24  HISTORICAL  INTRODUCTION 

In  the  year  1774  Scheele  made  his  great  discovery  of  chlorine 
gas,  which  he  termed  dephlogisticated  muriatic  acid  ;  in  the  same 
year  he  showed  that  baryta  was  a  peculiar  earth  ;  shortly  after- 
wards he  proved  the  separate  existence  of  molybdic  and  tungs- 
tic  acids,  whilst  his  investigations  of  prussian  blue  led  to  the 
isolation  of  hydrocyanic  acid,  of  which  he  ascertained  the 
properties.  It  was,  however,  especially  in  the  domain  of 
animal  and  vegetable  chemistry  that  Scheele's  most  numerous 
discoveries  lay,  as  will  be  seen  by  the  following  list  of  organic 
acids  first  prepared  or  distinctly  identified  by  him : — tartaric, 
oxalic  (by  the  action  of  nitric  acid  on  sugar),  citric,  malic,  gallic, 
uric,  lactic,  and  mucic.  In  addition  to  the  identification  of  each 
of  these  as  distinct  substances,  Scheele  discovered  glycerin,  and 
we  may  regard  him  not  only  as  having  given  the  first  indication 
of  the  rich  harvest  to  be  reaped  by  the  investigation  of  the 
compounds  of  organic  chemistry,  but  as  having  been  the  first 
to  discover  and  make  use  of  characteristic  reactions  by  which 
closely  allied  substances  can  be  detected  and  separated,  so 
that  he  must  be  considered  one  of  the  chief  founders  of  an- 
alytical chemistry. 

We  have  now  brought  the  history  of  our  science  to  the  point 
at  which  Lavoisier  placed  it  in  the  path  which  it  has  ever  since 
followed.  Before  describing  the  overthrow  of  the  phlogistic 
theory  it  may  be  well  shortly  to  review  the  position  of  the 
science  before  the  great  chemist  began  his  labours  about  one 
hundred  years  ago.  Chemistry  had  long  ceased  to  be  the  slave 
of  the  alchemist  or  the  doctor ;  all  scientific  chemists  had 
adopted  Boyle's  definition,  and  the  science  was  valued  for  its 
own  sake  as  a  part  of  the  great  study  of  nature.  Stahl  had  well 
defined  chemistry  to  be  the  science  which  was  concerned  with 
the  resolution  of  compound  bodies  into  their  simpler  consti- 
tuents, and  with  the  building  up  of  compounds  from  their 
elements  ;  so  that  the  distinction  between  pure  and  applied 
chemistry  was  perfectly  understood.  Geber's  definition  of  a 
metal  as  a  fusible,  malleable  substance,  capable  of  mixing 
with  other  metals,  was  still  accepted  ;  gold  and  silver  were  con- 
sidered to  be  pure  or  noble  metals,  whilst  the  other  malleable 
metals,  copper,  tin,  iron,  and  lead,  were  called  the  base  metals. 
Mercury,  on  the  other  hand,  was  thought  to  be  only  a  metal-like 
body  until  it  was  frozen  in  1759.  After  that  date  it  was  con- 
sidered to  be  a  true  metal  in  a  molten  state  at  the  ordinary 
temperature  Arsenic,  antimony,  bismuth,  and  zinc,  from  being 


LAVOISIEK  25 


brittle,  were  classed  as  semi-metals,  and  to  these  well-known 
bodies  were  added  cobalt  in  1735,  nickel  in  1751,  and  man- 
ganese in  1774,  whilst  platinum  was  recognised  as  a  peculiar 
metal  in  1750,  and  molybdenum  and  tungsten  were  discovered 
about  1780.  The  several  metals  were  supposed  to  be  compounds 
of  phlogiston  with  metallic  calces,  whilst  sulphur,  phosphorus, 
and  carbon  were  looked  upon  as  compounds  of  phlogiston  with 
the  acids  of  these  elements.  Of  the  simple  gases  the  following 
were  known  :  inflammable  air  (hydrogen),  supposed  to  be  either 
pure  phlogiston  or  phlogisticated  water  ;  dephlogisticated  or  fire- 
air  (oxygen) ;  phlogisticated  air  (nitrogen) ;  and  dephlogisticated 
muriatic  acid  (chlorine).  When  the  metals  dissolve  in  acids  the 
phlogiston  was  thought  to  escape  (as  inflammable  air)  either  in 
the  pure  state  or  combined  with  water.  It  was  also  known  that 
when  a  metal  is  calxed,  an  increase  of  weight  occurs,  but  this 
was  explained  either  by  the  metal  becoming  more  dense,  which, 
in  the  opinion  of  some,  would  produce  an  increase  of  weight,  or 
by  the  absorption  of  fiery  particles,  or  again  by  the  escape  of 
phlogiston,  a  substance  which  instead  of  being  attracted  is  re- 
pelled by  the  earth.  In  short,  confusion  and  difference  of 
opinion  in  the  quantitative  relations  of  chemistry  reigned  su- 
preme, and  it  was  not  until  Lavoisier  brought  his  great  powers 
to  bear  on  the  subject  that  light  was  evoked  from  the  darkness 
and  the  true  and  simple  nature  of  the  phenomena  was  rendered 
evident. 

In  the  year  1743  Lavoisier  was  born.  Carefully  educated, 
endowed  with  ample  means,  Lavoisier,  despising  the  usual  occu- 
pations of  the  French  youth  of  his  time,  devoted  himself  to 
science,  his  genius,  aided  by  a  careful  mathematical  and  physical 
training,  rendering  it  possible  for  him  to  bring  about  a  complete 
revolution  in  the  science  of  chemistry.  Before  his  time  quanti- 
tative methods  and  processes  were  considered  to  be  purely 
physical,  though  they  now  are  acknowledged  to  be  chemical, 
and  of  all  these,  the  determination  of  the  weights  of  bodies 
taking  part  in  chemical  change,  as  ascertained  by  the  balance, 
is  the  most  important.  Others,  indeed,  before  him,  had  made 
quantitative  investigations.  Black  and  Cavendish  almost  ex- 
ceeded Lavoisier  in  the  exactitude  of  their  experiments,  but  it 
is  to  the  French  philosopher  that  the  glory  of  having  first 
distinctly  asserted  the  great  principle  of  the  indestructibility 
of  matter  belongs.  Every  chemical  change,  according  to  him, 
consists  in  a  transference  or  an  exchange  of  a  portion  of  the 


26  HISTORICAL  INTRODUCTION 

material  constituents  of  two  or  more  bodies ;  the  sum  of  the 
weights  of  the  substances  undergoing  chemical  change  always 
remains  constant,  and  the  balance  is  the  instrument  by  which 
this  fundamental  fact  is  made  known. 

In  his  first  important  research  (1770)  Lavoisier  employs 
the  balance  to  investigate  the  question,  much  discussed  at  the 
time,  as  to  whether  water  on  being  heated  becomes  converted 
into  earth.  For  one  hundred  and  one  days  1  he  heated  water  in 
a  closed  and  weighed  vessel  ;  at  the  end  of  the  experiment  the 
weight  of  the  closed  vessel  remained  unaltered,  but  on  pouring 
out  the  water  he  found  that  the  vessel  had  lost  17'4  grains, 
whilst  on  evaporating  the  water,  he  ascertained  that  it  had  dis- 
solved 20*4  grains  of  solid  matter.  Taking  the  excess  of  3'0 
grains  as  due  to  unavoidable  experimental  errors,  he  concludes 
that  water  when  heated  is  not  converted  into  earth.  Shortly 
after  this,  the  same  question  was  examined  independently  by 
Scheele,  who  obtained  the  same  results  by  help  of  qualitative 
analysis,  which  showed  that  the  water  had  taken  up  a  constituent 
of  the  glass,  viz.,  the  alkaline  silicates. 

When  he  became  acquainted  with  the  novel  and  unexpected 
discoveries  of  Black,  Priestley,  and  Cavendish,  a  new  light  burst 
upon  the  mind  of  Lavoisier,  and  he  threw  himself  instantly 
with  fresh  ardour  into  the  study  of  specially  chemical 
phenomena.  He  saw  at  once  that  the  old  theory  was  in- 
capable of  explaining  the  facts  of  combustion,  and  by  help 
of  his  own  experiments,  as  well  as  by  making  use  of  the 
experiments  of  others,  he  succeeded  in  finding  the  correct 
explanation,  destroying  for  ever  the  theory  of  phlogiston,  and 
rendering  his  name  illustrious  as  having  placed  the  science  of 
chemistry  on  its  true  basis.  On  looking  back  in  the  history  of 
our  science  we  find  indeed  that  others  had  made  experiments 
which  could  only  be  explained  by  this  new  theory,  and  in  cer- 
tain isolated  instances  the  true  explanation  may  have  previously 
occurred  to  the  minds  of  others.  Thus  in  1774  Bayen  showed 
that  calx  of  mercury  loses  weight,  evolving  a  gas  equal  in  weight 
to  what  is  lost,  and  he  concludes  that  either  the  theory  of  phlo- 
giston is  incorrect,  or  this  calx  can  be  reduced  without  addition 
of  phlogiston.  This,  however,  in  no  way  detracts  from  Lavoisier's 
glory  as  having  been  the  first  to  carry  out  the  true  ideas  consist- 
ently and  deliberately  through  the  whole  science.  It  is  the 
systematic  application  of  a  truth  to  every  part  of  a  science  which 

1  (Euvres  de  Lavoisier,  2,  22. 


LAVOISIER  27 


constitutes  a  theory,  and  this  it  was  that  Lavoisier  and  no  one 
else  accomplished  for  chemistry. 

When  a  man  has  done  so  much  for  science  as  Lavoisier,  it 
seems  almost  pitiful  to  discuss  his  shortcomings  and  failings. 
But  it  is  impossible  in  any  sketch  of  the  history  of  chemistry  to 
ignore  the  question  how  far  Lavoisier's  great  conclusions,  the 
authorship  of  which  no  one  questions,  were  drawn  from  his 
own  discoveries,  or  how  far  he  was  indebted  to  the  original 
investigations  of  his  contemporaries  for  the  facts  upon  which 
his  conclusions  are  based.  The  dispute  has  recently  assumed 
fresh  interest.  Certain  chemists  consider  that  to  him  alone  the 
foundation  of  modern  chemistry  is  to  be  ascribed,  both  as  . 
regards  material  and  deduction,  whilst  others,  affirming  that 
Lavoisier  made  use  of  the  discoveries  of  his  predecessors, 
and  especially  of  the  discovery  of  oxygen  by  Priestley, 
without  acknowledgment,  assert  that  he  went  so  far  as  to 
claim  for  himself  a  participation  in  this  discovery  to  which 
he  had  no  right  whatever,  and  insist  that  until  he  had  thus 
obtained,  from  another,  the  key  to  the  problem,  his  views  upon 
the  question  of  combustion  were  almost  as  vague  as  those  of 
the  phlogistonists  themselves.  To  enter  into  a  full  discussion 
of  the  subject  would  lead  us  into  a  historical  criticism  which 
would  outrun  our  space.  Suffice  it  to  say  that  many  of  the 
charges  which  have  been  brought  against  Lavoisier's  good 
faith  unfortunately  turn  out  upon  investigation  to  be  well 
founded,  so  that  whilst  we  must  greatly  admire  the  clear  sight 
of  the  philosopher,  we  cannot  feel  the  same  degree  of  respect  for 
the  moral  character  of  the  man. 

His  investigations  on  the  phenomena  of  combustion  began  in 
the  year  1772.  In  a  first  memoir1  Lavoisier  finds  not  only  that 
^hen  sulphur  and  phosphorus  are  burnt  no  loss  of  weight 
occurs,  but  that  an  increase  of  weight  is  observed.  Hence  he 
concludes  that  a  large  quantity  of  air  becomes  fixed.  This  dis- 
covery leads  him  to  the  conclusion  that  a  similar  absorption  of 
air  takes  place  whenever  a  body  increases  in  weight  by  combus- 
tion or  calcination.  In  order  to  confirm  this  view,  he  reduces 
litharge  with  charcoal/and  finds  that  a  considerable  quantity 
of  air  is  liberated.  This,  he  asserts,  appears  to  him  to  be 
one  of  the  most  interesting  experiments  made  since  the  time 
of  Stahl. 

Lavoisier's   next  publication   was  his    Opuscles  physiques   ct 
1  Sur  la  Cause  de  I' Augmentation  des  Poids.     (Euvres,  2,  99- 


28  HISTORICAL  INTRODUCTION 

chymiques,  commenced  in  1774.  In  these  memoirs  he  first 
examines  the  kind  of  air  given  off  in  the  processes  of  breathing, 
combustion,  and  fermentation.  The  views  which  he  expresses  are 
similar  to  those  put  forward  long  before  by  Black,  to  whom 
he  repeatedly  refers  as  the  originator  of  them,  this  acknowledg- 
ment of  his  indebtedness  to  the  Scottish  philosopher  being 
repeated  in  the  letters  from  Lavoisier  to  Black  which  have 
been  already  referred  to,1  in  one  of  which  the  following  passage 
occurs : — "  Plus  confiant  dans  vos  idSes  que  dans  les  miennes 
propres,  accoutume  d  vous  regarder  comme  mon  maitre"  &c. 

In  the  year  1774  he  describes  experiments  on  the  calcination 
of  lead  and  tin,  which  he,  like  Boyle,  heats  in  closed  glass 
globes  :  so  long  as  the  vessel  is  closed  it  does  not  change  in 
weight,  but  when  the  neck  of  the  flask  is  broken,  air  rushes  in, 
and  the  weight  increases.  He  further  shows  that  only  a  por- 
tion of  the  air  is  taken  up  by  the  molten  metal,  and  that  the 
residual  air  is  different  from  common  air,  and  also  from  fixed  air. 
From  these  statements  it  is  clear  that  Lavoisier  considered  that 
the  air  consists  of  two  different  elastic  fluids,  but  that  he  was 
not  acquainted  with  Priestley's  discovery  of  oxygen.  Nor  were 
his  views  at  this  time  so  precise  or  well  defined  as  we  should 
gather  from  reading  his  papers  published  in  the  memoirs 
of  the  French  Academy  for  1774.  The  explanation  is 
simple  enough,  inasmuch  as  owing  to  the  careless  and  tardy 
manner  in  which  the  memoirs  of  the  French  Academy  were 
at  that  time  edited,  changes  in  the  original  communications 
were  frequently  made  by  the  writers  before  publication,  so  that 
the  papers  printed  in  the  memoirs  were  corrected  to  suit 
alteration  in  view  or  in  fact  which  had  become  known  to  the 
authors  between  the  times  of  reading  and  of  publication. 
Thus,  for  instance,  it  is  clear  that  the  paper2  detailing  the 
results  of  his  experiments  on  the  calcination  of  the  metals 
above  referred  to,  which  was  read  before  the  Academy  in  Nov. 
1774,  does  not  express  the  same  views  which  we  find  given 
in  the  extended  description  of  his  experiments  contained 
in  the  volume  of  the  memoirs  for  1774,  which  however 
was  not  published  till  1778.  So  that  although  Lavoisier  in 
1774  considered  air  to  be  made  up  of  several  different  elastic 
fluids,  it  is  certain  that  he  was  not  then  acquainted  with  the 
kind  of  air  which  was  absorbed  in  calcination,  that  his  views 

1  British  Association  Reports,  1871,  190. 

2  Journal  de  Physique  for  Dec.  1774. 


THE  DISCOVERY  OF  OXYGEN  29 

on  the  subject  were  in  reality  very  similar  to  those  expressed 
a  century  before  by  Jean  Key  (1630),  Mayow  (1669),  and  later, 
by  Pott  (1750),  and  that  they  were  far  from  being  as  precise 
and  true  as  we  should  gather  them  to  have  been  from  the  perusal 
of  his  extended  memoir,  printed  in  1778  and  corrected  so  as 
to  harmonise  with  the  position  of  the  science  at  that  date. 

It  is  not  until  we  come  to  a  paper,  Sur  le  nature  du 
principe  qui  se  combine  avec  les  me'taux  pendant  leur  calcina- 
tion, first  read  in  1775  and  re-read  on  Aug.  8,  1778,  that 
we  find  a  distinct  mention  of  oxygen  gas,  which  he  first  termed 
" I'air  e'minemment  respirable,"  or  "  I' air  pur,"  or  "  I' air  vital," 
and  that  we  see  that  the  whole  theory  of  combustion  is  clear  to 
Lavoisier.  He  shows  that  this  gas  is  necessary  for  the  cal- 
cination of  metals,  he  prepares  it  from  precipitation  per  se, 
as  Priestley  had  previously  done,  and  in  the  year  1778  we 
find  the  first  mention  of  oxygen  or  the  acidifiant  principle.  The 
name  was  given  to  it  because  he  observed  that  combined  with 
carbon  this  substance  forms  carbonic  acid,  with  sulphur  vitriolic 
acid,  with  nitrous  air  nitric  acid,  with  phosphorus  phosphoric 
acid,  although  with  the  metals  in  general  it  produces  the 
metallic  calces.  In  his  Elements  de  Chimie,  published  in  1782, 
we  find  the  following  words  under  oxygen  gas  : — "  Get  air  que 
nous  avons  (ttcouwrt  presque  en  mSme  temps,  Dr.  Priestley,  M. 
Scheele  et  moi."  x  Now  there  is  no  doubt  whatever  that  in 
October,  1774,  Dr.  Priestley  informed  Lavoisier,  in  Paris,  of  the 
discovery  he  had  lately  made,  and  that  Lavoisier  was  at  that 
time  unacquainted  with  the  fact  that  precipitatum  per  se  yields 
this  new  gas  on  heating.  Hence  we  cannot  admit  Lavoisier's 
claim  to  the  joint  discovery  of  oxygen,  a  claim,  it  is  to  be 
remembered,  not  made  until  eight  years  after  the  event  had 
occurred.  In  corroboration  of  this  conclusion  we  find  in 
Priestley's  last  work,  published  in  1800,  and  singularly  enough 
entitled  The  Doctrine  of  Phlogiston  Established,  the  following 
succinct  account  of  the  matter.  "  Now  that  I  am  on  the  subject  of 
the  right  of  discoveries,"  he  says,  "  I  will  as  the  Spaniards  say, 
leave  no  ink  of  this  kind  behind  in  my  ink-horn,  hoping  it  will  be 
the  last  time  I  shall  have  any  occasion  to  trouble  the  public  about 
it.  M.  Lavoisier  says  (Elements  of  Chemistry,  English  edition, 
p.  36)  '  This  species  of  air  (meaning  dephlogisticated)  was 
discovered  almost  at  the  same  time  by  Mr.  Priestley,  M.  Scheele, 
and  myself.'  The  case  was  this  :  having  made  the  discovery 

1  (Euvres,  1,  38. 


30  HISTORICAL  INTRODUCTION 

some  time  before  I  was  in  Paris  in  1774,  I  mentioned  it  at  the 
table  of  M.  Lavoisier,  when  most  of  the  philosophical  people  in 
the  city  were  present ;  saying  that  it  was  a  kind  of  air  in  which 
a  candle,  burned  much  better  than  in  common  air,  but  I  had 
not  then  given  it  any  name.  At  this  all  the  company,  and  Mr.  and 
Mrs.  Lavoisier  as  much  as  any,  expressed  great  surprise  ;  I  told 
them  then  I  had  gotten  it  from  precipitatum per  se  and  also  from 
red  lead.  Speaking  French  very  imperfectly,  and  being  little 
acquainted  with  the  terms  of  chemistry,  I  said  plomb  rouge  and 
was  not  understood  till  M.  Macquer  said, '  I  must  mean  minium! 
M.  Scheele's  discovery  was  certainly  independent  of  mine,  though 
I  believe  not  made  quite  so  early." 

The  two  memoirs  in  which  Lavoisier  clearly  puts  forward  his 
views  on  the  nature  of  combustion  and  respiration  are,  first,  one 
read  before  the  Academy  in  1775,  Sur  la  combustion  en  gtne'ral, 
and  second,  one  entitled  Reflexions  sur  la  Phlogistique,  published 
by  the  Academy  in  1783.  In  the  first  of  these  memoirs  he 
does  not  attempt  to  substitute  for  Stahl's  doctrine  a  rigorously 
demonstrated  theory,  but  only  an  hypothesis  which  appears  to 
him  more  conformable  to  the  laws  of  nature,  and  less  to  con- 
tradict known  facts.  In  the  second  memoir  he  develops  his 
theory,  denying  the  existence  of  any  "  principle  of  combust- 
ibility," as  upheld  by  Stahl,  stating  that  the  metals,  and  such 
substances  as  carbon,  sulphur,  &c.,  are  simple  bodies  which  on 
combustion  enter  into  combination  with  oxygen,  and  concluding 
that  Stahl's  supposition  of  the  existence  of  phlogiston  in  the 
metals,  &c.,  is  entirely  gratuitous,  and  more  likely  to  retard  than 
to  advance  the  progress  of  science. 

The  triumph  of  the  antiphlogistic  (Lavoisierian)  doctrines 
was,  however,  not  complete  until  the  discovery  of  the  compound 
nature  of  water  by  Cavendish  in  1783  became  fully  known. 
The  experiment  concerning  the  combination  of  hydrogen  (phlo- 
giston) and  oxygen  to  form  water  was  at  once  repeated  and 
confirmed  by  Lavoisier  and  Laplace  on  the  24th  June,  1783, 
and  then  Lavoisier  was  able  satisfactorily  to  explain  the  changes 
which  take  place  when  metals  dissolve  in  acids,  and  to  show 
that  the  metals  are  simple  bodies  which  take  up  oxygen  on 
combustion,  or  on  solution  in  acid,  the  oxygen  being  derived 
in  the  latter  case  either  from  the  acid  or  from  the  water 
present. 

Here,  again,  if  we  investigate  the  position  occupied  by  Lavoisier 
respecting  the  discovery  of  the  composition  of  water  we  shall  see 


CAVENDISH,  WATT,  AND  LAVOISIER  31 

that,  not  content  with  the  glory  of  having  been  the  first  to 
give  the  true  explanation  of  the  phenomena,  he  appears  to  claim 
for  himself  the  first  quantitative  determination  of  the  fact,1 
although  it  is  clear  that  he  had  been  previously  informed  by 
Blagden  of  Cavendish's  experiments.2 

The  verdict  concerning  the  much- vexed  question  as  to  the 
rival  claims  of  Cavendish,  Watt,  and  Lavoisier,  cannot  be  more 
forcibly  or  more  concisely  given  than  in  the  following  words  of 
Professor  Kopp — Cavendish  first  ascertained  the  facts  upon 
which  the  discovery  of  the  composition  of  water  was  based, 
although  we  are  unable  to  prove  that  he  first  deduced  from  these 
facts  the  compound  nature  of  water,  or  that  he  was  the  first 
rightly  to  recognise  its  constituent  parts.  Watt  was  the  first  to 
argue  from  these  facts  the  compound  nature  of  water,  although 
he  did  not  arrive  at  a  satisfactory  conclusion  respecting  the 
nature  of  the  components ;  whilst  Lavoisier,  also  from  these 
facts,  first  clearly  recognised  and  stated  the  true  nature  of  the 
components  of  water. 

Although  at  this  period  the  experimental  basis  of  the  true 
theory  of  combustion  was  complete,  it  was  some  time  before  the 
clear  statements  of  Lavoisier  were  accepted  by  chemists.  Many 
of  those  who  were  most  distinguished  by  their  discoveries 
remained  to  the  last  wedded  to  the  old  ideas,  but  by  degrees,  as 
fresh  and  unprejudiced  minds  came  to  study  the  subject,  the 
new  views  were  universally  adopted. 

In  considering  this  great  discussion  from  our  present  point 
of  view,  we  cannot  but  recognise  in  the  phlogistic  theory  the 
expression  of  an  important  fact,  of  which,  however,  the  true  in- 
terpretation was  unknown  to  the  exponents  of  the  theory.  The 
phlogistonists  assert  that  something  which  they  term  phlogiston 
escapes  when  a  body  burns  ;  the  antiphlogistonists  prove,  on  the 
other  hand,  that  no  escape  of  material  substance  then  occurs,  but 
that  on  the  contrary,  an  addition  of  oxygen  (or  some  other 
element)  always  takes  place.  In  thus  correcting  from  one  aspect 
the  false  statement  of  the  followers  of  Stahl,  Lavoisier  and  his 
disciples  to  some  extent  overlooked  an  interpretation  which  may 

1  (Euvres,  2,  338. 

2  For  an  exhaustive  discussion  of  this  subject  we  must  refer  the  reader  to 
George  Wilson's  Life  of  Cavendish,  1849,  as  well  as  to  Prof.  H.  Kopp's  Beitrdge 
zur  Geschichte  der  Chemie.     Die  JSntdcckung  der  Zusammensetzung  des  Wassers. 
Vieweg  und  Sohn,  1875.     See  also  Grimaux,  Lavoisier,  F.  Alcan,  Paris.     Thorpe, 
Brit.  Ass.  Reports,   1890,    p.    761.,  and    Berthelot,  La    Revolution   Chimiquey 
F.  Alcan,  Paris,  1890. 


32  HISTORICAL  INTRODUCTION 

truly  be  placed  upon  the  statements  of  the  phlogistonists,  for  if  in 
place  of  the  word  "  phlogiston/'  we  read  "  energy,"  this  old  theory 
becomes  the  expression  of  the  latest  development  of  scientific 
investigation.  We  now  know  that  when  two  elements  combine, 
Energy,  generally  in  the  form  of  heat,  is  usually  evolved, 
whilst  in  order  to  resolve  the  compound  into  its  constituent 
elements  an  expenditure  or  absorption  of  an  equal  amount 
of  energy  is  requisite. 

The  fact  that  every  distinct  chemical  compound  possesses  a 
fixed  and  unalterable  composition,  was  first  proved  by  the 
endeavour  to  fix  the  composition  of  certain  neutral  salts. 
Bergman  from  the  year  1775,  and  Kirwan  from  1780,  were 
occupied  with  this  experimental  inquiry,  but  their  results  did 
not  agree  sufficiently  well  to  enable  chemists  to  come  to  a  satis- 
factory conclusion,  and  it  was  to  Oavendish  that  we  owe  the  first 
proof  that  the  combining  proportion  between  base  and  acid  follows 
a  distinct  law,  whilst  to  him  we  also  owe  the  introduction  of  the 
word  "  equivalent "  into  the  science.  It  is,  however,  to  Richter 
(1762-1807)  that  we  are  indebted  for  the  full  explanation  of  the 
fact,  that  when  two  neutral  salts  undergo  mutual  decomposition, 
the  two  newly-formed  salts  are  also  neutral.  He  shows  in  his 
"  Stochiometrie,"  that  the  proportions  by  weight  of  different  bases 
which  saturate  the  same  weight  of  a  given  acid  will  also  saturate 
a  different  but  a  constant  weight  of  a  second  acid.  So  that  if 
we  have  determined  what  weight  of  a  given  base  is  required  to 
saturate  a  given  weight  of  several  different  acids,  and  also  if  we 
know  the  weights  of  the  different  bases  which  are  needed  for  the 
neutralization  of  a  given  weight  of  any  one  of  these  acids,  we 
can  calculate  in  what  proportion  each  of  these  bases  will  unite 
with  any  one  of  these  acids.  Richter  also  showed  that  when  the 
different  metals  are  separately  dissolved  in  the  same  quantity 
of  sulphuric  acid,  each  one  takes  up  the  same  quantity  of 
oxygen  ;  or,  as  we  may  now  express  it,  the  varying  quantities  of 
these  different  oxides  which  neutralize  one  and  the  same 
quantity  of  any  acid,  all  contain  the  same  quantity  of  oxygen. 
These  important  observations  attracted  but  little  attention  or 
consideration  from  Richter's  contemporaries,  all  of  whom  were 
busily  engaged  in  carrying  on  the  phlogistic  war  in  which  he 
himself  took  an  active  part  in  defence  of  the  older  doctrine. 

The  investigations  of  Richter  and  his  predecessors  had 
reference  mainly  to  the  proportions  by  weight  in  which  acids 
and  bases  unite,  which,  according  to  Lavoisier's  theory,  are  not 


"ESSAI  DE  STATIQUE  CHIMIQUE  "  33 

simple  substances,  whilst  Lavoisier  recognised  the  fact  that  the 
elements  themselves  combine  in  definite  proportions  by  weight. 
In  opposition  to  this  view  of  combination  in  definite  unalterable 
•quantities,  L.  Claude  Berthollet  published  in  1803  his  cele- 
brated Essai  de  statique  Chimique,  in  which  he  refers  the 
phenomena  of  chemistry  to  certain  fundamental  properties  of 
matter,  endeavouring  to  explain  chemical  changes  by  the  motions 
of  the  particles  of  matter  on  the  same  principles  as  Newton's 
theory  of  gravitation  accounts  for  the  simpler  motions  of  the 
heavenly  bodies.  Considering  chemical  change  from  this 
mechanical  point  of  view,  Berthollet  pointed  out  the  cir- 
cumstances under  which  we  can  accomplish  the  highest 
development  of  the  science,  namely,  prediction  of  phenomena ; 
.and,  if,  in  his  assumed  identity  of  the  laws  of  gravitation 
and  chemical  action,  he  was  mistaken,  the  aim  which  he  set 
before  himself  is  that  which  has  remained,  and  will  ever 
remain,  the  highest  ideal  of  the  science.  The  influence  which 
Berthollet's  views  exercised  on  the  progress  of  the  science  was 
less  powerful  than  it  otherwise  would  have  been  owing  to  the 
fact  that  he,  considering  chemical  combination  to  be  based  upon 
purely  mechanical  laws,  was  obliged  to  admit  that  an  alter- 
ation of  the  conditions,  such  as  mass  and  temperature,  must 
generally  produce  an  alteration  in  the  composition  of  the 
chemical  compound,  and  was,  therefore,  forced  to  the  conclusion 
that  combination  may  take  place,  as  a  rule,  between  variable 
proportions  of  the  elements,  fixity  of  proportion  being  the 
exception,  due  to  some  special  physical  property  of  the  com- 
pound containing  those  proportions,  such  as  insolubility  or 
elasticity.  The  opposite  view  that  combination  only  takes 
place  in  a  small  number  of  definite  fixed  proportions  was 
defended  by  his  countryman  Proust,  and  this  led  to  a  keen 
debate  between  the  two  French  philosophers  which  lasted  from 
the  year  1801  to  the  year  1808.  In  the  end,  however,  Proust 
proved  conclusively  that  Berthollet's  views  were  incorrect 
inasmuch  as  he  showed  that  when  one  metal  gives  rise  to  two 
oxides,  the  weight  of  the  metal  which  combines  with  the  same 
quantity  of  oxygen  to  form  the  various  oxides  is  a  different 
but  a  fixed  quantity,  so  that  combination  does  not  take  place  by 
the  gradual  addition  of  one  element,  but  by  sudden  increments. 
This  observation  ought  in  fact  to  have  led  to  the  recognition  by 
Proust  of  the  law  of  combining  proportions,  but  his  analyses 
4 


34  HISTORICAL  INTRODUCTION 


were  not  sufficiently  accurate  for  this  purpose,1  so  that  neither 
Proust  nor  Richter  arrived  at  the  true  expression  of  the  facts 
of  chemical  combination,  and  it  was  reserved  for  John  Dalton, 
(1766 — 1844)  clearly  to  state  the  great  law  of  chemical  com- 
bination in  multiple  proportions,  and  to  found  upon  this  a  theory 
which  fully  explains  the  observed  facts. 

Democritus,  and  after  him  Epicurus  and  Lucretius,  had  long 
ago  taught  that  matter  is  made  up  of  small  indivisible  particles, 
and  the  idea  of  the  atomic  constitution  of  matter,  and  even  the 
belief  that  chemical  combination  consists  in  the  approximation 
of  the  unlike  particles,  had  been  already  expressed  by  Kirwan  in 
1783^  as  well  as  by  Higgins  in  1789.  Dalton  was,  however,  the 
first  to  propound  a  truly  chemical  atomic  theory,  the  only  one 
hitherto  proposed  which  explains  the  facts  of  chemical  combina- 
tion in  a  satisfactory  manner.  The  cardinal  point  upon  which 
Dalton's  atomic  theory  rests,  and  in  which  it  differs  from  all 
previous  suggestions,  is  that  it  is  a  quantitative  theory  respecting 
the  constitution  of  matter,  whereas  all  others  are  simply  quali- 
tative views.  For  whilst  all  previous  upholders  of  an  atomic 
theory,  including  even  Higgins,  had  supposed  that  the  relative 
weights  of  the  atoms  of  the  various  elements  are  the  same, 
Dalton  at  once  declared  that  the  atoms  of  the  different  elements 
are  not  of  the  same  weight  ;  and  that  the  relative  atomic  weights 
of  the  elements  are  the  proportions  by  weight  in  which  the  elements 
combine,  or  some  multiple  or  submultiple  of  these. 

In  1803  Dalton  published  his  first  table  of  atomic  weights  of 
certain  elements  and  their  compounds,  as  an  appendix  to  a  paper 
read  before  the  Manchester  Literary  and  Philosophical  Society, 
Oct.  23,  1803,  on  the  absorption  of  gases  by  water  and  other 
liquids.  As  a  reason  for  introducing  these  numbers,  Dalton  states 
that  the  different  solubility  of  gases  in  water  depends  upon  the 
weight  and  number  of  the  ultimate  particles  of  the  several  gases. 
"  The  inquiry,"  he  continues,  "  into  the  relative  weights  of  the 
ultimate  particles  of  bodies  is  a  subject,  as  far  as  I  know,  entirely 
new ;  I  have  lately  been  prosecuting  this  inquiry  with  remark- 
able success.  The  principle  cannot  be  entered  upon  in  this 
paper,  but  I  shall  subjoin  the  results  as  far  as  they  appear 
ascertained  by  my  experiments." 

1  Journal  de  Physique,  59,  260  and  321. 


JOHN  DALTON  35 


Daltoris  First  Table  of  the  Relative    Weights  of  the   Ultimate 
Particles  of  Gaseous  and  other  Bodies. 

Hydrogen .....  1  Nitrous  oxide     .     .     .  137 

Azot 4'2      Sulphur 14-4 

Carbon 4'3  Hypo-nitric  acid      .     .15-2 

Ammonia 5'2  Sulphuretted  hydrogen  15 '4 

Oxygen 5'5  Carbonic  acid      .     .     .  15'3 

Water 6'5      Alcohol 151 

Phosphorus    .     .     .     .  7'2  Sulphureous  acid    .     .19*9 

Phosphuretted  hydrogen  8*2  Sulphuric  acid    .     .     .  25*4 

Nitrous  gas    ....  0*3  Carburetted  hydrogen, 

Ether 9*6  from  stagnant  water.     6'3 

Gaseous  oxide  of  carbon  9*8  Olefiant  gas   .     .     .     .     5'3 

Thus  then,  at  the  end  of  a  paper  on  a  physical  subject,  does 
Dalton  make  known  a  principle  the  discovery  of  which  at  once 
placed  the  science  of  chemistry  upon  its  true  basis,  and  has 
rendered  the  name  of  its  discoverer  second  only  to  that  of 
Lavoisier  amongst  the  founders  of  the  Science. 

It  is  not  easy  to  follow  in  detail  the  mental  or  experimental 
processes  by  which  Dalton  arrived  at  this  great  theory.  Certain 
it  is,  however,  that  the  idea  which  lay  at  its  foundation  had  long 
been  in  his  mind,  which  was  essentially  of  a  mathematical  and 
mechanical  turn,  and  that  it  was  by  his  own  experimental  deter- 
minations, and  not  by  combining  any  train  of  reasoning  derived 
from  the  previous  conclusions  of  other  philosophers,  that  he  was 
able  to  prove  the  correctness  of  his  theory.  Singularly  self-reliant, 
accustomed  from  childhood  to  depend  on  his  own  exertions, 
Dalton  was  a  man  to  whom  original  work  was  a  necessity.1  In 
the  preface  to  the  second  part  of  his  New  System  of  Chemical  Phil- 
osophy, published  in  1810,  he  clearly  shows  his  independence 
and  even  disregard  of  the  labours  of  others,  for  he  says — "  Having 
been  in  my  progress  so  often  misled  by  taking  for  granted  the 
results  of  others,  I  have  determined  to  write  as  little  as  possible 
but  what  I  can  attest  by  my  own  experience." 

As  early  as  1802,  in  an  experimental  inquiry  into  the  propor- 
tions in  which  the  several  gases  constituting  the  atmosphere 
occur,  Dalton  clearly  points  out  "  that  the  elements  of  oxygen 
may  combine  with  a  certain  portion  of  nitrous  gas  "  (our  nitric 

1  Lonsdule's  Life  of  Dalton.     Longmans,  1874. 


36  HISTORICAL  INTRODUCTION 

oxide)  "  or  with  twice  that  portion,  but  with  110  intermediate 
quantity,"  and  this  observation  was  clearly  the  first  which  led  to 
the  possibility  of  drawing  up  the  table  already  given.1  In  that 
table,  it  will  be  seen  that  the  relative  weights  of  the  smallest 
particle  of  nitrous  gas  is  given  as  9*3,  that  of  Azot  (nitrogen) 
being  4'2,  and  that  of  oxygen  5*5.  Dalton  clearly  intending  by 
this  to  express  that  the  gas  is  a  compound  of  one  atom  of  nitro- 
gen with  one  atom  of  oxygen,  whilst  the  substance  to  which 
he  gives  the  name  of  hypo- nitric  acid  (now  called  nitrogen 
peroxide),  is  a  compound  in  which  one  atom  of  nitrogen  is 
combined  with  two  of  oxygen,  and  therefore  having  the  rela- 
tive atomic  weight  of  15'2.2 

The  first  public  announcement  of  the  atomic  theory,  and  of  the 
law  of  combination  in  multiple  proportions  upon  which  it  was 
founded,  was,  singularly  enough,  not  made  by  Dalton  himself,  but 
by  his  friend,  Professor  Thomas  Thomson,  of  Glasgow,  who  pub- 
lished in  1807  an  account  of  Dalton's  discovery  in  the  third  edition 
of  his  System  of  Chemistry.  In  the  following  year  (1808)  Dal- 
ton made  known  his  own  views  in  the  remarkable  book  entitled 
A  New  System  of  Chemical  Philosophy,  in  which  (Part  i.  p.  213)  he 
says — "  It  is  one  great  object  of  this  work  to  show  the  importance 
and  advantage  of  ascertaining  the  relative  weights  of  the 
ultimate  particles,  both  of  simple  and  compound  bodies,  the 
number  of  simple  elementary  particles  which  constitute  one 
compound  particle,  and  the  number  of  less  compound  particles 
which  enter  into  the  formation  of  one  more  compound  particle." 
Thomson  states  that  during  the  years  1803  and  1804  Dalton 
was  occupied  with  the  examination  of  the  composition  of  the 
two  gaseous  hydro-carbons,  marsh  gas  and  olefiant  gas,  and  the 
results  of  this  examination  led  him  to  the  adoption  of  the 
atomic  theory.  He  found  that  both  these  bodies  consist  solely 
of  carbon  and  hydrogen,  and  that  the  first  of  these  gases  contains 
twice  as  much  hydrogen  to  a  given  quantity  of  carbon  as  the 
second.  Hence  he  concluded  that  olefiant  gas  contains  one 
atom  of  carbon  combined  with  one  of  hydrogen,  whereas  marsh 
gas  consists  of  one  atom  of  carbon  combined  with  two  atoms 

1  Manchester  Memoirs,  2nd  Series,  1,  250. 

2  Certain  inaccuracies  in  the  values  of  the  weights  of  some  of  the  compounds 
occur  in  this  table  ;  thus,    4 '2  +  5  "5  =  97,  whilst  9 '3  appears  opposite  nitrous 
oxide.     Whether  these  are  merely  printer's  errors  or  are  to  be  explained  in  some 
other  way  can  now  only  be  conjectured.     See  Roscoe  on  Dalton's  First  Table  of 
Atomic  Weights.     Manchester  Lit.   and  Phil.   Soc.   Mem.     3rd  Series,  5,   269, 
1874-5. 


DALTON'S  ATOMIC  THEORY  37 


of  hydrogen.  The  same  idea  and  method  of  investigation  he 
then  applied  to  the  oxides  of  carbon,  oxides  of  sulphur,  oxides 
of  nitrogen,  to  ammonia  and  other  bodies,  and  he  showed  that  the 
composition  of  these  might  be  most  simply  explained  by  the 
assumption  that  one  atom  of  one  element  is  attached  to  1,  2,  3, 
&c.  atoms  of  another.  The  novelty  and  importance  of  his  view 
of  the  composition  of  chemical  compounds  induced  Dalton  to 
introduce  a  method  of  graphic  representation  of  the  atoms  of 
the  elements,  and  the  system  he  adopted  was  as  follows : — 

Relative  Weight 
Symbol.          of  the  atom. 

Oxygen O  7 

Hydrogen 0  1 

Nitrogen 0  5 

Carbon 9  5 

Water ©  O  8 

Ammonia 00  6 

Carbonic  oxide O  0  12 

Carbon  dioxide O  •  O  19 

Olefiant  gas 00  6 

Marsh  gas ©  •  0  7 

Nitrous  oxide O  0  0  17 

Nitric  oxide O  0  12 

Nitrous  acid  f  O  0  O I          26 


Acetic  acid ^  ©' O  O  26 


These  atomic  weights,  it  is  evident,  are  far  from  being  those 
which  we  now  accept  as  correct,  indeed  they  are  different  from 
those  given  in  his  first  table,  for  Dalton  not  only  frequently 
altered  and  amended  these  numbers,  according  as  his  experi- 
ments showed  them  to  be  faulty,  but  even  distinctly  asserts 
the  doubtful  accuracy  of  some.  Chemists  at  that  time  did  not 
possess  the  means  of  making  accurate  determinations,  and  when 
we  become  acquainted  with  the  rough  methods  which  Dalton 
adopted,  and  the  imperfect  apparatus  he  had  to  employ,  we  can- 
not but  be  struck  with  the  clearness  of  his  vision  and  the 
boldness  of  grasp  which  enabled  him,  thus  poorly  equipped,  to 
establish  a  doctrine  which  further  investigation  has  only  more 
firmly  established,  and  which,  from  that  time  forward,  has  served 


38  HISTORICAL  INTRODUCTION 

as    the  pole   star  round   which    all    other    chemical    theories 
revolve. 

Amongst  those  to  whose  labours  we  are  indebted  for  advancing 
Dalton's  atomic  theory  are  Thomas  Thomson  and  Wollaston,  but 
before  all,  the  great  Swedish  chemist  Berzelius.  to  whom  we 
owe  the  first  really  exact  values  for  these  primary  chemical 
constants.  With  remarkable  perseverance  he  ascertained  the 
exact  composition  of  a  large  number  of  compounds,  and  was,  there- 
fore, able  to  calculate  the  combining  weights  of  many  elements, 
thus  laying  the  foundation-stones  of  the  science  as  it  at  present 
exists.  In  1818  Berzelius  published  his  theory  of  chemical  pro- 
portions, and  of  the  chemical  action  of  electricity,  and  in 
these  remarkable  works  he  made  use  of  the  chemical  symbols  and 
formulae  such  as  we  now  employ,  to  denote  not  only  the  qualita- 
tive, but  also  the  quantitative  composition  of  chemical  compounds. 
From  this  time  forward  it  was  satisfactorily  proved  and  generally 
acknowledged  that  the  elementary  bodies  combine  together 
either  in  certain  given  proportions  by  weight,  or  in  simple 
multiples  of  these  proportions  ;  and,  through  the  researches  of 
Berzelius  and  others,  the  list  of  elements,  which  at  the  time 
of  Lavoisier  amounted  to  twenty-three  in  number,  was  now  con- 
siderably increased. 

Next  in  order  comes  Humphry  Davy's  discovery  of  the  com- 
pound nature  of  the  alkalis  (1808),  proving  that  they  are  not 
simple  substances  but  oxides  of  peculiar  metals,  and  thus  entirely 
revolutionizing  the  views  of  chemists  as  to  the  constitution  of  a 
large  and  important  class  of  compounds,  including  the  salts  of 
the  alkaline  earths.  The  discussion  in  1810  as  to  the  constitu- 
tion of  chlorine — then  termed  oxygenated  muriatic  acid — decided 
by  Davy  and  Gay-Lussac  in  favour  of  its  elementary  nature,  was 
likewise  a  step  of  the  greatest  importance  and  of  wide  applica- 
tion. In  1811  iodine  was  discovered  by  Courtois,  and  most 
carefully  investigated  by  Gay-Lussac,  who  proved  the  close 
analogy  existing  between  this  element  and  chlorine.  The  dis- 
covery of  many  other  elements- now  opened  out  fresh  fields  for 
investigation,  and  gave  the  means  of  classifying  those  already 
known.  The  names  and  properties  of  these  will  be  found  in 
the  portions  of  this  book  specially  devoted  to  their  description. 

If  Dalton,  as  we  have  seen,  succeeded  in  placing  the  laws  of 
chemical  combination  by  weight  on  a  firm  basis,  to  Gay-Lussac 
belongs  the  great  honour  of  having  discovered  the  law  of 
the  combination  of  gaseous  bodies  by  volume.  In  the  year  1805 


HUMBOLDT  AND  GAY-LUSSAC  39 

Gay-Lussac  and  Alexander  von  Humboldt  found  that  one  volume 
of  oxygen  combines  with  exactly  two  volumes  of  hydrogen  to 
form  water,  and  that  these  exact  proportions  hold  good  at  what- 
ever temperature  the  gases  are  brought  into  contact.  This 
observation  was  extended  by  Gay-Lussac,  who  in  1808  published 
his  celebrated  memoir  on  the  combination  of  gaseous  bodies,1  in 
which  he  proves  that  gases  not  only  combine  in  very  simple 
relations  by  volume,  but  also  that  the  alteration  of  volume  which 
these  gases  undergo  in  the  act  of  combination  may  be  expressed  by 
a  very  simple  law.  Hence  it  follows  that  the  densities  of  gases 
must  bear  a  simple  relation  to  their  combining  weights.  The  true 
explanation  of  these  facts  was  first  given  by  Avogadro  in  1811, 
and  his  hypothesis  is  now  universally  admitted  both  by 
chemists  and  physicists.  According  to  the  Italian  philo- 
sopher the  number  of  smallest  particles  or  molecules  con- 
tained in  the  same  volume  of  every  kind  of  gas  is  the  same, 
similar  circumstances  of  pressure  and  temperature  being  of 
course  presupposed. 

The  discovery  by  Gay-Lussac  of  the  laws  of  volume- 
combination,  together  with  Avogadro's  explanation  of  the  law, 
served  no  doubt  as  most  valuable  supports  of  Dalton's  atomic 
theory,  but  the  truth  of  this  latter  theory  was  still  further 
asserted  by  a  discovery  made  by  Dulong  and  Petit  in  1819. 
These  French  chemists  determined  the  specific  heat  of  thirteen 
elementary  bodies,  and  found  that  the  numbers  thus  obtained, 
when  compared  with  the  atomic  weights  of  the  same  bodies, 
showed  that  the  specific  heats  of  the  several  elements  are 
inversely  proportional  to  their  atomic  weights,  or  in  other 
words,  the  atom  of  each  of  these  elements  possesses  the  same 
capacity  for  heat.  Although  subsequent  research  has  shown 
that  this  law  does  not  apply  in  every  case,  it  still  remains  a 
valuable  means  of  controlling  the  atomic-weight  determina- 
tions of  many  elements. 

In  the  same  year  a  discovery  of  equal  importance  was  an- 
nounced by  Mitscherlich — that  of  the  law  of  Isomorphism. 
According  to  this  law,  chemically  analogous  elements  can  re- 
place each  other  in  many  crystalline  compounds,  either  wholly 
or  in  part,  without  any  change  occurring  in  the  crystalline  form 
of  the  compound.  This  law,  like  that  of  atomic  heats,  has  proved 
of  great  value  in  the  determination  of  atomic  weights. 

Gradually  the  new  basis  given  by  Dalton  to  our  science  was 
1  Memoires  d'Arcueil,  2,  207. 


40  HISTORICAL  INTRODUCTION 


widely  extended  by  these  discoveries  and  by  the  researches  of 
other  chemists,  and  a  noble  structure  arose,  towards  the  com- 
pletion of  which  a  numerous  band  of  men  devoted  the  whole 
energies  of  their  lives. 

Especially  striking  was  the  progress  made  during  these  years 
in  the  domain  of  Organic  Chemistry,  or  the  chemistry  of  the 
substances  found  in,  or  obtained  from,  vegetable  or  animal 
bodies.  Dalton  had  in  vain  endeavoured  to  obtain  analytical 
results  to  prove  that  the  complicated  Organic  bodies  followed  the 
same  laws  as  the  more  simple  Inorganic  compounds.  It  is  to 
Berzelius  that  we  owe  the  proof  that  this  is  really  the  case,  and 
his  exact  analyses  placed  organic  chemistry  in  this  respect  on  a 
firm  and  satisfactory  basis.  There  still  remained,  however,  much 
doubt  as  to  the  strict  identity  of  the  laws  according  to  which 
organic  and  inorganic  compounds  were  severally  formed.  Most 
of  the  compounds  met  with  in  mineral  chemistry  could  be  easily 
prepared  by  the  juxtaposition  of  their  constituents ;  they  were 
of  comparatively  simple  constitution,  and  could  as  a  rule  be  pre- 
pared by  synthesis  from  their  constituent  elements.  Not  so 
with  organic  bodies ;  they  appeared  to  be  produced  under  cir- 
cumstances wholly  different  from  those  giving  rise  to  mineral 
compounds ;  the  mysterious  phenomena  of  life  seemed  in  some 
way  to  influence  the  production  of  these  substances  and  to 
preclude  the  possibility  of  their  artificial  preparation.  A  great 
step  was  therefore  made  in  our  science  when,  in  1828,  Wohler 
artificially  prepared  urea,  a  body  which  up  to  that  time  had  been 
thought  to  be  a  product 'peculiar  to  animal  life.  This  discovery 
broke  down  at  once  the  supposed  impassable  barrier  between 
organic  and  mineral  chemistry,  pointed  out  the  rich  harvest  of 
discovery  since  so  largely  developed,  especially  by  Liebig,  in  the 
synthesis  of  organic  substances,  and  paved  the  way  to  the  know- 
ledge which  we  have  gained,  chiefly  through  the  labours  of  the 
last-named  chemist,  that  the  science  of  Physiology  consists 
simply  in  the  Chemistry  and  Physics  of  the  body. 


GENEEAL  PRINCIPLES  OF  THE  SCIENCE 

i  MATTER  is  capable  of  assuming  three  different  states  or  con- 
ditions : — the  solid,  the  liquid,  and  the  gaseous.  Of  these,  the 
first  two  have,  for  obvious  reasons,  been  recognised  from  the 
earliest  ages  as  accompanying  very  different  kinds  of  substances, 
It  is,  however,  only  within  a  comparatively  short  time  that  men 
have  come  to  understand  that  just  as  there  are  many  distinct 
kinds  of  solids  and  liquids,  so  there  are  many  distinct  kinds  of 
gases  (Van  Helmont).  These  may,  indeed,  be  colourless  and  in- 
visible, but,  nevertheless,  they  can  readily  be  shown  to  differ  one 
from  another.  Thus,  Black,  in  1752,  collected  a  peculiar  gas, 
which  wenow  know  as  carbonic  acid  gas,  or  carbon  dioxide,  obtained 
by  the  action  of  dilute  acids  on  marble  ;  to  this  gas  he  gave  the 
name  of  "  fixed  air,"  because  it  is  fixed  in  the  alkaline  carbonates, 
which  at  that  time  were  called  the  mild  alkalis,  in  contradis- 
tinction to  the  caustic  alkalis.  This  invisible  gas  does  not,  like 
air,  support  the  combustion  of  a  taper,  and,  unlike  air,  it  renders 
clear  lime-water  turbid ;  it  is  also  much  heavier  than  air,  as  can 
be  shown  by  pouring  it  downwards  from  one  vessel  to  another, 
by  drawing  it  out  of  a  vessel  by  means  of  a  syphon,  or  by  pouring 
it  into  a  beaker  glass  previously  equipoised  at  one  end  of  the 
beam  of  a  balance  (see  Fig.  2).  That  the  gas  has  actually  been 
poured  out  is  seen  either  by  a  burning  taper  being  extinguished 
when  dipped  into  the  beaker  glass,  or  by  adding  some  clear 
lime-water,  which  then  turns  milky. 

In  1766,  Cavendish  showed  that  the  gas  termed  by  him  in- 
flammable air,  and  obtained  by  the  action  of  dilute  acids  on 
metallic  zinc  or  iron,  is  also  a  peculiar  and  distinct  substance, 
to  which  we  now  give  the  name  of  hydrogen  gas.  It  is  so  much 
lighter  than  air  that  it  may  be  poured  upwards,  and  takes  fire 
when  a  light  is  brought  in  contact  with  it,  burning  with  a  pale 
blue  flame.  Soap-bubbles  blown  with  hydrogen  ascend  in  the 


42  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

air,  and  if  hydrogen  be  poured  upwards  into  the  equipoised  bell- 
jar  hung  mouth  downwards  on  the  arm  of  the  balance  (Fig.  3), 
the  equilibrium  will  be  disturbed,  and  the  arm  with  the  bell-jar 
will  rise. 

2  On  August  1st,  1774,  Priestley  heated  some  red  precipi- 
tate (oxide  of  mercury)  and  obtained  from  it  a  new  colour- 
less gas  called  oxygen,  and  this,  although  invisible,  possesses 


FIG.  2. 

properties  quite  different  from  those  of  air,  carbonic  acid  gas,  or 
hydrogen  gas.  A  red  hot  chip  of  wood  is  at  once  rekindled 
when  plunged  into  this  gas,  and  bodies  such  as  iron  wire  or  steel 
watch-spring,  which  do  not  burn  in  the  air,  burn  with  brilliancy 
in  oxygen. 

These  examples  suffice  to  show  that  invisible  gases  exist 
which  differ  in  the  widest  degree  from  each  other,  though  many 
more  illustrations  of  the  same  principle  might  be  given. 


THE  EXPERIMENTAL  METHOD  43 

3  The  method  which  we  have  had  to  adopt  in  order  thus  to 
discern  differences  between  these  invisible  gases,  is  termed  the 
Experimental  Method.  Experiments  may  be  said  to  be  ques- 
tions put  to  nature,  and  a  science  is  termed  experimental,  as 
opposed  to  observational,  when  we  are  able  so  to  control  and 
modify  the  conditions  under  which  the  phenomena  occur  as  to 
produce  results  which  are  different  from  those  which  are 


otherwise  met  with.  Chemistry  is,  therefore,  one  of  several 
experimental  sciences,  each  of  which  has  the  study  of  natural 
phenomena  for  its  aim.  These  sciences  are  most  intimately  con- 
nected, or,  rather,  the  division  into  separate  sciences  is  quite 
arbitrary,  so  that  it  is  not  possible  exactly  to  say  where  the 
phenomena  belonging  to  one  science  begin  and  those  appertaining 
to  another  science  end.  Nature  is  a  connected  whole,  and  the 
divisions  which  we  are  accustomed  to  make  of  natural  phenomena 


44  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

into  separate  sciences  serve  only  to  aid  the  human  mind  in  its 
efforts  to  arrange  a  subject  which  is  too  vast  in  its  complete 
extent  for  the  individual  to  grasp.  Although  it  may  not  be 
possible  exactly  to  define  the  nature  of  the  phenomena  which 
we  class  as  chemical,  as  distinguished  from  those  termed  physical, 
it  is  not  difficult,  by  means  of  examples,  to  obtain  a  clear  idea 
of  the  kind  of  observations  with  which  the  chemist  has  to  do. 
Thus,  for  instance,  it  is  found  that  when  two  or  more  given 
substances  are  brought  together,  under  certain  conditions  they 
may  change  their  properties,  and  a  new  substance,  differing 
altogether  from  the  original  ones,  may  make  its  appearance. 
Or,  again,  a  given  substance  may,  when  placed  under  certain 
conditions,  yield  two  or  more  substances  differing  entirely  from 
the  original  one.  in  their  essential  properties.  In  both  these 
cases  the  change  which  occurs  is  termed  a  chemical  change ;  if 
several  distinct  substances  have  coalesced  to  form  one  new 
substance,  an  act  of  chemical  combination  is  said  to  occur  ;  if  one 
substance  is  made  to  yield  two  or  more  distinct  new  bodies,  a 
chemical  decomposition  has  taken  place.  These  acts  of  chemical 
union  and  disruption  occur  alike  amongst  solid,  liquid,  and 
gaseous  bodies  ;  they  are  regulated  in  the  first  place  by  the 
essential  nature  of  the  substances,  and  secondly,  by  the  circum- 
stances or  conditions  under  which  they  are  brought  together. 
It  is  also  to  be  observed  that  these  actions  of  chemical  union 
in  the  first  place  do  not  occur  when  the  component  materials, 
are  situated  at  a  distance  from  each  other,  close  contact 
being  necessary  in  order  that  such  changes  should  take 
place  ;  whilst  secondly,  we  almost  invariably  notice  that  such 
a  combination  is  attended  with  an  evolution  of  heat  and,, 
sometimes,  of  light. 

4  Some  simple  illustrations  of  chemical  action  may  here  be 
cited  : — If  powdered  sulphur  and  fine  copper-filings  be  well 
mixed  together,  a  green-coloured  powder  will  result,  in  which, 
however,  a  powerful  microscope  will  show  the  particles  of  sulphur 
lying  by  the  side  of  the  particles  of  copper.  On  heating  this  green 
powder  in  a  test  tube,  the  mass  suddenly  becomes  red-hot,  and, 
on  cooling,  a  uniform  black  powder  is  found.  This  is  neither 
copper  nor  sulphur,  but  a  chemical  compound  of  the  two,  in 
which  no  particle  of  either  of  the  substances  can  be  seen,  how- 
ever high  a  magnifying  power  be  employed,  but  from  which,  by 
the  employment  of  certain  chemical  means,  both  copper  and 
sulphur  can  again  be  extracted.  Here  then  we  have  a  case 


CHEMICAL  ACTION  45 


of  chemical  combination.  An  experiment  similar  to  that  made  by 
Priestley  when  he  discovered  oxygen,  may  serve  as  an  illustra- 
tion of  a  chemical  decomposition.  21 '6  grams l  of  red  oxide  of 
mercury  are  heated  in  a  small  retort  provided  with  a  receiver, 
and  a  gas  delivery  tube  passing  to  the  top  of  a  graduated  cylinder 
filled  with  water  to  the  beginning  of  the  graduations,  and  standing 
in  the  pneumatic  trough  over  water  (Fig.  4).  On  heating  the  red 
oxide  by  means  of  the  Bunsen  gas  flame,  it  first  becomes  dark- 
coloured  and  then  soon  begins  to  decompose  into  metallic 
mercury,  which  collects  in  small  bright  drops  in  the  neck  of  the 
retort  gradually  running  down  in  the  receiver,  and  into  oxygen 
gas,  which  passes  through  the  delivery  tube  and  collects  in  the 
graduated  cylinder.  After  the  heat  has  been  continued  for 
some  time,  the  whole  of  the  red  powder  will  have  disappeared, 


FIG.  4. 

.having  been  changed  by  heat  into  metallic  mercury  and  oxygen. 
On  allowing  the  retort  to  cool,  a  volume  of  1120  cubic  centi- 
metres of  gas  has  been  collected,  and  this,  on  application  of  the 
red-hot  chip  test,  is  shown  to  be  oxygen.  If  this  experiment 
is  properly  conducted,  the  volume  of  oxygen  obtained  is  always 
found  to  be  the  same  from  the  same  weight  of  oxide  (provided 
the  temperature  and  pressure  at  which  the  gas  is  measured  are 
the  same  in  the  different  experiments),  viz.,  1120  cubic  centi- 
metres from  21 '6  grams  of  oxide. 

Another  interesting  case  of  chemical  change  is  the  decompo- 

1  For  a  table  of  equivalent  values  of  the  common  English  weights  and  measures 
with  those  of  the  metrical  system,  see  Appendix  to  this  volume. 


46 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


sition  of  water  by  galvanic  electricity  as  discovered  by  Nicholson 
and  Carlisle  in  1800.  For  the  purpose  of  exhibiting  this  we 
only  need  to  pass  a  current  of  electricity  from  four  or  six 
Grove's  or  Bun  sen's  elements  by  means  of  two  platinum 
poles  through  some  water  acidulated  with  sulphuric  acid 
(Fig.  5).  The  instant  contact  is  made,  bubbles  of  gas  begin  to 
ascend  from  each  platinum  plate  and  collect  in  the  graduated 
tubes,  which  at  first  are  filled  with  the  acidulated  water.  After 


FIG.  5. 


a  little  time  it  will  be  seen  that  the  plate  which  is  in  con- 
nection with  the  zinc  of  the  battery  evolves  more  gas  than  the 
one  which  is  in  contact  with  the  platinum  or  carbon  of  the 
battery  ;  and  after  the  evolution  has  continued  for  a  few  minutes 
one  tube  will  be  seen  to  contain  twice  as  much  gas  as  the 
other.  On  examination,  the  larger  volume  of  gas  will  be  found 
to  be  hydrogen,  and  will  take  fire  and  burn  when  a  light 


INDESTRUCTIBILITY  OF  MATTER  47 


is  brought  to  the  end  of  the  tube  in  which  it  was  collected 
whilst  the  smaller  volume  of  gas  is  seen  to  be  oxygen, 
a  glowing  chip  of  wood  being  rekindled  when  plunged 
into  it. 

5  In  many  cases  of  chemical  action,  the  products  are  gaseous, 
whilst  one  or  more  of  the  materials  acted  upon  are  solid  or  liquid. 
Hence,  in  these  cases,  a  disappearance  or  apparent  loss  of  matter 
occurs.     It  has,  however,  been  shown  by  many  accurate  experi- 
ments that  in  these  cases  the  loss  of  matter  is  only  apparent,  so 
that  chemists  have  come  to  the  conclusion  that  matter  is  in- 
destructible, and  that  in  all  cases  of  chemical  action  in  which 
matter  disappears,  the  loss  is  apparent  only,  the  solid  or  liquid 
being  changed  into  an  invisible  gas,  the  weight  of  which  is,  how- 
ever, exactly  identical  with  that  of  its  component  parts.     We 
only  require  to  allow  a  candle  to  burn  for  a  few  minutes  in 
a  clean  flask  filled  with  air  in  order  to  show  that  the  materials 
of  the  candle,  hydrogen  and  carbon,  unite  with  the  oxygen  of 
the  air  to  form,  in  the  first  place,  water,  which  is  seen  in  small 
drops  bedewing  the  bright  sides  of  the  flask,  and  in  the  second, 
carbon  dioxide  or  carbonic  acid  gas,  whose  presence  is  revealed 
to  us  by  lime-water  being  turned  milky.     The  fact  that  the  sum 
of  the  weights  of  the  products  of  combustion  (water  and  carbon 
dioxide)  is  greater  than  the  loss  of  weight  sustained  by  the  candle 
is  clearly  shown  by  an  experiment  made  by  means  of  the  appa- 
ratus (Fig.  6),  which  consists  of  a  tube  equipoised  on  the  arm  of 
a  balance.     In  the  long  vertical  tube  a  taper  is  placed,  the  other 
end   of  the  system  being  attached  to  a  gasholder  filled  with 
water,  which,  on  being  allowed  to  run  out,  causes  a  current  of 
air  to  pass  through  the  tube,  and  thus  maintains  the  combustion 
of  the  taper.     The  water  and  carbonic  acid  gas  which  are  formed 
are  absorbed  by  the  bent  tube,  which  contains  caustic  potash. 
After  the  taper  has  burnt  for  a  few  minutes,  the  apparatus  is 
disconnected  from  the  gasholder  and  allowed  to  vibrate  freely, 
when  it  will  be  found  to  be  appreciably  heavier  than  it  was 
before  the  taper  had  burnt,  the  explanation  being  that  the  excess 
of  weight  is  due  to  the  combination  of  the  carbon  and  hydrogen 
of  the  wax  with  the  oxygen  of  the  air. 

6  Another  series  of  experiments  which  also  show  plainly  the 
fact  of  the  indestructibility  of  matter,  and  are  of  historical  in- 
terest, are  those  by  which  it  has  been  clearly  demonstrated  that 
the  air  consists  of  two  different  gases,  oxygen  and   nitrogen. 
The  fact  of  the  composite  nature  of  air  was  proved  by  Priestley 


48 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


in  1772  by  setting  fire,  by  means  of  a  burning  glass,  to 
charcoal  contained  in  a  vessel  of  air.  He  showed  that  fixed  air 
(carbonic  acid  gas)  was  produced,  and  that  on  the  absorption  of 
this  fixed  air  by  lime-water,  one-fifth  of  the  original  bulk  of 
the  air  disappeared,  and  a  colourless  gas  remained,  which  did 
not  support  combustion  or  respiration.  It  was  not,  however, 
till  the  year  1775,  after  he  had  discovered  oxygen,  that  Priest- 


FIG.   6. 


ley  distinctly  stated  that  this  gas  was  contained  in  common 
air,  and  about  the  same  time  Scheele  came  to  an  identical 
conclusion  from  independent  experiments.  But  the  method, 
by  which  the  existence  of  oxygen  in  the  air  was  first  demon- 
strated in  the  clearest  way,  is  that  adopted  by  Lavoisier,  and 
described  in  his  Traitt  de  Chimie.1  Into  a  glass  balloon  (Fig.  7) 
having  a  long  straight  neck,  Lavoisier  brought  4  ounces  of  pure 
mercury  ;  he  then  bent  the  neck  so  that  when  the  balloon  rested 

1     Part  i. ,  chap.  iii. 


LAVOISIER'S  EXPERIMENTS 


49 


on  the  top  of  the  furnace,  the  end  of  the  bent  neck  appeared 
above  the  surface  of  the  mercury  contained  in  the  trough,  thus 
placing  the  air  in  the  bell-jar  in  communication  with  that  in 
the  balloon.  The  volume  of  the  air  (reduced  to  28  inches  of 
mercury  and  a  temperature  of  10°)  contained  in  the  bell-jar  and 
balloon  amounted  to  50  cubic  inches.  The  mercury  in  the  balloon 
was  now  heated,  by  a  fire  placed  in  the  furnace,  to  near  its  boil- 
ing point.  For  the  first  few  hours  no  change  occurred,  but  then 
red-coloured  specks  and  scales  began  to  make  their  appearance. 
Up  to  a  certain  point  these  increased  in  number,  but  after  a 
while  no  further  formation  of  this  red  substance  could  be  noticed. 


After  heating  for  twelve  days  the  fire  was  removed,  and  the 
volume  of  the  air  was  seen  to  have  undergone  a  remarkable 
diminution :  the  volume,  measured  under  the  same  conditions  as 
before,  having  been  reduced  from  50  to  between  42  and  43  cubic 
inches.  The  red  particles  were  next  carefully  collected,  and  on 
weighing,  were  found  to  amount  to  45  grains.  These  45  grains 
were  next  introduced  into  a  small  retort  connected  with  a 
graduated  glass  cylinder  (Fig.  8),  and  on  heating  they  yielded 
41 J  grains  of  metallic  mercury  and  from  7  to  8  cubic  inches  of 
a  gas  which  was  found  to  be  pure  oxygen.  Thus,  the  whole  of 
the  oxygen,  whether  measured  by  volume  or  by  weight,  which 
was  withdrawn  from  the  air  by  the  mercury,  was  obtained  again 
when  the  oxide  formed  was  decomposed  by  heat. 

5 


50 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


7  It  is  not  merely  to  the  investigation  of  changes  occurring  in 
the  essential  properties  of  inorganic  or  mineral  matter  that  the 
chemist  has  to  direct  his  attention.  The  study  of  many  of  the 
phenomena  observed  in  the  vegetable  or  animal  world  also  claim 
his  notice.  So  much  so,  indeed,  is  this  the  case  that  the  science 
of  physiology  has  been  denned  as  the  physics  and  chemistry 
of  the  body.  The  simplest  as  well  as  the  most  complicated 
changes  which  accompany  life  are,  to  a  great  extent,  dependent 
upon  chemical  laws,  and,  although  we  are  still  unable  fully  to 
explain  many  of  these  changes,  yet  each  year  brings  us  addi- 
tional aid,  so  that  we  may  expect  some  day  to  possess  an  exact 


L 


& 


FIG.  8. 

knowledge  of  the  chemistry  of  life.  In  order  to  be  convinced 
that  vital  actions  are  closely  connected  with  chemical 
phenomena,  we  only  need  to  blow  the  air  from  our  lungs  through 
clear  lime-water  to  see  from  the  ensuing  turbidity  of  the  water 
that  carbonic  acid  gas  is  evolved  in  large  quantities  during  the 
process  of  the  respiration  of  animals,  and  when  we  further 
observe  that  the  higher  animals  are  all  warmer  than  surrounding 
objects,  we  come  to  the  conclusion  that  the  process  of  respiration 
is  accompanied  by  oxidation,  and  that  the  breathing  animal 
resembles  the  burning  candle,  not  only  in  the  products  of  this 
combustion,  viz.,  water  and  carbon  dioxide,  but  in  the  heat 


\  OF        v  /y 

ELEMENTAEY  AND  COMPOUND  BODIES 


51 


which  that  combustion  evolves,  the  difference  being  that  in  the 
one  case  the  oxidation  goes  on  quickly  and  is  confined  to  one 
spot  (the  wick  of  the  candle)  whereas  in  the  other  it  goes  on 
slowly  and  takes  place  throughout  the  body.  In  like  manner 
the  living  plant  is  constantly  undergoing  changes,  which  are  as 
necessary  for  its  existence  as  the  act  of  breathing  is  for  animals. 
One  of  the  most  fundamental  of  these  changes  is  readily  seen  if 
we  place  some  fresh  green  leaves  in  a  bell-jar  filled  with  spring 
water  and  expose  the  whole  to  sunlight.  Bubbles  of  gas  are 
observed  to  rise  from  the  leaves,  and  these,  when  collected,  prove 
to  be  oxygen.  In  presence  of  the  sunlight  the  green  leaf  has 
decomposed  the  carbonic  acid  gas  held  in  solution  in  the  spring 
water,  assimilating  the  carbon  for  the  growth  of  its  body  and 
liberating  the  oxygen  as  a  gas.  Nor,  indeed,  are  the  investiga- 
tions of  the  chemist  now  confined  to  the  organic  and  inorganic 
materials  of  the  earth  which  we  inhabit.  Recent  research  has 
enabled  him,  in  conjunction  with  his  colleague  the  physicist,  to 
obtain  a  knowledge  of  the  chemistry  as  well  as  of  the  physics 
of  the.  sun  and  far  distant  stars,  and  thus  to  found  a  truly 
cosmical  science. 

8  It  is  the  aim  of  the  chemist  to  examine  the  properties  of 
all  the  different  substances  which  occur  in  nature,  so  far  as  they 
act  upon  each  other,  or  can  be  made  to  act  so  as  to  produce 
something  different  from  the  substances  themselves;  to 
ascertain  the  circumstances  under  which  such  chemical  changes 
occur,  and  to  discover  the  laws  upon  which  they  are  based. 
In  thus  investigating  terrestrial  matter  it  is  found  that  all  the 
various  forms  of  matter  with  which  we  are  surrounded,  or 
which  have  been  examined,  can  be  divided  into  two  great 
classes. 

I.  ELEMENTARY  BODIES.  —  Elements,  or  simple  substances,  out 
of  which  no  other  two  or  more  essentially  differing  substances 
have  been  obtained. 

II.  COMPOUND  BODIES,  or  compounds,  out  of  which  two  or 
more  essentially  differing  substances  have  been  obtained. 

Only  twenty-three  elements  were  known  during  the  lifetime 
of  Lavoisier  ;  now  we  are  acquainted  with  not  less  than  sixty- 
nine.  Of  these,  and  their  compounds  with  each  other,  the  whole 
mass  of  our  globe,  solid,  liquid,  and  gaseous,  is  composed,  and 
these  elements  contribute  the  material  out  of  which  the  fabric 
of  our  science  is  built.  The  science  of  chemistry  has  for  its 
aim  the  experimental  examination  of  the  elements  and  their 


52 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


compounds,     and     the    investigation    of    the    laws    of    their 
combination  one  with  another. 

The  following  is  an  alphabetical  list  of  the  elementary  bodies 
known  at  present  (1893)  : — 

LIST  OF  ELEMENTS.* 


Atomic  weight. 

Atomic  weight 

Aluminium   .    . 

26-9 

Molybdenum 

.    .     95-2 

Antimony  .    .    . 

119-4 

Nickel      .    . 

.    .    58-6 

Arsenic      .    .    . 

74-4 

Niobium 

.    .    93-5 

Barium      .    .    . 

135-97 

Nitrogen 

.    .    13-94 

Beryllium      .    . 

8-99 

Osmium  .    . 

.    .  189-3 

Bismuth    .    .    . 

206-4 

Oxygen    .    . 

.    .     15-88 

Boron     .... 

10-74 

Palladium    . 

.    .  104-7 

Bromine    .    .    . 

79-36 

Phosphorus 

.    .    30'8 

Cadmium  .    .    . 

111-3 

Platinum     . 

.    .  193-3 

Caesium     .    .    . 

131-9 

Potassium    . 

.    .    38-85 

Calcium     .    .    . 

39-7 

Rhodium 

.    .  102-0 

Carbon       .    .    . 

11-91 

Rubidium    . 

.    .     84-8 

Chlorine     ... 

3519 

Ruthenium 

.    .  103-0 

Cerium      .    .    . 

139-0 

Samarium   . 

.    .  149-0 

Chromium     .    . 

51-7 

Scandium    . 

.    .    43-8 

Cobalt    .... 

58-6 

Selenium     . 

.    .    78-5 

Copper  .... 

62-8 

Silver  .    .    . 

107-13 

Didymium  ^' 

'  142-5 
'  139-7 

Silicon     .    . 
Sodium    .    . 

.    .     28-2 
.    o    22-87 

Erbium      .    .    . 

165-0 

Strontium    . 

.    .    86-8 

Fluorine     .    .    . 

18-9 

Sulphur  .    . 

.    .    31-82 

Gallium     .    .    . 

69-4 

Tantalum    . 

.    .  181-0 

Germanium  .    . 

71-8 

Tellurium    . 

.    .  124-0 

Gold  

195-7 

Thallium 

202*6 

Hydrogen      .    . 

1-0 

Thorium 

•        •     £d\J  ^  \j 

.    .  230-7 

Indium      .    .    . 

112-8 

Thulium      . 

.    .  169-7 

Iodine    .... 

125-91 

Tin  .... 

117-2 

Iridium      .    .    . 

191-7 

Titanium     . 

.    .    47-7 

Iron   

55-6 

Tungsten     . 

.    .  182-6 

Lanthanum   .    . 

137-5 

Uranium 

.    .  237-6 

Lead  

205-4 

Vanadium 

50'8 

Lithium     .    .    . 

6-98 

Ytterbium   . 

•        •         *J  \J  O 

.    .  172-0 

Magnesium    .    . 

24-2 

Yttrium  .    . 

.    .    88-0 

Manganese    .    . 

54-6 

Zinc     .    .    . 

65*0 

o 

Mercury    .    .    . 

198-9 

Zirconium    . 

.    .    90-0 

See  also  Appendix. 


LIST  OF  ELEMENTS  53 


In  addition  to  the  above  the  existence  of  many  other  elements, 
discovered  in  certain  rare  Norwegian  and  American  minerals, 
has  been  announced,  among  which  are  Decipium,  Holmium, 
Iduminium,  Norwegium,  and  Yttrium-a.  These,  however,  have 
not  as  yet  been  very  perfectly  investigated,  and  their  atomic 
weights  and  chemical  relationships  remain  undetermined. 

For  the  sake  of  convenience  it  is  customary  to  divide  the 
elements  into  two  classes — the  Metals  and  the  Non-Metals,  or 
Metalloids,  a  distinction  which  was  first  made  about  the  time  of 
Lavoisier,  when  only  a  few  elements  were  known.  Now, 
however,  the  division  is  a  purely  arbitrary  one,  as  it  is  not 
possible  to  draw  an  exact  line  of  demarcation  between  these  two 
groups,  so  that  there  are  cases  in  which  an  element  has  been 
considered  as  a  metal  by  some  chemists  and  as  a  non-metal  by 
others.  To  the  first  class  belong  such  substances  as  gold,  silver, 
mercury,  and  tin;  to  the  second  substances  which  are  gaseous  at 
the  ordinary  temperature,  such  as  hydrogen,  nitrogen,  and 
oxygen,  together  with  certain  solid  bodies,  as  carbon  and  sulphur. 
The  number  of  metals  is  much  larger  than  that  of  the  non- 
metals  :  we  are  acquainted  with  fifty-four  metals,  and  with 
only  fifteen  non-metals. 

9  In  this  treatise  the  elements  will  be  considered  in  the  follow- 
ing order,  although  another  system  of  classification,  termed  the 
natural  system,  based  upon  the  chemical  properties  of  the 
elements,  will  be  discussed  and  explained  in  a  later  volume. 
The  natural  system  is,  however,  as  yet  imperfect,  and  its  value 
can  only  be  properly  appreciated  when  a  fuller  knowledge  of  the 
properties  of  the  elements  has  been  gained. 

ARRANGEMENT  OF  THE  ELEMENTS. 
I. — NON-METALS. 

Symbol.  Symbol. 

Hydrogen    .    .    .    .  H  Nitrogen N 

Phosphorus     .    .    .    .  P 

Fluorine F  Arsenic As 

Chlorine      .    .    .    .  Cl 

Bromine      .    .    .    .  Br  Boron      B 

Iodine I 

Oxygen O 

Sulphur S  Carbon C 

Selenium     .    .    .    .  Se  Silicon Si 

Tellurium    ,  .  Te 


54 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


II. — METALS. 


Symbol. 

Potassium  .    .    .    .  K 

Sodium Na 

Lithium Li 

Kubidium  ....  Rb 
Caesium Cs 

Calcium Ca 

Strontium   .    .    .    .  Sr 
Barium    ,  .  Ba 


Beryllium  . 
Magnesium 
Zinc  .  .  . 
Cadmium  . 

Lead  .  .  . 
Thallium 


Copper 

Silver 

Mercury 


Yttrium 
Cerium   .    . 
Lanthanum 
Didymium 


Erbium  .  . 
Ytterbium  . 
Thulium  . 
Samarium  . 

Aluminium 
Scandium    , 
Indium    . 
Gallium   . 


Be 
Mg 
Zn 
Cd 

Pb 

Tl 

Cu 
Ag 
Hg 

Y 

Ce 
La 
Di 

Er 
Yb 

Tu 

Sa 

,  Al 

Sc 

,  In 

,  Ga 


Symbol. 

Manganese      ....  Mn 
Iron    .......  Fe 

Cobalt Co 

Nickel Ni 

Chromium  .  .  .  .  Cr 
Molybdenum  ....  Mo 
Tungsten  .....  W 
Uranium U 

Tin Sn 

Titanium    .....  Ti 

Zirconium Zr 

Thorium Th 

Vanadium V 

Antimony Sb 

Germanium    ....  Ge 

Bismuth Bi 

Tantalum Ta 

Niobium Nb 

Gold Au 

Platinum Pt 

Palladium Pd 

Rhodium Rh 

Iridium   .  .  Ir. 


Ruthenium 
Osmium  . 


.  Ru 
.  Os 


Of  these  elements  only  four  occur  largely  in  the  air,  about 
thirty  have  been  detected  in  the  sea,  whilst  all  the  sixty-nine 
are  found  irregularly  distributed  throughout  the  solid  mass  of 
our  planet. 

Some  are  very  abundant,  and  are  widely  distributed,  whilst 


ARRANGEMENT  OF  THE  ELEMENTS  55 

others  have  hitherto  been  found  only  in  such  minute  quantities 
and  so  seldom,  that  even  their  properties  have  not  yet  been 
satisfactorily  examined.  Thus  oxygen  is  found  throughout  the 
air,  sea,  and  solid  earth  in  such  quantities  as  to  make  up  nearly 
half  the  total  weight  of  the  crust  of  our  planet,  whilst  the  com- 
pounds of  caesium,  although  tolerably  widely  distributed,  occur 
only  in  very  minute  quantity,  and  those  of  erbium  have  as  yet 
been  met  with  only  in  very  small  quantities,  and  in  very  few 
localities. 

In  order  to  obtain  an  idea  as  to  which  elements  form  the  main 
portion  of  the  solid  crust  of  the  earth,  we  may  examine  the 
composition  of  all  the  different  kinds  of  granitic  or  eruptive 
rocks  which  constitute  by  far  the  greater  part  of  the  earth's 
crust.  From  analyses  made  by  Bunsen  we  find  that  all  granitic 
rocks  possess  a  composition  varying  between  the  limits  given  in 
the  following  table,  so  that  these  numbers  give  a  fair  idea  of 
what  is  known  of  the  average  chemical  composition  of  the  solid 
globe.  All  the  other  elements  occur  in  quantities  less  than 
any  of  those  mentioned  in  the  table. 

The    Composition  of  the  ^Earth's   Solid  Crust   in  100  parts 
ly  weight. 

Oxygen.     .  44'0  to  487  Calcium.     .  .  G'6to0'9 

Silicon     .     .  22-8  „  36'2  Magnesium.  .  27  „  01 

Aluminium.  9'9  „     61  Sodium  .     .  .  2'4  „  2'5 

Iron   ...  9'9  „     2'4  Potassium  .  .  17  „  31 

10  In  considering  for  the  first  time  the  subject  of  the 
elements,  the  question  will  at  once  suggest  itself — Are  these 
sixty-nine  all  the  elements  which  make  up  our  earth,  or 
is  it  likely  that  other  hitherto  undiscovered  elements  exist  ? 
Judging  from  analogy,  remembering  what  has  previously 
occurred,  and  looking  to  the  incomplete  state  of  our  know- 
ledge concerning  the  composition  of  the  earth's  crust,  we 
may  fairly  conclude  that  it  is  all  but  certain  that  other 
elementary  bodies  remain  to  be  discovered.  Every  improve- 
ment in  our  methods  of  examination  leads  to  the  detec- 
tion either  of  new  elements  or  of  old  ones  in  substances 
in  which  they  had  previously  been  overlooked.  Thus  by 
the  methods  of  Spectrum  Analysis  no  less  than  five 
(caesium,  rubidium,  thallium,  indium,  and  gallium)  new 
elements  have  been  discovered,  and  the  existence  of  several 


56  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

others  rendered  highly  probable  especially  among  the  rare 
earths.  By  help  of  this  method  we  are  also  enabled  to  come 
to  certain  conclusions  respecting  the  distribution  and  occur- 
rence of  these  same  elements  in  some  of  the  heavenly  bodies, 
and  we  learn  that  many  of  the  metals,  and  even  non-metals, 
which  are  well  known  to  us  on  the  earth,  are  found  in  the  sun 
and  the  fixed  stars.  The  conclusion  that  the  terrestrial  ele- 
ments exist  beyond  the  bounds  of  our  planet  is  borne  out  by  the 
chemical  examination  of  the  meteoric  stones  which  are  constantly 
falling  upon  the  surface  of  the  earth.  In  hundreds  of  these  which 
have  been  examined,  no  single  case  of  the  discovery  of  an  un- 
known element  has  occurred.  The  substances  of  which  meteorites 
have  been  found  to  consist  are  iron,  nickel,  oxygen,  calcium, 
silicon,  carbon,  and  other  well-known  terrestrial  elements. 

Another  question  which  may  here  be  asked  is — Are  these 
elements  really  undecomposable  substances  ?  and  to  this  it  may 
be  replied,  that  so  far  as  our  chemical  knowledge  enables  us  to 
judge,  we  may  assume,  with  a  considerable  degree  of  probability, 
that  by  the  application  of  more  powerful  means  than  are  at 
present  known,  chemists  will  succeed  in  obtaining  still  more 
simple  bodies  from  the  so-called  elements.  Indeed,  if  we  examine 
the  history  of  our  science,  we  find  frequent  examples  occurring  of 
bodies  which  only  a  short  time  ago  were  considered  to  be  elemen- 
tary which,  upon  more  careful  examination,  have  been  shown  to 
be  compounds. 

11  A  very   remarkable  fact  observed   in   the  case   of  many 
elements  is  that  they  are  capable  of  existing  in  more  than  one 
distinct  condition,  presenting  totally  different  physical  qualities. 
One  of  the  most  striking  examples  of  these  allotropic  modifica- 
tions or  conditions  of  matter  (aXXo?,  another — T^OTTO?,  away  or 
mode)  occurs  with  carbon,  which  exists  as  Diamond,  Graphite, 
and  Charcoal,  bodies  which  as  regards  colour,  hardness,  specific 
gravity,  &c.,  bear  certainly  but  a   slight  resemblance  to  each 
other,  but  which,  when  they  are  burnt  in    oxygen,  all  give  the 
same  relative  weight  of  the  same  product,  viz.,  carbonic  acid, 
thereby  proving  their  chemical  identity. 

12  The  Balance. — As  it  is  the  aim  of  the  chemist  to  examine 
the  properties  of  the  elements  and  their  compounds,  and  as  the 
weight-determination  of  a  substance  is  of  the  greatest  import- 
ance, it  becomes  necessary  for  him  to  ascertain  with  great  pre- 
cision the  proportion  by  weight  in  which  these  several  elements 
combine,  as  well  as  that  in  which  any  one  of  them  occurs  in  a 


THE  BALANCE 


57 


given  compound,  and  for  this  purpose  the  Balance  is  employed. 
By  means  of  this  instrument  the  weight  of  a  given  substance  is 
compared  with  the  unit  of  weight.  It  consists  essentially  of  a 
light  but  rigid  brass  beam  (Fig.  9),  suspended  on  a  fixed  hori- 
zontal axis  situated  at  its  centre  ;  and  this  beam  is  so  hung  as  to 
assume  a  horizontal  position  when  unloaded.  At  each  end  of  the 
beam  scale-pans  are  hung,  one  to  receive  the  body  to  be  weighed 
and  the  other  for  the  weights.  When  each  pan  is  equally 
weighted  the  beam  must  still  retain  its  horizontal  position,  but 
when  one  pan  is  more  heavily  weighted  than  the  other,  the  beam 
will  incline  on  the  side  of  the  heavier  pan.  The  balance  is,  there- 
fore, a  lever  with  equal  arms,  and  it  is  evident  that  the  weight  of 
the  substance  relative  to  the  unit  weight  employed  is  the  sum  of 


FIG.  9. 


the  weights  necessary  to  bring  the  balance  into  equilibrium.  The 
two  important  requisites  in  a  balance  are  (1)  accuracy,  (2)  sen- 
sibility, and  these  can  only  be  gained  by  careful  construction, 
It  needs  but  little  consideration  to  see  that  in  a  delicate  balance 
the  friction  of  the  various  parts  must  be  reduced  to  a  minimum. 
This  is  usually  accomplished  by  suspending  the  beam  by  means  of 
an  agate  knife-edge,  working  on  an  agate  plane,  whilst  the  pans  are 
attached  to  each  end  of  the  beam  by  a  somewhat  similar  arrange- 
ment shown  in  Fig.  10.  The  position  of  the  axis  of  suspension 
relatively  to  the  centre  of  gravity  of  the  beam  is  likewise  a  matter 


58 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


of  consequence.  If  the  axis  of  suspension  and  the  centre  of 
gravity  in  a  balance  were  coincident,  the  beam  would  remain 
stationary  in  all  positions  in  which  it  might  be  placed.  If  the 
axis  of  suspension  be  placed  below  the  centre  of  gravity  the 
beam  would  be  in  a  condition  of  unstable  equilibrium.  Hence 
the  only  case  in  which  the  balance  can  be  used  is  that  in  which 
the  point  or  axis  of  suspension  is  above  the  centre  of  gravity, 
for  in  this  case  alone  will  the  beam  return  to  a  horizontal 
position  after  making  an  oscillation,  and  in  this  case  the  balance 
may  be  considered  as  a  pendulum,  the  whole  weight  of  the 
beam  and  pans  being  regarded  as  concentrated  at  the  centre 
of  gravity.  In  order  that  the  weight  of  the  substance  and  the 
sum  of  the  measuring  weights  in  the  scale-pan  may  be  equal, 
it  is  evident  that  the  axis  of  suspension  must  be  exactly  in  the 
centre  of  the  beam,  or  in  other  words,  that  the  balance  must 
have  arms  of  equal  length.  It  is  also  necessary  that  the 
balance  should  have  great  sensibility;  that  is,  that  it  may  be 


FIG.  10. 

moved  by  the  smallest  possible  weight;  for  this  end  it  is  like- 
wise requisite  that  the  vertical  distance  of  the  centre  of  gravity 
below  the  axis  of  suspension  should  be  as  small  as  possible.  As 
the  whole  weight  of  the  instrument  may  be  regarded  as  con- 
centrated at  the  centre  of  gravity,  it  evidently  requires  a  less 
force  to  act  at  the  end  of  the  beam  to  move  the  instrument 
when  the  distance  of  the  centre  of  gravity  from  the  point  of 
suspension  of  the  balance  is  small,  than  when  that  distance  is 
greater,  inasmuch  as  in  the  latter  case  the  weight  has  to  be 
lifted  through  a  longer  arc.  The  sensibility  of  the  balance  is 
also  increased,  both  by  increasing  the  length  of  the  beam  and 
by  diminishing  the  weight  of  the  beam  and  of  the  load.  When, 
however,  the  beam  is  made  either  too  long  or  too  light  it  ceases 


PROPERTIES  OF  GASES 


59 


to  be  rigid,  and  a  serious  source  of  error  is  introduced.  If  great 
accuracy  in  the  weighing  is  desired,  it  is  advisable  to  have 
recourse  to  the  method  of  weighing  by  vibration1  by  which 
the  excursions  of  the  moving  beam  are  accurately  observed 
instead  of  its  approach  to  the  horizontal  position.  By  combining 
this  method  of  vibrations  with  that  of  double  weighing,  which 
consists  in  reversing  the  position  of  the  weights  in  the  two 
pans,  it  is  possible  for  a  good  balance  which  is  loaded  with  a 
kilogram  in  each  pan,  to  turn  with  0*0007  grm.  or  about 


the 


1400000 


th  of  the  weight  in  either  pan. 


PROPERTIES  OF  GASES. 

13  The  chemist  has  to  deal  with  matter  in  all  its  various 
states ;  solids,  liquids,  and  gases  alike  being  the  objects  of  his 
examination.  The  study  of  gases  in  particular  has  led  to  most 
important  results  in  the  theoretical  branch  of  the  science,  the 
system  of  formulas  now  employed  being  in  fact  founded  upon 
observations  made  upon  matter  in  the  state  of  gas. 


RELATION  OF  VOLUME  TO  PRESSURE.    BOYLE'S  LAW. 

14  The  gaseous  condition  of  matter  is  well  defined  to  be  that 
in  which  it  is  capable  of  indefinite  expansion.  If  a  quantity 
of  gas  as  small  as  we  please  is  placed  in  a  closed  vacuous 
space,  however  large,  the  gas  will  distribute  itself  uniformly 
throughout  that  space.  The  relation  between  the  volume  and 
pressure  of  a  gas,  the  temperature  remaining  constant,  is 
expressed  by  the  well-known  law  of  Boyle  (1662),  viz.,  that 
the  volume  of  the  gas  varies  inversely  as  the  pressure,  or,  in 
other  words,  the  pressure  of  a  gas  is  proportional  to  its  density  ; 
so  that  the  pressure  exerted  on  the  containing  vessel  by  two 
portions  of  any  given  gas  is  the  sum  of  the  pressures  which 
each  portion  would  exert  if  present  by  itself.  Dalton  extended 
this  law,  inasmuch  as  he  showed  that  if  different  gases,  which 
do  not  act  chemically  on  each  other,  are  mixed  together,  the 
pressure  exerted  is  likewise  the  sum  of  the  separate  pressures 
of  the  different  gases. 

1  See  Prof.  W.  H.  Miller,  Phil  Tram.  1856,  763;  and  article  "Balance," 
Watts'  Dictionary,  1st  Edition. 


60  GENERAL  PRINCIPLES  OF  THE  SCIENCE 


RELATION  OF  VOLUME  TO  TEMPERATURE.    DALTON'S  LAW. 

15  Another  simple  numerical  law,  which  characterises  the 
gaseous  condition,  is  known  as  the  law  of  Dalton,1  but  often 
referred  to  as  the  law  of  Charles  or  of  Gay-Lussac.2  This 
states  that  all  gases  heated  under  constant  pressure  expand  by 
an  equal  fraction  of  their  volume  at  0°  centigrade  for  equal 
increments  of  temperature,  one  volume  at  0°  becoming  1*3665 
at  100° ;  so  that  the  coefficient  of  expansion  of  gases  is  O003665 
or  nearly  ^T^  for  an  increase  from  0°  to  1°  centigrade.  If  the 
temperature  of  the  gas  be  lowered,  the  volume  contracts  in  the 
same  proportion.  When,  on  the  other  hand,  the  volume  of  a 
gas  is  kept  constant  and  the  temperature  raised,  the  pressure 
of  the  gas  increases  in  this  same  ratio;  so  that  a  mass  of  gas 
which  at  0°  has  a  pressure  of  1,  has  at  100°  a  pressure  of 
1*3665  if  it  be  not  allowed  to  increase  in  volume. 

The  behaviour  of  substances  in  the  gaseous  state  as  regards 
pressure  and  temperature  is  distinguished  by  its  simplicity  and 
uniformity  from  that  of  the  solid  and  liquid  forms  of  matter. 
For  in  the  case  of  solids  and  liquids  the  effect  on  the  volume 
of  alteration  of  pressure  as  well  as  of  temperature  is  different 
for  every  substance,  whilst  gases  are  all  uniformly  affected. 
Hence  we  are  led  to  conclude  that  the  gaseous  form  of  matter 
is  that  in  which  the  constitution  is  most  simple,  and  this  result 
is  borne  out  by  many  other  considerations. 


THE  KINETIC  THEORY  OF  GASES. 

1 6  The  doctrine  that  heat  is  only  a  kind  of  motion  is  one  which 
is  now  generally  admitted,  so  that  a  hot  body  may  be  regarded  as 
possessing  a  store  of  energy,  some  portion  of  which  at  any  rate 
may  be  made  use  of  to  accomplish  actual  work.  The  energy  of 
motion  is  termed  Kinetic  (from  /ctveco,  I  move),  and  this  energy 
is  communicated  when  the  body  possessing  it  comes  to  rest  by 
contact  with  some  other  body.  The  other  form  of  energy 

1  Dalton,  Manchester  Memoirs,  v.   535  (1801). 

2  The  experiments  of  Gay-Lussac  were  published  at  a  later  date  than  those  of 
Dalton.      The  results  obtained  by  Charles  were   never  published,    but  were 
verbally  communicated  to   Gay-Lussac.     Gay-Lussac,  Ann.  Chim.  [1],  43,  137.- 
Dixon,  Memoirs  Lit.  and  Phil.  Soc.  of  Manchester  [4],  4,  36. 


THE  KINETIC  THEORY  OF  GASES  61 

depending  on  position  with  respect  to  other  bodies  and  not 
upon  the  condition  of  matter  is  termed  Potential  energy.  It 
has  been  shown  that  in  a  hot  body  a  very  considerable  portion 
of  the  energy  arises  from  a  motion  of  the  parts  of  the  body ;  so 
that  every  hot  body  is  in  motion,  but  this  motion  is  not  one 
affecting  the  motion  of  the  mass  as  a  whole  but  only  that  of  the 
molecules  or  small  portions  of  the  body.  These  molecules  may 
consist  of  a  collection  or  system  of  smaller  parts  or  atoms 
which  partake  as  a  whole  of  this  general  motion  of  the  molecule. 
The  subject  of  the  motion  of  the  smallest  particles  of  matter 
attracted  the  attention  of  the  ancients,  and  Lucretius  held  that 
the  different  properties  of  matter  depended  upon  such  a  motion. 
Daniel  Bernoulli  was  the  first  to  conceive  the  idea  that  the  pres- 
sure of  the  air  could  be  explained  by  the  impact  of  its  par- 
ticles on  the  walls  of  the  containing  vessel,  whilst  in  the  year 
1848,  Joule  l  showed  that  these  views  were  correct,  and  cal- 
culated the  mean  velocity  which  the  molecules  must  possess 
in  order  to  bring  a.bout  the  observed  pressure.  Since  the 
above  date,  Clausius,  Maxwell,  and  other  physicists,  have 
extended  and  completed  the  dynamical  theory  of  gases. 
Many  of  the  phenomena  observed  in  gases  and  also  in  liquids, 
especially  diffusion,  prove  that  the  large  number  of  small 
particles  or  molecules  of  which  these  forms  of  matter  are 
made  up  are  in  a  constant  condition  of  change  or  agitation, 
and  the  hotter  a  body  is  the  greater  is  the  amount  of  this 
agitation.  According  to  the  kinetic  theory,  these  molecules 
are  supposed  to  move  with  great  velocity  amongst  one  another, 
and,  when  not  otherwise  acted  on  by  external  forces,  the  direc- 
tion of  this  motion  is  a  rectilinear  one,  and  the  velocity  uniform. 
The  molecules,  however,  come  into  frequent  contact  with  one 
another,  or  as  Maxwell  describes  it,  encounters  between  two 
molecules  occur.  In  these  encounters,  and  also  when  the  mole- 
cules strike  the  surface  of  the  containing  vessel,  no  loss  of 
energy  takes  place,  provided  of  course  that  everything  is  at 
the  same  temperature,  so  that  the  total  energy  of  the  enclosed 
system  remains  unaltered. 

17  From  these  principles,  assuming  simply  that  the  molecules 
have  weight  and  are  in  motion,  and  applying  the  usual  laws  of 
masses  in  motion,  the  experimental  laws  of  gases,  already 
alluded  to,  as  well  as  others,  may  be  deduced.  The  pressure  of 
a  gas  is  thus  due  to  the  impacts  of  its  molecules  upon  the  walls 
1  Brit.  Assoc.  Reports,  1848,  2nd  Part.  p.  21. 


62  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

of  the  containing  vessel ;  hence,  when  twice  as  many  molecules 
are  crowded  into  a  given  space,  by  the  compression  of  the  gas 
to  half  of  its  original  volume,  the  frequency  of  the  impacts,  and, 
therefore,  also  the  resulting  pressure  will  be  doubled,  or,  in  other 
words,  the  pressure  is  inversely  proportional  to  the  volume 
(Boyle's  Law).  The  temperature  is  measured  by  the  kinetic 
energy  of  the  molecules,  which  is  equal  to  the  product  of  half 
their  mass  into  the  square  of  their  velocity  i.e.,  \  mv2.  Increase 
of  temperature,  therefore,  means  increase  of  the  velocities  of  the 
molecules,  and  hence  if  the  volume  of  a  gas  be  kept  constant 
and  its  temperature  raised,  both  the  force  of  impact  of  each 
molecule  against  the  wall  of  the  vessel  and  the  number  of 
impacts  per  second  will  be  increased,  the  pressure  rising  in  pro- 
portion to  the  square  of  the  velocity,  or  in  other  words  in  direct 
proportion  to  the  rise  of  temperature  (Dalton's  Law).  The  tem- 
perature at  which  the  velocity  of  the  molecules,  and,  therefore, 
also  the  kinetic  energy,  would  become  equal  to  nothing,  is  called 
the  absolute  zero  of  temperature  and  is  found  by  calculation 
to  be  —273°  centigrade,  temperatures  reckoned  from  this  zero 
being  termed  absolute  temperatures. 

When  two  gases  are  at  the  same  pressure,  the  total 
kinetic  energy  of  the  molecules  in  equal  volumes  must  be 
the  same ;  if  their  temperatures  are  also  equal,  the  kinetic 
energy  of  each  molecule  must  also  be  equal,  and  it  hence 
follows  that  the  number  of  molecules  in  equal  volumes  of  the 
two  gases  must  be  the  same.  Assuming  then  that  the  tem- 
perature of  a  gas  represents  the  kinetic  energy  of  its  molecules 
it  follows  that  equal  volumes  of  all  gases  under  similar  conditions 
of  temperature  and  pressure  contain  equal  numbers  of  mole- 
cules. This  is  the  statement  known  to  chemists  as  Avogadro's 
theory  (p.  98),  which  is  thus  shown  to  rest  upon  a  sound 
physical  foundation. 

1 8  The  velocities  of  the  molecules  can  be  calculated  for  any 
given  temperature  when  we  know  the  density  of  the  gas  and  its 
pressure  at  that  temperature.  As  will  be  seen  from  the  table 
given  below  their  magnitude  is  very  considerable,  and  this 
accounts  for  the  great  velocity  with  which  disturbances,  such  as 
sound  waves  or  explosions,  are  propagated  through  gases. 


VELOCITY  OF  MOLECULES  OF  GASES        63 

Velocity  at  0° 

in 
metres  per  second. 

Hydrogen 1843 

Ammonia 628 

Oxygen 461 

Carbon  dioxide 392 

Chlorine 310 

Hydriodic  acid 230 

These  numbers  represent  the  mean  velocity  of  the  molecules, 
since  it  is  supposed  that  owing  to  encounters  the  velocity  is 
not  absolutely  uniform  but  varies  about  an  average  value; 
in  hydrogen  for  example  at  0°  C.  and  760mm.  some  of  the 
molecules  are  moving  at  a  much  slower  rate  than  1843  metres 
per  second,  whilst  others  are  moving  much  more  rapidly.  In 
addition  to  the  energy  of  translation  which  the  molecule  as  a 
whole  possesses,  the  atoms  of  which  it  is  composed  and  which 
are  capable  of  motion  relatively  to  each  other  have  also  a 
certain  amount  of  kinetic  energy.  When  a  gas  is  heated,  there- 
fore, both  of  these  kinds  of  energy  are  increased,  or  in  other 
words  the  motion  of  the  atoms  within  the  molecule  is  increased 
at  the  same  time  as  the  velocity  of  the  molecule  itself. 

Clerk- Maxwell  and  others  have  calculated  that  the  actual  num- 
ber of  molecules  which  we  must  conclude  to  be  present  in  one 
cubic  centimetre  of  a  gas  at  the  standard  temperature  and  pres- 
sure is  no  less  than  21  trillions  (21,000,000,000,000,000,000 
or  21  x  1018)  ;  the  weight  of  a  single  molecule  of  hydrogen 
being,  therefore,  about  O'OOO  000  000  000  000  000  04  milli- 
grams or  (0-04  x  10-18). 


DIFFUSION  OF  GASES. 

19  Early  in  the  history  of  the  chemistry  of  gases  it  was 
observed  that  when  gases  of  different  specific  gravities,  which 
exert  no  mutual  chemical  action,  are  once  thoroughly  mixed, 
they  do  not  of  themselves  separate  in  the  order  of  their  several 
densities  by  long  standing.  On  the  contrary,  they  remain 
uniformly  distributed  throughout  the  mass.  Priestley l  proved 
this  by  very  satisfactory  experiments ;  but  he  believed  that  if 
the  different  gases  were  very  carefully  brought  together,  the 
1  Observations  on  Air,  2,  441. 


64  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

heavier  one  being  placed  beneath  and  the  lighter  one  being 
brought  on  to  the  top  without  being  mixed  with  the  other,  they 
would  then,  on  being  allowed  to  stand,  not  mix,  but  continue 
separate  one  above  the  other.  Dalton,1  in  1803  proceeded  to 
investigate  this  point,  and  he  came  to  the  conclusion  that  a 
lighter  gas  cannot  rest  upon  a  heavier,  as  oil  upon  water,  but 
that  the  particles  of  the  two  gases  are  constantly  diffusing 
through  each  other  until  an  equilibrium  is  reached,  and  this 
without  any  regard  to  their  specific  gravities.  This  conclusion 
Dalton  regarded  as  a  necessary  consequence  of  his  theory  of  the 
constitution  of  matter,  according  to  which  the  particles  of  all 
gaseous  bodies  exert  a  repulsive  influence  on  each  other,  and 
each  gas  expands  into  the  space  occupied  by  the  other  as  it 
would  into  a  vacuum.  In  fact,  however,  it  does  not  so  expand, 
for  the  rate  at  which  a  gas  diffuses  into  another  gas  is  many 
thousand  times  slower  than  that  at  which  it  rushes  into  a 
vacuum.2  As  was  usual  with  him,  the  apparatus  used  by  Dalton 
in  these  experiments  was  of  the  simplest  kind.  It  consisted  of 
a  few  phials  and  tubes  with  perforated  corks.  "  The  tube 
mostly  used  was  one  10  inches  long  and  of  ^V  inch  bore  ;  in 
some  cases  a  tube  of  30  inches  in  length  and  J  inch  bore  was 
used ;  the  phials  held  the  gases  which  were  the  subject  of 
experiment  and  the  tube  formed  the  connection.  In  all  cases  the 
heavier  gas  was  in  the  lower  phial  and  the  two  were  placed  in 
a  perpendicular  position,  and  suffered  to  remain  so  during  the 
experiment  in  a  state  of  rest;  thus  circumstanced  it  is  evident 
that  the  effect  of  agitation  was  sufficiently  guarded  against ;  for 
a  tube  almost  capillary  and  10  inches  long,  could  not  be  instru- 
mental in  propagating  an  intermixture  from  a  momentary  com- 
motion at  the  commencement  of  each  experiment."  The  gases 
experimented  on  were  atmospheric  air,  oxygen,  hydrogen,  nitro- 
gen, nitrous  oxide  and  carbonic  acid;  and  after  the  gases  had 
remained  in  contact  for  a  certain  length  of  time  the  composition 
of  that  contained  in  each  phial  was  determined,  and  invariably 
showed  that  a  passage  of  the  heavier  gas  upwards  and  the  lighter 
gas  downwards,  had  occurred.  Similar  experimental  results 
were  also  obtained  by  Berthollet  in  1809.3 

The  passage  of  gases  through  fine  pores  was  likewise  observed 
by  Priestley 4  in  the  case  of  unglazed  earthenware  retorts  which 
although  perfectly  air-tight  so  as  not  to  allow  of  any  escape  by 

1  Manch.  Memoirs,  1805,  p.  259.  2  Phil.  Mag.  4,  26,  409. 

3  Him.  d'Arcueil,  2,  463.  <  Observations,  <fcc.,  2,  414. 


DIFFUSION  OF  GASES  65 


blowing  in?  allowed  the  vapour  of  water  to  pass  out  whilst  air 
came  in,  even  where  the  gas  in  the  retort  was  under  a  greater 
pressure  than  that  outside.  Dalton  was  the  first  to  explain  this 
fact  as  being  due  to  precisely  the  same  cause  as  that  which 
brings  about  the  exchange  of  gases  in  the  phials  connected  with 
the  long  tubes,  only  that  here  we  have  a  large  number  of  small 
pores  instead  of  one  (the  bore  of  the  tube)  of  sensible  magnitude. 

20  In  the  year  1823  Dobereiner1  made  the  remarkable  observa- 
tion that  hydrogen  gas  collected  ove.r  water  in  a  large  flask  which 
happened  to  have  a  fine  crack  in  the  glass,  escaped  through  the 
crack  into  the  air,  whilst  the  level  of  the  water  rose  in  the  flask 
to  a  height  of  nearly  three  inches  above  its  level  in  the  trough. 
Air  placed  in  the  same  flask  did  not  produce  a  similar  effect,  nor 
was  this  rise  of  the  water  observed  with  the  flask  full  of  hydro- 
gen when  it  was  surrounded  with  a  bell-jar  filled  with  the  same 
gas. 

As  in  the  former  instance,  the  discoverer  of  the  fact  was  un- 
able to  explain  the  phenomenon,  and  it  was  not  until  1832  that 
Thomas  Graham 2  in  repeating  Dobereiner's  experiments  showed 
that  no  hydrogen  could  escape  by  the  crack  without  some  air 
coming  in,  and  enunciated  the  law  of  gaseous  diffusion  founded 
on  the  results  of  his  experiments,  viz.,  that  the  rate  at  which  gases 
diffuse  is  not  the  same  for  all  gases  but  that  their  relative  rates 
of  diffusion  are  inversely  proportional  to  the  square  roots  of  their 
densities,  so  that  hydrogen  and  oxygen  having  the  relation  of 
their  densities  as  1  to  16  the  relative  rates  of  diffusion  are  as 
4  to  1. 

Instead  of  using  cracked  vessels  Graham  employed  a  diffusion 
tube  consisting  of  a  glass  tube  open  at  each  end  and  about  six  to 
fourteen  inches  in  length  and  half  an  inch  in  diameter;  a  wooden 
cylinder  is  introduced  into  the  tube  so  as  to  fill  it  with  the  exception 
of  half  an  inch  at  one  end,  and  this  unoccupied  space  is  filled  with 
a  plug  of  plaster  of  Paris;  the  cylinder  being  withdrawn  after 
the  paste  of  plaster  has  set.  With  such  a  tube  divided  into 
volumes  of  capacity,  filled  with  gas  and  placed  over  water,  the 
rate  of  the  rise  or  depression  of  the  water  could  be  easily 
observed  and  the  composition  of  the  gas  both  before  and 
after  the  experiment  ascertained.  In  this  way  the  relative 
dirfusibility  of  various  gases  was  determined,  the  results  of 
Graham's  experiments  being  shown  in  the  following  table. 

1  Ann.  Chim.  Phys.  1823,  24,  332. 

2  Edin.  Phil.  Trans.  12,  1834,  222.     Phil.  Mag.  1833,  2,  175. 

6 


66 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


DIFFUSION  OF  GASES. 


Square 

i 

Velocity  of 

density. 

isj  density. 

of  air  =  1. 

Hydrogen  
JVlarsh  2fas  .  . 

0-06949 
0-559 

0-2636 
0-7476 

3-7935 
1-3375 

3-83 
1-344 

Steam     
Carbonic  oxide 
Nitro°'en 

0-6235 
0:9678 
0-9713 

0-7896 
0-9837 
0-9856 

1-2664 
1-0165 
1-0147 

1-1149 
1-0143 

Ethylene 

0-978 

0-9889 

1-0112 

1-0191 

Nitric  oxide  .... 
Oxvsren  . 

1-039 
1-1056 

1-0196 
1-0515 

0-9808 
0-9510 

0-9487 

Sulphuretted  hydrogen. 
Nitrous  oxide  .... 
Carbon  dioxide  .  .  . 
Sulphurous  acid  .  .  . 

1-1912 
1-527 
1-52901 

2-247 

1-0914 
1-2357 
1-2365 
1-4991 

0-9162 
0-8092 
0-8087 
0-6671 

0-95 
0-82 
0-812 
0-68 

The  observed  velocities  of  diffusion  agree  very  closely  with 
those  obtained  by  calculation.  This  is,  however,  only  the  case 
when  the  porous  plate  through  which  the  diffusion  takes  place 
is  very  thin.  If  the  plate  be  thick  the  gases  have  to  pass 
through  a  series  of  capillary  tubes,  and  the  rate  of  diffusion  is 
considerably  diminished  by  the  friction. 

The  passage  of  the  gases  through  capillary  tubes  has  been 
termed  transpiration  of  gases,  and  this  proceeds  according  to 
other  laws  than  those  of  diffusion,  as  in  transpiration  we  have  to 
do  with  a  motion  of  the  mass  of  the  gas,  whereas  in  diffusion  the 
motion  is  purely  molecular.  Thus  when  allowed  to  pass  through 
capillary  tubes  the  rate  of  transpiration  of  equal  volumes  of  the 
following  gases  was  found  by  Graham  to  be  represented  by  the 
numbers  : — 

Oxygen     ....     TOO 

Hydrogen      .     .     .     0*44 

Carbon  dioxide .     .     0'72 

The  numbers  bear  no  relation  to  the  square  roots  of  the 
densities  of  the  gases. 

Of  all  substances,  that  which  is  best  adapted  for  exhibiting  the 
laws  of  diffusion  is  a  thin  plate  of  artificial  graphite.  With  a 
porous  plate  of  graphite  0'5  mm.  in  thickness  Graham  1  obtained 

1  Phil.  Trans.  1863,  392. 


DIFFUSION  OF  GASES  67 

the   following  times  of  diffusion  into  air  under  a  pressure  of 
100  mm.  of  mercury. 

Time  of  molecular          Square  root  of 
passage.  density  0  =  1. 

Hydrogen.     .     .     .     0'2472     .  .     .     0'2509 

Oxygen     ....     1-0000     .  .     .     1-0000 

Carbon  dioxide  .     .     11886     .  .     .     T1760 

When  the  same  gases  were  allowed  to  diffuse  into  a  vacuum 
the  following  were  the  results  : — 

Time  of  molecular        Square  root  of 
passage.  density  0  =  1. 

Hydrogen   ....  0'2505  .  .  .  0'2509 

Air 0-9501  .  .  .  0'9507 

Oxygen I'OOOO  .  .  .  I'OOOO 

Carbon  dioxide    .     .  1-1860  .  .  .  T1760 

Hence  it  appears  that  a  plate  of  artificial  graphite  is  practically 
impermeable  to  gas  by  transpiration  but  is  readily  penetrated  by 
gases  when  in  molecular  or  diffusive  movement,  whether  the  gases 
pass  under  pressure  into  air  or  into  a  vacuum,  and  this  sub- 
stance, therefore,  serves  as  a  kind  of  "  pneumatic  sieve  "  which 
permits  the  passage  of  the  molecules  but  not  the  masses  of  the 
gas. 

21  The  phenomena  of  diffusion  can  be  strikingly  demonstrated 
by  the  following  experiments : — 

First.  To  one  end  of  a  glass  tube  about  1  metre  in  length  and 
1  cm.  in  diameter,  having  a  bulb  blown  on  to  it,  a  cylindrical 
porous  cell  (such  as  those  used  for  galvanic  batteries)  is  fixed  by 
means  of  a  caoutchouc  cork.  The  other  end  of  the  tube  is  drawn 
out  to  a  fine  point  and  bent  round  as  shown  in  Fig.  11.  If  now 
a  vessel  filled  with  hydrogen  be  held  over  the  porous  jar  this 
gas  will  enter  more  quickly  than  the  air  can  issue,  viz.,  in  the  pro- 
portion of  the  inverse  square  roots  of  their  densities,  that  is,  as 
X/14'4  to  1,  or  as  3*8  volumes  to  one  volume,  so  that  the  pressure 
in  the  porous  cell  will  increase  and  the  coloured  water  placed  in 
the  bulb  will  be  driven  out  in  the  form  of  a  fountain  through  the 
narrow  jet. 

A  second  experiment  showing  the  mode  in  which  one  gas  may 
be  separated  from  another  by  diffusion  (termed  atmolysis  from 
drfjios  vapour  and  Xuo>  I  loosen)  is  the  following.  A  slow  current 
of  the  detonating  gas  obtained  by  the  electrolysis  of  water,  and 


68 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


consisting  of  2  volumes  of  hydrogen  to  one  volume  of  oxygen,  is 
allowed  to  pass  through  a  common  long  clay  tobacco-pipe,  the 
gas  on  issuing  from  the  pipe  being  collected  over  water  in  a 
pneumatic  trough.  On  bringing  the  gas,  thus  collected,  in 


m 


FIG.  11. 


contact  with  a  flame  it  no  longer  detonates.  On  the  contrary, 
it  will  rekindle  a  glowing  chip  of  wood,  thus  showing  that  in 
its  passage  through  the  porous  pipe  the  greater  portion  of  the 
lighter  hydrogen  has  escaped  by  diffusion  through  the  pores  of 
the  clay,  whilst  the  heavier  oxygen  has  not  passed  through. 


DIFFUSION  OF  GASES 


69 


A  third  experiment  to  illustrate  the  law  of  diffusion  is  one 
which  possesses  interest  from  another  point  of  view,  inasmuch 
as  it  has  been  proposed  to  employ  the  arrangement  for  giving 
warning  of  the  outbreak  of  the  dangerous  and  explosive 
gas  termed  fire-damp  by  the  coal-miners.  Fire-damp  or 
marsh  gas  is  lighter  than  air,  and  134  volumes  of  this 
gas  will  diffuse  through  a  porous  medium  in  the  same  time 
as  100  volumes  of  air  will  do.  Hence  if  a  quantity  of 
fire-damp  surround  the  porous  plate,  the  volume  within  the 
vessel  will  become  larger,  and  this  increase  of  volume  may  be 


FIG.  12. 


made  available  either  to  drive  out  water  as  in  the  first  experi- 
ment or  to  alter  the  level  of  a  column  of  mercury  so  as  to 
make  contact  with  a  connected  battery  and  then  to  ring  a 
warning  bell.  The  latter  form  of  apparatus  is  seen  in  Fig.  12. 
Holding  a  beaker-glass  (A)  filled  with  hydrogen  or  common 
coal-gas  over  the  plate  of  porous  stucco  fastened  into  the  tube- 
funnel  an  increase  of  volume  occurs  inside  the  glass  tube  and 
a  consequent  depression  of  the  mercury  takes  place  in  the  bend  of 
the  tube  which  is  sufficient  to  make  metallic  contact  with  a 
second  platinum  wire  fused  through  the  glass  and  to  bring  the 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


•current  to  act  on  the  magnet  of  the  electric-bell  (B).  The  other 
tube  is  arranged  for  showing  that  a  dense  gas,  such  as 
carbonic  acid,  does  not  diffuse  through  the  porous  septum  so 
quickly  as  air  escapes.  By  immersing  the  porous  plate  in  a 
jar  (c)  filled  with  the  heavy  gas,  the  volume  inside  the  tube 
becomes  less,  the  level  of  the  mercury  in  the  bend  is  altered, 
contact  is  again  made  with  the  battery,  and  the  ringing  of  the 
bell  gives  notice  of  the  change. 

22  Effusion  of  Gases  is  the  name  given  by  Graham  to  the 
flow  of  gases  under  pressure  through  a  minute  aperture  in  a 
metallic  plate.  The  law  of  diffusion  is  found  to  hold  good 
with  regard  to  this  molecular  motion  of  gases,  the  times 
required  for  equal  volumes  of  different  gases  to  flow  through 
an  aperture  of  a  diameter  of  ^^  of  an  inch  having  been  found 
to  be  very  nearly  proportional  to  the  square  roots  of  their 
densities,  and  the  velocity  of  flow  to  be  inversely  as  the  square 
roots  of  their  densities. 

This  law,  which  is  true  for  the  flow  of  all  fluids  through  a 
small  aperture  in  a  thin  plate  has  been  applied  by  Bunsen  l 
for  the  purpose  of  determining  the  specific  gravity  of  gases, 
the  method  serving  admirably  when  only  small  quantities  of 
the  gas  can  be  obtained. 


DEVIATIONS  FROM  THE  LAWS  OF  BOYLE  AND  DALTON. 

23  These  laws  are  only  approximately  true,  for  it  is  found  that 
no  gas  exactly  follows  them  under  all  conditions  of  temperature 
and  pressure,  although  under  moderate  variations  of  these 
conditions  the  deviations  are  but  slight.  The  ideal  gas 
which  would  agree  with  these  laws  under  all  conditions  is  called 
a  perfect  gas,  and  this  term  is  often  also  applied  to  gases  which 
approach  this  state.  The  effect  of  high  pressures  upon  certain 
gases  is  shown  in  the  following  table  exhibiting  the  experi- 
mental results  obtained  by  Natterer.2 

-     l  Gasometry,  p.  121.  2  Pogg.  Ann.  94,  436. 


DEVIATIONS  FROM  BOYLE'S  LAW 


71 


Table  of  Deviations  from  the  Law  of  Boyle. 
Pressure  in                  Number  of  volumes  of  gas  compressed  into  one  volume. 

Atmospheres.     Hydrogen. 

Oxygen. 

Nitrogen. 

Air.     Nitric  Oxide. 

1 

1 

1 

1 

1 

1 

50 

50 

50 

50 

50 

50 

100 

98 

100 

99 

100 

100 

500 

396 

439 

381 

396 

412 

1,000 

623 

595 

519 

527 

544 

1,500 

776 

— 

590 

607 

617 

2,000 

899 

— 

641 

661 

669 

3,600 

1,040 

— 

710 

800 

•      : 

Two  factors  seem  to  be  involved  in   producing  deviations  of 
this  character.     In  the  first  place,  as  the  density  of  the  gas 


•40 


30 


40  80 

P.  in  Atmospheres 
A  Hydrogen. 


120 


160 


200 


240 


280 


320 


B  Ethylene. 
FIG.  13. 


C  Carbon  dioxide. 


increases,  the  volume  occupied  by  the  molecules  themselves, 
which  we  must  suppose  to  be  unalterable  by  pressure,  bears  a 
greater  proportion  to  the  distance  between  them,  which  can  be 
lessened  by  compression,  and  consequently  the  volume  diminishes 
less  rapidly  than  when  the  density  is  smaller.  Secondly,  as  the 


72  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

molecules  approach  one  another  more  closely,  they  tend  to  cohere, 
and  the  volume  is  thereby  diminished  more  rapidly  than  the 
pressure  increases.  The  actual  effect  produced  is  generally  due 
to  a  combination  of  these  two  causes  ;  thus  the  volume  of  carbon 
dioxide  at  a  temperature  of  100°  decreases  more  rapidly  than  we 
should  expect  from  Boyle's  law  until  a  pressure  of  160  atmo- 
spheres is  reached,  after  which  it  decreases  less  rapidly.  The 
behaviour  of  several  gases  is  shown  in  Fig.  13,  on  the  preceding 
page.  In  every  case  the  value  of  the  product  of  pressure  and 
volume  should  remain  invariable,  according  to  the  law  of  Boyle, 
whilst  the  actual  values  are  seen  to  deviate  considerably  from 
this  theoretical  constancy. 

Deviations  of  a  similar  character  are  observed  when  the  density 
of  a  gas  is  increased  by  lowering  its  temperature. 

If  any  gas  be  subjected  to  a  greatly  increased  pressure  and  its 
temperature  simultaneously  greatly  reduced,  a  point  is  at  last 
reached  at  which  the  gas  undergoes  sudden  contraction  and 
becomes  converted  into  a  liquid.  Just  before  liquefaction  the 
gas  may  be  considered  as  the  vapour  of  a  liquid,  so  that  carbon 
dioxide  at  a  high  pressure  and  a  low  temperature  may  be 
compared  to  steam  just  about  to  condense. 

THE  CONTINUITY  OF  THE  GASEOUS  AND  LIQUID  STATES  OF 

MATTER. 

24  It  is  matter  of  everyday  experience  that  the  tension  of  the 
vapour  of  water  and  of  other  liquids  heated  with  excess  of  the 
liquid  in  closed  vessels,  increases  in  a  Very  rapid  ratio  with 
increase  of  temperature,  and  that  the  density  of  the  steam  or 
vapour  in  such  a  case  undergoes  a  similar  rapid  increase.  Thus 
at  231°  the  weight  of  a  cubic  metre  of  steam  is  -gV  part  of  the 
weight  of  the  same  bulk  of  water  at  4°,  the  point  of  maximum 
density,  the  weight  of  steam  at  100°,  being  only  TTVir  of  that  of 
the  same  bulk  of  water,  so  that  at  a  temperature  not  very  far 
above  230°  the  weight  of  the  vapour  will  become  equal  to  that 
of  the  liquid.  The  result  of  this  must  be  that  under  these  circum- 
stances a  change  from  the  gaseous  to  the  liquid  state  is  not 
accompanied  by  any  condensation,  and  in  such  a  case  the  distinc- 
tions we  have  been  in  the  habit  of  drawing  between  these 
conditions  of  matter  cease  to  have  any  meaning.  So  long  ago  as 
]  822,  Cagniard  de  la  Tour 1  made  experiments  upon  the  action  of 

1  Ann.  Chim.  Phys.  [2],  21,  127,  22,  410. 


THE  CRITICAL  TEMPERATURE 


73 


liquids  sealed  up  in  glass  tubes  of  a  capacity  but  little  greater 
than  that  of  the  liquid.  When  a  tube  one-fourth  filled  with 
water  was  heated  up  to  about  360°  the  water  entirely  disap- 
peared, the  tube  appearing  empty,  and  as  the  vapour  cooled  a 
point  was  reached  at  which  a  kind  of  cloud  made  its  appearance, 
and  a  few  moments  later  the  liquid  was  again  visible.  Cagniard 
de  la  Tour  considered  that  the  substance  when  thus  heated 
assumes  the  gaseous  condition ;  but  Dr.  Andrews  1  has  shown 
that  in  such  an  experiment  the  properties  of  the  liquid 
and  those  01  the  vapour  constantly  approach  one  another,  so  that 
above  .a  given  temperature  the  properties  of  the  two  states  can- 
not be  distinguished.  Hence  it  follows  that  at  all  temperatures 
above  this  particular  one  no  increase  of  pressure  can  bring 
about  the  change  by  condensation  which  we  term  liquefaction. 
This  temperature  is  called  by  Andrews  the  critical  point.  For 
many  gases,  the  critical  points  are  situated  at  temperatures 
which  render  the  observation  of  this  fact  easy.  Thus,  for 
example,  the  critical  temperature  for  carbon  dioxide  (or  carbonic 
acid  gas)  is  30°'92,  and  at  all  temperatures  above  this  point  no 
condensation  from  gas  to  liquid  occurs ;  so  that  if  the  pressure 
on  the  gas  be  gradually  increased  up  to  150  atmospheres  a 
steady  diminution  of  volume  occurs  as  the  pressure  augments, 
and  no  sudden  diminution  of  volume  will  occur  in 
any  stage  of  it.  The  temperature  may  then  be  gradually 
allowed  to  fall  until  the  carbon  dioxide  has  reached  a  temperature 
below  the  critical  one  when  it  is  found  to  be  a  liquid ;  thus 
beginning  as  a  gas  and  by  a  series  of  gradual  changes,  pre- 
senting nowhere  any  abrupt  alteration  of  volume  or  sudden 
evolution  of  heat,  it  ends  by  being  an  undoubted  liquid.  This 
clearly  shows  that  the  properties  of  a  gas  can  be  continually  and 
imperceptibly  changed  into  those  of  a  liquid. 

The  tension  of  the  substance  at  the  critical  temperature  is 
known  as  the  critical  pressure. 

The  critical  temperatures  of  a  few  gases  and  liquids  are  given 
in  the  following  table  : — 

1  Phil.  Trans.  1869,  Part  2,  575. 


74  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

Oxygen -113° 

Ethylene +13° 

Carbon  dioxide  .     .     .  +30°'9 

Nitrous  oxide     .     .     .  +36° 

Chlorine +141° 

Alcohol +234° 

Benzene  +281° 


LIQUEFACTION  OF  GASES. 

25  The  first  instance  of  a  substance,  which,  under  ordinary 
•conditions,  is  known  as  a  gas,  being  transformed  by  pressure  into 
a  liquid  is  chlorine  gas.  This  gas  was  first  liquefied  under  pres- 
sure by  Northmore  l  in  1806.  Faraday  investigated  the  subject 
fully  shortly  afterwards  2  showing  that  many  other  gases  such  as 
sulphur  dioxide,  carbonic  acid,  euchlorine,  nitrous  oxide, 
cyanogen,  ammonia,  and  hydrochloric  acid  can  also  be 
reduced  to  the  liquid  state.  In  these  experiments  Faraday  em- 
ployed bent  tubes  made  of  strong  glass,  in  the  one  limb  of 
which,  being  closed  at  the  end,  materials  were  placed  which  on 
being  heated  will  yield  the  gas ;  the  open  limb  of  the  tube  was 
then  hermetically  sealed  and  the  gas  evolved  by  heating  the 
other  end.  The  pressure  or  tension  exerted  by  the  gas  itself, 
when  thus  generated  in  a  closed  space  is  sufficient  to  condense 
a  portion  into  the  liquid  state.  The  following  table  shows  the 
maximum  tensions  of  some  of  these  more  readily  liquefiable 
gases  at  0° : — 

Table  of  Tensions. 

Atmospheres.  Atmospheres. 

Sulphur  dioxide  .     .     1'53  Sulphuretted  hydrogen  10' 00 

Cyanogen  ....     2'37  Hydrochloric  acid  .     .  26'20 

Hydriodic  acid    .     .     3*97  Nitrous  oxide     .     .     .  32*00 

Ammonia  .  v .     .     .     4*40  Carbon  dioxide  .     .     .   38'50 
Chlorine     ....     8'95 

If,  therefore,  any  of  the  above  gases  at  0°  be  exposed  to  pres- 
sures exceeding  those  given  in  the  table  they  will  condense  to 
liquids,  their  critical  temperatures  being  all  above  0°. 

The  liquefaction  of  gases  can  be  brought  about  not  merely  by 

1  Northmore,  Nicholson's  Journal,  12,  368  ;  13,  232. 
"  Phil.  Trans.  1823,  160.     Ibid,  1823,  189. 


LIQUEFACTION  OF  GASES  75 

simple  exposure  to  high  pressure  but  also  to  low  temperature  ; l 
thus,  if  we  reduce  the  temperature  of  sulphur  dioxide,  under 
the  ordinary  atmospheric  pressure,  to  -  10°  it  liquefies,  and  when 
the  temperature  sinks  to  —  76°,  the  liquid  freezes  to  an  ice-like 
mass. 

26  Until  the  important  researches  of  Pictet  2  and  Cailletet  3 
certain  gases,  such  as  oxygen,  hydrogen,  and  nitrogen,  had 
resisted  all  attempts  to  reduce  them  to  the  liquid  or  solid  state, 
because  their  critical  temperatures  had  never  been  reached,  and 
to  these  the  name  of  the  permanent  gases  was  given. 

The  meeting  of  the  French  Academy  of  the  24th  December, 
1877,  was  a  memorable  one.  On  that  day  the  Academicians 
were  told  that  Cailletet  had  succeeded  in  liquefying  both  oxygen 
and  carbon  monoxide  at  his  works  at  Chatillon-sur-Seine,  and 
that  the  former  gas  had  also  been  liquefied  by  Raoul  Pictet  at 
Geneva. 

These  experimenters  soon  succeeded  in  condensing  the  other 
gases  already  named,  with  the  exception  of  hydrogen,  which 
has,  however,  since  yielded  to  the  lower  temperatures  produced 
by  later  workers,  and  thus  we  are  able  to  give  experimental 
proof  of  the  view  which  has  been  frequently  expressed  that  all 
bodies  without  exception  possess  the  power  of  cohesive  attraction. 
These  important  results  were  arrived  at  independently  by  both 
observers,  each  having  made  the  question  the  subject  of  many 
years'  study  and  experiment.  It  is  difficult,  on  reading  the 
description  of  these  experiments,  to  know  which  to  admire  most, 
the  ingenious  and  well-adapted  arrangement  of  the  apparatus 
employed  by  Pictet,  or  the  singular  simplicity  of  that  used  by 
Cailletet.  The  latter  gentleman  is  one  of  the  greatest  of  French 
ironmasters,  whilst  the  former  is  largely  engaged  as  a  manu- 
facturer of  ice-making  machinery,  and  the  experience  and 
practical  knowledge  gained  by  each  in  his  own  business  have 
materially  assisted  to  bring  about  one  of  the  most  interesting 
results  in  the  annals  of  scientific  discovery. 

The  process  successfully  adopted  in  each  case  consisted  in 
simultaneously  exposing  the  gas  to  a  very  high  pressure  and  to 
a  very  low  temperature. 

The  increase  of  pressure  was  effected  by  Pictet  by  evolving 
the  gas  in  a  wrought-iron  vessel  strong  enough  to  withstand  an 

1  Faraday,  Phil.  Trans.  1845,  155. 

2  Compt.  Rend,  85,  1214,  1220. 

3  Ibid.  815. 


76  GENERAL  PRINCIPLES  OF  THE  SCIENCE 


enormous  tension;  whilst  in  Cailletet's  arrangement  the  same 
end  was  brought  about  by  a  hydraulic  press.  For  the  purpose  of 
obtaining  a  low  temperature  the  first  experimenter  made  use  of 
the  rapid  evaporation  of  liquid  carbon  dioxide,  thus  producing 
a  constant  temperature  of  — 130°.  Cailletet,  on  the  other  hand, 
effected  the  same  end  by  suddenly  diminishing  the  pressure 
upon  the  compressed  gas.  This  sudden  expansion  gives  rise  to 
a  rapid  diminution  of  temperature  caused  by  the  transference 
of  heat  into  the  motion  of  the  particles  of  the  expanding  gas 
(chaleur  de  detente).  So  great  is  the  amount  of  heat  thus 
absorbed  that  the  temperature  of  the  particles  sinks  below  the 
critical  point  of  the  gases,  and  a  condensation  occurs,  the  finely 
divided  liquid  oxygen  or  nitrogen  appearing  as  a  mist  in  the 
tube. 


PICTET'S  METHOD  OF  LIQUEFYING  GASES. 

27  A  vertical  section  and  a  ground  plan  of  Pictet's  apparatus 
are  shown  in  Figs.  14  and  15.  p  and  p'  are  two  pumps, 
p'  being  an  exhausting,  and  p  a  compressing  pump,  such  as 
are  used  in  the  ice-making  machines.  These  are  employed 
respectively  for  the  volatilization  and  condensation  of  liquid 
sulphur  dioxide,  contained  in  the  outer  inclined  double- 
jacketed  tube  (R)  Fig.  15 ;  and  both  pumps  are  so  arranged 
that  there  is  the  largest  possible  amount  of  difference  of  pres- 
sure always  kept  up  between  the  two  cylinders.  In  the  tube 
(R)  the  pressure  is  so  regulated  that  the  liquid  sulphur  dioxide 
evaporates  at  a  temperature  of  —  65°.  The  gaseous  sulphur  dioxide 
passes  through  the  pumps  and  is  condensed  to  a  liquid  by  a  stream 
of  cold  water  which  surrounds  the  reservoir  (c)  at  a  tempera- 
ture of  +  25°,  and  under  a  pressure  of  2'75  atmospheres.  The 
liquid  then  flows  back  through  the  small  tube  (z)  into  the 
tube  (R).  (o)  and  (o)  are  two  smaller  pumps  which  act  upon 
liquid  carbon  dioxide  which  is  contained  in  the  tube  (s).  These 
pumps  are  so  arranged  that  the  evaporation  of  the  liquid  takes 
place  from  the  tube  (s)  at  a  temperature  of  —  140°,  this  being 
surrounded  by  the  liquid  sulphur  dioxide,  and  flowing  under 
a  pressure  of  five  atmospheres  through  the  tube  (s)  into  the 
tube  or  cylinder  surrounding  the  tube  (A).  All  these  portions 
of  the  apparatus  are  made  of  strong  cold-drawn  copper  tubes 
able  to  resist  a  high  pressure.  (B)  is  a  strong  wrought  iron 


PICTET'S  EXPERIMENTS 


77 


78 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


PICTET'S  EXPERIMENTS 


retort  employed  for  the  evolution  of  the  gas  about  to  be  con- 
densed. The  gas  thus  generated  passes  into  the  long  thin 
copper  condensation-tube  (A),  four  metres  in  length,  which  is 
surrounded  by  a  bath  of  liquid  carbon  dioxide  at  a  tem- 
perature varying  from  —  120°  to  —  140°.  The  end  of  this 
condensation-tube  is  provided  with  a  well-fitted  stopcock  (v) 
and  a  Bourdon's  manometer  at  (m),  capable  of  indicating  a 
pressure  up  to  800  atmospheres.  With  this  apparatus  oxygen 
was  first  condensed  on  the  22nd  December,  1877. 

The  following  description  of  the  experiment  will  render 
intelligible  the  working  of  the  process  : 

(1.)  9  A.M.  —  The  pumps  for  condensing  and  rarefying  the 
sulphur  dioxide  were  set  to  work. 

(2.)  9.30.  —  The  temperature  of  the  upper  tube  was  —  55°. 
The  pumps  for  condensing  and  rarefying  the  carbon  dioxide 
were  started. 

(3.)  10.40.  —  Temperature  inside  the  tubes  —60°;  pressure, 
5  atmospheres.  800  litres  of  carbon  dioxide  have  been 
liquefied. 

(4.)  11.0  —  The  shell  containing  the  chlorate  of  potash  is 
now  heated. 

(5.)  11.15.  —  The  temperature  of  the  carbon  dioxide  sinks 
to  —130°.  The  pressure  of  oxygen  in  the  copper  tube  =  5 
atmospheres.  The  pressure  then  began  gradually  to  rise, 
and  at  last  it  remained  constant,  as  is  seen  in  the  following 
table  : 

(6.)  12.10  P.M.  —  Pressure  of  oxygen  50  ats.  ;  temp,  as  before. 

(7.)  12.36  .....  100  „ 

(8.)  12.37  .....  200  „ 

•  (9.)  12.38  .....  460  „ 

(10.)  12.40  .....  525  „ 

(11.)  12.42  .....  526  „ 

(12.)  12.44  .....  525  „ 

(13.)  1.0  .....  471  „ 

(14.)  1.5  .....  475  „ 

The  pressure  now  remained  constant.  The  whole  of  the 
interior  of  the  glass  tube  was  filled  with  liquid  oxygen.  On 
opening  the  stopcock  at  the  end  of  the  oxygen  tube,  a  lustrous 
jet  of  liquid  oxygen  issued  with  great  violence,  whilst  around 
it  was  a  haze  of  particles  of  what  was  taken  to  be  solid  oxygen. 


80 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


CAILLETET'S  EXPERIMENTS 


81 


The  general  arrangement  and  appearance  of  Pictet's  apparatus 
is  shown  in  Fig.  16  in  which  the  end  of  the  oxygen  condensa- 
tion-tube (A),  the  stopcock  (v),  and  the  manometer  (m)  are 


seen. 


CAILLETET'S  PROCESS  FOR  LIQUEFYING  GASES. 

28  The  apparatus  employed  by  M.  Cailletet1  for  the  lique- 
faction of  oxygen  is  shown  in  Fig.  17.  The  first  part  of 
this  apparatus  consists  of  a  powerful  hydraulic  press,  the 
soft  steel  cylinder  (A)  of  which  is  fixed  by  the  bands  (BB) 
on  to  a  horizontal  cast-iron  bed.  A  steel  piston  works  into 


FIG.  17. 

this  cylinder  through  a  stuffing-box,  and  to  the  end  of  the 
piston  is  attached  the  screw  (F),  which  can  be  carried  backwards 
or  forwards  by  turning  the  wheel  (M)  either  in  one  direction  or 
the  other.  The  hydraulic  cylinder  is  filled  with  water  from  the 
vessel  (G),  and  this  communicates  with  the  interior  of  the 
cylinder  by  a  fine  opening,  which  can  be  perfectly  closed  at 
pleasure  by  means  of  a  conical  valve  attached  to  a  piston  worked 
by  the  wheel  (o).  On  withdrawing  this  piston  water  flows  into 
the  cylinder. 

The  second  part  of  the  apparatus  is  the  receiver  (Fig.  19). 

1  Compt.  Rend.,  10,  85,  815  ;  and  Ann.  Chim.  Phys.  [5],  15,  132. 

7 


82 


GENEEAL  PRINCIPLES  OF  THE  SCIENCE 


This  consists  of  a  glass  tube  with  reservoir  at  the  lower  end 
firmly  bedded  into  a  steel  head  (B,  Fig.  19),  sufficiently  strong 
to  resist  a  pressure  of  1,000  atmospheres.  This  receiver  is 
placed  in  direct  connection  with  the  hydraulic  pump  by  means 


FIG.  is. 


of  a  flexible  metallic  tube  (TU)  of  small  diameter.  A  steel 
head  is  firmly  screwed  on  to  the  upper  part  of  the  receiver  by 
the  screw  (E1)  and  this  head  carries  the  glass  tube  (T),  which 


FIG.  19. 


contains  the  gas  to  be  experimented  upon.  The  shape  and 
mode  of  fixing  this  tube  with  its  reservoir  of  gas  is  seen  in 
Fig.  18;  whilst  in  Fig.  19  the  same  is  shown  placed  in  position 
with  the  lower  portion  dipping  into  the  mercury  which  fills  the 


CAILLETET'S  EXPERIMENTS 


lower  part  of  the  steel  receiver.  As  the  glass  reservoir  is  exposed 
to  the  same  pressure  on  both  its  inside  and  outside  surfaces, 
its  dimensions  may  be  made  large  in  spite  of  the  extremely 
high  pressures  to  which  it  is  subjected  in  the  course  of  the 
experiments.  The  thin  tube,  on  the  other  hand,  which  passes 
out  above  the  steel  head  of  the  condenser  has  of  course  to 
support  the  pressure  necessary  for  the  condensation  of  the  gases, 
and  hence  it  must  be  made  of  strong  glass  with  a  capillary 
bore.  A  glass  cylinder  (M)  resting  on  tfee  iron  flange  (s)  serves 
to  enable  the  experimenter  to  surround  the  tube  either  with 
a  freezing-mixture  or  with  a  warm  liquid.  When  the  reservoir 


FIG.  20. 


has  been  filled  with  the  pure  dry  gas  under  examination  the 
end  of  the  tube  is  carefully  hermetically  sealed,  and  the  whole 
screwed  into  position.  Water  is  then  forced  into  the  receiver 
from  the  hydraulic  cylinder  ;  this  forces  the  mercury  into  the 
reservoir,  and  the  compressed  gas  condenses  in  the  capillary 
tube,  where  the  changes  which  occur  can  be  readily  observed. 
The  position  of  the  receiver  and  capillary  tube  is  shown  at 
a  and  ra,  Fig.  17.  The  pressure  is  measured  by  two  manometers 
(N  and  N1,  Fig.  17). 

With  this  apparatus  Cailletet  liquefied  ethylene  at  +  4°  under 
a  pressure  of  46  atmospheres;    acetylene  under  the  ordinary 


84  GENEEAL  PRINCIPLES  OF  THE  SCIENCE 

temperature  at  a  pressure  of  86  atmospheres  ;  nitric  oxide  and 
marsh  gas  required  to  be  cooled  to  —  11°,  and  these  became 
liquid  at  the  respective  pressures  of  104  and  108  atmospheres. 
Oxygen  and  carbon  monoxide  remained  gaseous  at  a  temperature 
of  —  29°  under  a  pressure  of  300  atmospheres,  as  did  nitrogen 
at  a  temperature  of  +13°  and  under  a  pressure  of  200 
atmospheres.  When,  however,  this  pressure  was  suddenly 
reduced  a  thick  mist  was  formed  in  the  tubes,  and  this  con- 
densed, forming  small  drops.  Hydrogen,  on  the  other  hand, 
appeared,  when  the  pressure  from  300  atmospheres  was  suddenly 
removed,  in  the  form  of  a  slight  mist,  but  dried  air  liquefied 
under  a  pressure  of  200  atmospheres  after  it  had  been  well 
cooled  with  liquid  nitrous  oxide.  Air  has  also  recently  been 
solidified  (Dewar). 

29  An  apparatus  for  exhibiting  the  liquefaction  of  the 
difficultly  condensible  gases  has  been  constructed  by  Messrs. 
Ducretet  et  Cie.  of  Paris.1  The  condensing  arrangements  of  this 
apparatus  are  seen  in  Fig.  20,  the  apparatus  of  the  receiver  and 
glass  tube  and  reservoir  being  identical  with  that  just  described 
(Fig.  20).  The  piston  of  the  hydraulic  cylinder  is  worked  by 
the  lever  (L),  and  by  this  means  a  pressure  of  200  atmospheres 
can  be  reached.  In  order  to  increase  this  pressure  up  to  300 
atmospheres  a  steel  plunger  can  be  slowly  forced  into  the 
cylinder  by  means  of  the  first  wheel.  The  second  wheel  works 
a  second  plunger,  by  the  withdrawal  of  which  the  pressure  can 
be  suddenly  diminished,  and  thus  the  temperature  so  reduced 
that  the  gas  is  liquefied  by  the  intense  cold  produced. 

The  method  now  adopted  for  the  liquefaction  of  oxygen,  air, 
nitrogen,  &c.,  is  a  modification  of  that  of  Pictet,  the  gas  being 
strongly  compressed  by  a  powerful  pump  and  simultaneously 
cooled  by  liquid  ethylene  boiling  under  reduced  pressure,  by 
means  of  which  temperatures  of  -100°  to  -150°  can  be 
maintained.2 

1  Rue  des  Feuillantines,  89. 

2  Wroblewski,  Compt.  Rend.  98,  304  ;  Olszewski,  Compt.  Rend.  98,  365,  101, 
338  ;  Dewar,  Proc.  Roy.  Institution,  13,  part  3,  p.  695. 


BOILING-POINTS  OF  LIQUIDS 


85 


BOILING-POINTS    OF    LIQUIDS. 

30  It  is  evident  from  what  has  been  said  that  the  distinction 
between  gas  and  vapour  is  only  one  of  degree,  for  a  vapour  is 
simply  a  gas  below  its  critical  temperature.  The  same  laws 
according  to  which  the  volumes  of  gases  vary  under  change  of 
temperature  and  pressure  apply  also  to  vapours,  at  any  rate 
when  they  are  examined  at  temperatures  considerably  above  their 
points  of  condensation.  When  a  gas  or  vapour  is  near  this  point 
its  density  increases  more  quickly  than  the  pressure,  and  as  soon 
as  the  point  is  reached  the  least  increase  of  the  pressure  brings 
about  a  condensation  of  the  whole  to  a  liquid.  The  tempera- 
ture at  which  the  liquid  again  assumes  the  gaseous  form  is 
termed  the  boiling-point  of  the  liquid,  and  at  this  tempera- 
ture the  tension  of  the  vapour  is  equal  to  the  superincumbent 
pressure.  The  following  table  gives  the  boiling-points  of 
some  well-known  bodies  under  a  pressure  of  760  mm.  of 
mercury  : 

Table  of  Boiling -Points. 

Propionic  acid 


Oxygen  .  .  .  .  - 
Nitrous  oxide 
Carbonic  acid  .  - 
Cyanogen  .  .  .  - 
Sulphurous  acid  - 
Ethyl  chloride  .  - 
Prussic  acid  .  . 

Ether 

Carbon  disulphide 
Chloroform  .  . 
Bromine  .... 
Alcohol  .... 
Benzene  .... 
Water  .... 
Acetic  acid 


105° 
78° 
21° 
10° 
12°-5 
26°'5 
34c-5 
43°'3 
61° 
63° 
78°-4 
80°-3 

100° 

118° 


Butyric  acid  .  . 
Aniline  .... 
Phenol  .... 
Napthalene  .  . 
Phosphorus  .  . 
Sulphuric  acid  . 
Mercury  .... 
Sulphur  .... 
Stannous  chloride 
Zinc  bromide  . 
Zinc  chloride  .  . 
Cadmium  .  .  . 
Zinc 


+  141° 
162° 

182° 
183° 
218° 
290° 
332° 
350° 
440° 
606° 
650° 
730° 
860° 
1049° 


31  Liquids  possess  a  notable  tension  below  their  boiling- 
points  ;  thus  water  gives  off  vapour  at  all  temperatures,  and  even 
slowly  evaporates  when  in  the  solid  state,  for  the  tension  of  the 
vapour  coming  from  ice  at  —  10°  is  O208  mm.  According  to  the 


86  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

experiments  of  Faraday,  there  is,  however,  a  limit  to  evapora- 
tion ;  thus  he  found  that  mercury,  which  gives  out  a  perceptible 
amount  of  vapour  during  the  summer,  emits  none  in  the  winter  ; 
and  that  certain  compounds  which  can  be  volatilized  at  150° 
undergo  no  evaporation  when  kept  for  years  at  the  ordinary 
temperature.  The  tension  of  the  vapour  of  a  liquid  is  constant 
for  a  given  temperature,  and  this  amount,  which  is  always 
reached  when  an  excess  of  the  liquid  is  present,  is  termed  the 
maximum  tension  of  the  vapour.  Dalton1  in  1801  discovered 
that  this  maximum  tension  or  density  of  a  vapour  is  not  altered 
by  the  presence  of  other  gases,  or,  in  other  words,  that  the 
quantity  of  a  liquid  which  will  evaporate  into  a  given  space  is 
the  same  whether  the  space  is  a  vacuum  or  is  filled  with 
another  gas.  The  same  philosopher  also  believed  that  the 
vapours  of  all  liquids  possessed  an  equal  tension  at  tempera- 
tures equally  distant  from  their  boiling-points.  Regnault 2 
has,  however,  shown  by  exact  experiments  that  the  above 
conclusions  can  only  be  considered  as  approximately  true,  inas- 
much as  he  found  that  about  2  per  cent,  more  vapour  ascends 
into  a  space  filled  with  gas  than  into  a  vacuum,  whilst  at 
considerable  but  equal  distances  from  the  boiling-point  the 
tensions  of  volatile  liquids  are  by  no  means  equal. 


LAWS    OF    CHEMICAL    COMBINATION. 

32  The  composition  of  a  chemical  compound  can  be  ascertained 
in  two  ways  :  (1)  By  separating  it  into  its  component  elements, 
an  operation  termed  analysis  (ava\vco,  I  unloose),  and  (2)  by 
bringing  the  component  elements  under  conditions  favourable  to 
combination,  an  operation  termed  synthesis  (ffwrldrjiu,  I  place 
together).  In  both  of  these  operations  the  balance  is  employed  ; 
the  weight  of  the  compound  and  of  the  components  in  each 
instance  must  be  ascertained,  except  indeed  in  the  case  of 
certain  gases  of  known  specific  gravity,  when  a  measurement  of 
the  volume  occupied  by  the  gas  may  be  substituted  for  a  deter- 
mination of  its  weight. 

It  is  one  of  the  aims  of  analytical  chemistry  to  ascertain  with 
great  precision  the  percentage  composition  of  all  chemical  sub- 

1  Manch.  Memoirs,  1st  series,  5,  535. 

2  Memoires  de  I'Acad.  des  Sciences,  21,  465. 


LAWS  OF  CHEMICAL  COMBINATION  87 

stances,  and  this  branch  of  inquiry  is  termed  quantitative 
analysis,  as  contradistinguished  from  that  which  has  only  to 
investigate  the  kind  of  material  of  which  substances  are  com- 
posed, and  which  is  hence  termed  qualitative  analysis. 


COMBINATION  BY  WEIGHT. 

33  The  first  great  law  discovered  by  the  use  of  the  balance,  is 
that  the  elements  combine  with  one  another  in  a  limited  number 
of  definite  proportions,  this  number  being  almost  invariably  found 
by  experiment  to  be  a  small  one.  When  two  elements  are 
brought  together  under  such  conditions  that  they  can  combine, 
it  is  always  found  that  one  or  more  of  a  small  number  of  com- 
pounds is  produced,  the  particular  substance  or  substances 
formed  depending  upon  the  special  circumstances  of  the  experi- 
ment. Thus  carbon  is  found  to  be  capable  of  uniting  with 
oxygen  in  two  different  proportions,  producing  two  distinct  sub- 
stances, carbonic  acid  gas  and  carbon  monoxide,  these  being  the 
only  compounds  of  carbon  with  oxygen  which  are  known.  Some 
elements,  on  the  other  hand,  only  form  one  compound  with  each 
other,  whilst  others  again  form  a  larger  number.  Each  one  of 
these  compounds  is  found  to  have  a  fixed  composition,  contain- 
ing the  elements  of  which  it  is  made  up  in  a  definite  proportion 
by  weight,  and  this  fixity  of  composition  is  used  as  a  character- 
istic of  a  chemical  compound  as  opposed  to  a  mere  mechanical 
mixture,  the  constituents  of  which  may  be  present  in  any  vari- 
able proportions.  In  whatever  way  the  conditions  under  which 
the  elements  are  made  to  combine  may  be  varied,  it  is  always 
found  that  they  unite  in  exactly  the  same  ratio,  unless,  as  some- 
times happens,  the  changed  conditions  are  favourable  to  the 
production  of  one  of  the  small  number  of  other  compounds 
which  can  be  formed  by  the  same  elements.  Thus,  for  instance, 
the  combination  of  silver  with  chlorine  has  been  brought  about 
in  no  less  than  four  different  ways,  but  in  every  case  it  was  found 
that  the  resulting  compound  contained  107*13  parts  of  "silver  for 
35 '  1 9  of  chlorin  e.1  The  combination  of  chlorine  with  phosphorus, 
on  the  other  hand,  takes  place  in  two  distinct  ratios,  so  that  when 
an  excess  of  phosphorus  is  present,  the  resulting  compound  con- 
tains 10*27  parts  of  this  element  for  35*19  parts  of  chlorine,  whilst 
if  the  latter  be  kept  in  excess,  this  weight  of  it  only  combines 
1  Stas,  Eecherches,  etc.  pp.  108,  210.  1865. 


88  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

with  6'16  parts  of  phosphorus.  These  are,  however,  the  only  two 
compounds  of  these  elements  which  are  known.  In  like  manner 
hydrogen  combines  with  oxygen  to  yield  water,  a  substance 
which  contains  88*81  parts  of  oxygen  to  11*19  of  hydrogen.  If 
these  elements  are  brought  together  in  proportions  differing 
from  those  in  which  they  are  present  in  water,  the  excess  of  one 
element  remains  in  the  free  state ;  thus,  if  98*8  L  parts  of  oxygen 
by  weight  be  brought  together  with  11-19  parts  of  hydrogen 
under  circumstances  in  which  they  can  combine,  88'81  parts  of 
the  oxygen  will  combine  with  all  the  hydrogen  to  form  100 
parts  of  water,  whilst  10  parts  of  oxygen  remain  in  the  free 
state. 

It  will  therefore  be  seen  that  the  chemical  combination  of 
two  or  more  elements  does  not  result  in  the  production  of  a 
series  of  compounds  varying  gradually  in  composition,  according 
to  the  conditions  of  the  experiment,  but  yields  one  or  more 
compounds,  each  of  which  contains  its  constituents  in  a  perfectly 
fixed  and  definite  ratio. 

34  As  has  been  said,  the  case  frequently  occurs  of  two  elements 
uniting  to  form  several  compounds,  for  each  of  which  the  law  of 
definite  proportion  holds  good.  The  special  relations  which 
exist  between  the  weights  of  the  two  elements  entering  into 
combination  were  first  discovered  by  John  Dalton,  and  indeed 
form  the  basis  of  his  atomic  theory.  Thus  the  two  elements, 
carbon  and  oxygen,  unite  to  form  two  distinct  compounds, 
carbonic  oxide  gas  and  carbonic  acid  gas,  and  100  parts  of  each 
of  these  bodies  are  found  by  analysis  to  contain  the  following 
weights  of  the  elements  : — 

Carbonic  Oxide  Gas.     Carbonic  Acid  Gas. 

Carbon      ....     42*86     .     .     .     27'27 
Oxygen     ....     5714     .     .     .     7273 


100-00  100-00 


Knowing  these  facts  Dalton  asked  himself  what  was  the 
relation  of  one  element  (say  of  the  oxygen)  in  both  compounds 
when  the  other  element  remains  constant  ?  He  thus  found  that, 
in  proportion  to  the  carbon,  the  one  compound  contained 
exactly  double  the  quantity  of  oxygen  which  the  other  contained; 
thus : — 


LAWS  OF  CHEMICAL  COMBINATION 


Carbonic  Oxide  Gas.  Carbonic  Acid  Gas. 

Carbon     .....     lO'O     .  .     .     lO'O 

Oxygen    .....     13'3     .  .     .     26'6 

23-3  36'6 


Thus  again,  analysis  showed  that  two  compounds  which  carbon 
forms  with  hydrogen,  viz.,  marsh  gas  and  olefiant  gas,  have  the 
following  percentage  composition  : — 

Marsh  Gas.  Olefiant  Gas. 

Carbon      ....     74'95     .     .     .     85'68 
Hydrogen      .     .     .     25'05     .     .     .     14'32 


100-00  100-00 


Dalton  then  calculated  how  much  hydrogen  is  combined  in 
each  compound  with  10  parts  by  weight  of  carbon,  and  he 
found  that  in  olefiant  gas  there  are  1*67  parts  by  weight  of 
hydrogen  to  10  of  carbon,  whilst  marsh  gas  contains  3'34  parts 
of  hydrogen  to  the  same  quantity  of  carbon,  or  exactly  double 
as  much. 

As  another  example  we  may  take  the  compounds  of  nitrogen 
and  oxygen,  of  which  no  less  than  five  are  known  to  exist.  The 
percentage  composition  of  these  five  bodies  is  found  by  experi- 
ment to  be  as  follows : — 


(1) 

(2) 

(3) 

(4) 

(5) 

Nitrogen  . 

.     63-71 

46-75 

36-91 

30-51 

25-99 

Oxygen    . 

.     36-29 

53-25 

63-09 

69-49 

74-01 

100-00  100-00  100-00  100-00  100-00 

If  then,  like  Dalton,  we  inquire  how  much  oxygen  is  con- 
tained in  each  of  these  five  compounds,  combined  with  a  fixed 
weight,  say  10  parts  of  nitrogen,  we  find  that  this  is  represented 
by  the  numbers  5'7,  1T4,  171,  22'8,  and  28'5.  In  other  words, 
the  relative  quantities  of  oxygen  are  in  the  ratio  of  the  simple 
numbers  1,  2,  3,  4,  and  5. 

35  The  above  examples  illustrate  the  relations  exhibited  in  the 
combination  of  two  or  more  of  the  elements  to  form  compounds, 
but  a  careful  examination  of  the  quantitative  composition  of  a 


90 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


whole  series  of  chemical  compounds  leads  to  a  further  conclusion 
respecting  the  nature  of  the  laws  of  chemical  combination 
which  is  of  the  highest  importance.  Let  us  examine  the  com- 
position of  any  given  series  of  compounds  as  determined  by 
analysis,  such  as  the  following  : 


CHLORIDES. 


Hydrogen  Chloride. 

Chlorine    ....     97'24 
Hydrogen.     .     .     .       276 


100-00 


Potassium  Chloride. 
Chlorine    ....     47'53 
Potassium  52'47 


100-00 


Sodium  Chloride. 
Chlorine    ....     60'61 
Sodium     ....     39-39 

100-00 


Silver  Chloride. 

Chlorine    ....     2473 

Silver   .  75-27 


100-00 


BROMIDES. 


Hydrogen  Bromide. 
Bromine    ....     98'76 
Hydrogen.     .     .     .       1'24 

100-00 


Sodium  Bromide. 
Bromine    ....     77'62 
Sodium  22-38 


loo-oo 


Potassium  Bromide. 

Bromine    ....     67*13 
Potassium      .     .     .     32'87 

100-00 

Silver  Bromide. 

Bromine   ....     42'55 
Silver  .  57'45 


100-00 


IODIDES. 


Hydrogen  Iodide. 

Iodine 99'21 

Hydrogen.     .     .     .       0'79 


100-00 


Potassium  Iodide. 

Iodine 76 '42 

Potassium  2358 


100-00 


LAWS  OF  CHEMICAL  COMBINATION 


91 


Sodium  Iodide. 

Iodine 84'62 

Sodium     .     .     .     .     15-38 

100-00 


Silver  Iodide. 

Iodine 54*03 

Silver  .  45-97 


100-00 


Arranged  in  this  way  we  do  not  notice  any  simple  relation 
•existing  between  the  components  of  this  series,  except  that  the 
quantity  of  hydrogen  is  always  smaller  than  that  of  the  chlorine, 
bromine,  or  iodine,  whilst  the  quantity  of  sodium  is  always 
smaller  than  that  of  potassium,  and  this  again  is  less  than  the 
quantity  of  silver. 

36  If,  however,  instead  of  examining  a  constant  weight  of 
the  several  compounds  we  ask  ourselves,  how  much  of  the 
one  constituent  in  each  compound  combines  with  a  constant 
weight  of  that  constituent  which  is  common  to  several,  we 
shall  obtain  at  once  a  clear  insight  into  the  law  of  the 
formation  of  the  compound.  In  the  series  of  hydrogen  com- 
pounds for  instance,  let  us  calculate  (by  simple  proportion)  how 
much  chlorine,  bromine,  and  iodine  combine  with  the  unit 
weight  of  hydrogen.  We  then  obtain  for  the  composition  of  these 
compounds : — 


Hydrogen  Chloride. 
Chlorine  .     .  3519 
Hydrogen      .     1-00 


Hydrogen  Bromide. 
Bromine    .    79 '36 
Hydrogen .      TOO 


Hydrogen  Iodide. 
Iodine    .      125*91 
Hydrogen    .     1*00 


3619 


80-36 


126-91 


Continuing  our  calculation,  let  us  next  ask  how  much  of  the 
metals,  potassium,  sodium,  and  silver,  unite  with  35  "19  parts  by 
weight  of  chlorine  to  form  chlorides  ;  with  79*36  parts  of  bro- 
mine to  form  bromides,  and  with  125*91  parts  of  iodine  to  form 
iodides.  The  result  is  as  follows  : — 


Potassium  Chloride. 
Chlorine      .     3519 
Potassium   .     38*85 


CHLORIDES. 

Sodium  Chloride. 

Chlorine    .     35*19 
Sodium          22-87 


Silver  Chloride. 
Chlorine    .     3519 
Silver  .     .  10713 


74-04 


58-06 


143-03 


92 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


Potassium  Bromide. 
Bromine      .     79*36 
Potassium   .     38 -85 


118-21 


BROMIDES. 

Sodium  Bromide. 
Bromine  .     79*36 
Sodium    .     22-87 

102-23 


Silver  Bromide. 
Bromine    .     79*36 
Silver    .     .  107'13 


186-49 


Potassium  Iodide. 
Iodine  .     .  125'91 
Potassium.     38*85 


16476 


IODIDES. 

Sodium  Iodide. 
Iodine   .     .  125'91 
Sodium .          22-87 


148-78 


Silver  Iodide. 

Iodine   .     .  125*91 
Silver         .  107*13 


233-04 


Now  for  the  first  time  a  remarkable  relation  becomes  apparent, 
for  it  is  clear  that  the  SAME  weights  of  the  metals,  potassium, 
sodium,  and  silver,  which  combine  with  35*19  parts  of  chlorine 
to  form  chlorides,  also  combine  with  79'36  parts  of  bromine  to 
form  the  bromides,  and  with  125*91  parts  of  iodine  to  form  the 
iodides.  In  other  words,  if  we  replace  the  35*19  parts  by 
weight  of  chlorine  in  each  of  these  compounds  by  79*36  parts 
of  bromine,  we  get  the  bromides,  and  if  by  125  '91  parts  of  iodine 
we  obtain  the  iodides  of  the  metals.  Hence  one  and  the  same 
weight  of  metals  (38*85  of  potassium,  22*87  of  sodium,  and  107*13 
of  silver)  has  the  power  of  forming  compounds  with  the  precise 
quantities  of  chlorine,  bromine,  and  iodine  respectively,  which 
unite  with  1  part  by  weight  of  hydrogen,  to  form  the  hydrides 
of  these  elements.  These  quantities  of  the  elements  in  question 
are  called  equivalent  quantities,  because  they  are  the  amounts  of 
them  which  will  combine  with  the  same  weight  of  some  other 
element. 

(    35*19  of  chlorine  ) 

38*85  of  potassium  combine  with^    79'36  „  bromine 

(  125*91  ,,  iodine 

22*87  „  sodium  „  „  „ 

107*13  „  silver 
1*00  „  hydrogen 


tlve1?' 


Similar   results  are   obtained   from  the  examination  of  the 
compounds  of  all  the  other  elements,  so  that  a  number  may  be 


LAWS  OF  CHEMICAL  COMBINATION 


93 


-assigned  to  each  element  which  is  termed  the  combining  weight 
or  equivalent  weight  of  the  element. 

37  Taking  an  example  from  another  group  of  chemical  com- 
pounds we  find  that  the  well-known  oxides  of  hydrogen,  lead, 
copper,  mercury,  and  cadmium  possess  the  following  percentage 
composition : — 


Water. 

Hydrogen 
Oxygen     . 


1119 

88-81 

100-00 


OXIDES. 

Lead  Oxide. 
Lead    .     .     92*82 
Oxygen    .       718 


Copper  Oxide. 

Copper    .     79-82 
Oxygen    .     2018 


100-00 


Mercury  Oxide. 
Mercury  .  .  .  . 
Oxygen  .  .  .  . 


Cadmium  Oxide. 


92-60 

7-40 

100-00 


Cadmium 
Oxygen     . 


100-00 


87-49 
12-51 


100-00 


Whilst   the   corresponding   sulphides  exhibit   the   following 
•composition  : 

SULPHIDES. 

Sulphuretted  Hydrogen.  Lead  Sulphide.  Copper  Sulphide. 

Hydrogen     .     6'01      Lead.     .     .     86'58     Copper  .     .     6676 
-Sulphur  .     .  93-99      Sulphur      .     1342     Sulphur      .     33*24 


100-00 


Mercury  Sulphide. 
Mercury    ....     86'20 
Sulphur    ....     13-80 

100-00 


100-00 


100-00 


Cadmium  Sulphide. 
Cadmium      .     .     .     77'73 
Sulphur    ....     22-27 


100-00 


38  If,  as  before,  we  now  compare  the  quantity  of  each  element 
united  with  one  and  the  same  weight  of  oxygen,  taking  7"94 
parts  of  this  element,  because  this  is  the  amount  of  it  which 
combines  with  one  part  of  hydrogen  and  is,  therefore,  the 
•equivalent  weight  of  oxygen,  we  get  the  following  numbers  :— 


94 


GENERAL  PRINCIPLES  OF  THE  SCIENCE 


Water.  Lead  Oxide. 

Hydrogen     .     TOO  Lead    .     .     102*68 

Oxygen    .     .     7'94  Oxygen     .         7*94 

8-94  110-62 


Copper  Oxide. 

Copper  .     .     31-41 
Oxygen .     .       7*94 


39-35 


Mercury  Oxide. 
Mercury   . 
Oxygen     . 


99-45 

7-94 

107-39 


Cadmium  Oxide. 

Cadmium      .     .     .     55'63 
Oxygen     ....       7 '94 

63-57 


And,  if  we  investigate  the  sulphides,  we  find  that  one  and  the 
same  weight  of  sulphur,  viz.  15'64  parts  by  weight  unites  with 
weights  of  these  elements  to  form  sulphides,  which  are  identi- 
cal with  the  amounts  that  combined  with  7 '94  parts  by  weight 
of  oxygen  to  form  oxides.  Thus  we  have  :— 


Sulphuretted  Hydrogen. 

Hydrogen     .     TOO 
Sulphur  .     .15-64 


16-64 


Lead  Sulphide. 

Lead    .     .     102*68 
Sulphur    .       15-64 

118-32 


Copper  Sulphide. 
Copper  .     .     31-41 
Sulphur      .     15-64 

47-05 


Mercury  Sulphide. 
Mercury    ....     99'45 
Sulphur    ....     15-64 

115-09 


Cadmium  Sulphide. 
Cadmium      .     .     .     55*63 
Sulphur    .     .     .     .     15-64 


71*27 


Hence  we  see  again  that  the  amounts  of  these  elements  which 
unite  with  an  equivalent  of  oxygen  also  combine  with  an 
equivalent  of  sulphur,  so  that 

1  part  by  weight  of  hydrogen  combines  with  -J 

I  15'64  of  sulphur 

102*68  parts  by  weight  of  lead  combine  with  15*64  of  sulphur 
31*41      „      „        „       „  copper 
99*45      „      „        „        „  mercury 
55*63      „      „  „  cadmium  „  „ 


and   these   are,  therefore,   the    equivalent    weights   of    these 
elements. 


DALTON'S  ATOMIC  THEORY 


95 


39  When  one  element  combines  with  another  in  more  than  one 
proportion  it  is  said  to  have  more  than  one  equivalent,  and 
since  the  amounts  of  one  element  which  combine  with  a  fixed 
weight  of  a  second  are  in  a  simple  ratio  to  one  another,  it 
follows  that  the  several  equivalents  of  an  element  must  also 
stand  in  a  simple  ratio  to  one  another.  Iron  for  example 
forms  several  different  compounds  with  oxygen,  two  of  which 
have  the  following  composition  as  determined  by  analysis  : — 


Iron 

Ferrous  Oxide. 

.....    77-78 

Ferric  Oxide. 

70-00 

Oxygen 

22-22 

30-00 

100-00 


100-00 


Calculating  the  amount  of  iron  combined  with  the  equivalent 
(7*94  parts)  of  oxygen  we  find 

(l)  (2) 

Iron 2778  18'52 

Oxygen 7'94  7*94 


35-72 


26-46 


These  amounts  of  iron  are,  however,  in  the  simple  ratio  of 
2 :  3,  27*78  being  the  equivalent  of  iron  in  ferrous  oxide  and 
18*52  in  ferric  oxide. 

40  It  will  be  seen,  therefore,  that  combination  always  takes 
place  between   certain   definite  and   constant  proportions   of  the 
elements  or  between  multiples  of  these. 

41  At  a  time  when  the  question  of  combination  in  a  limited 
number  of  definite  proportions  was  still  under  discussion,  John 
Dalton's  speculative  mind  conceived  an  hypothesis  which  clearly 
explained  the  law  of  combination  in  constant  proportions,  and 
solved  the  question  as  to  the  nature  of  the  compounds  formed 
by  the  union  of  two  or  more  elements  in  several  different  pro- 
portions.    The  hypothesis  known   as  Dalton's  Atomic  Theory 
may   be   said   to   have    become    one   of    the   most   important 
foundation   stones   of   the    science,    and   to   have   exerted    an 
influence    on    its   progress   greater   than    that    of    any   other 
generalization,  with,  perhaps  the  single  exception  of  Lavoisier's 
explanation  of  the  phenomena  of  combustion,  and  the  discovery 
of  the  indestructibility  of  matter. 


96  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

The  Atomic  Theory,  then,  follows  the  doctrines  of  the  Greek 
philosophers  so  far  as  it  supposes  that  matter  is  not  continuous, 
but  made  up  of  extremely  small  individual  particles  termed 
atoms  (a  privative  and  re^va)  I  cut) ;  but  differs  from  that  of 
the  ancients  and  becomes  truly  a  chemical  atomic  theory  inas- 
much as  it  supposes  the  atoms  of  different  elements  not  to 
possess  the  same  weights,  but  to  be  characterised  by  different 
weights.  Thus  the  atom  of  oxygen  is  15*88  times  as  heavy  as 
the  atom  of  hydrogen,  and  the  weights  of  the  atoms  of  oxygen 
.and  chlorine  are  as  15 '88  to  35 '19.  Dalton  assumed,  in  the 
second  place,  that  chemical  combination  consists  in  the 
approximation  of  the  individual  atoms  to  each  other.  Having 
made  these  assumptions  he  was  able  to  explain  why  combination 
always  takes  place  between  certain  amounts  of  the  elements  or 
between  multiples  of  these  amounts  ;  since  combination  being 
•supposed  to  take  place  between  some  number  of  atoms  of  each 
of  the  elements  which  unite,  it  follows  that  the  amounts  which 
combine  must  be  some  finite  multiple  of  the  weights  of  the 
atoms. 

It  is  thus  clear  that  the  atomic  theory  explains  the  formation 
of  all  compounds  which  are  found  to  exist,  but  it  is  equally 
evident  that  it  in  no  way  decides  how  many  compounds  can  be 
formed  by  any  two  or  more  elements.  This  at  present  can  only 
be  learned  by  experiment,  but  we  are  not  without  indications 
that  a  time  approaches  when  this  further  problem  will  receive  a 
theoretical  solution. 

42  Although  the  atomic  theory  satisfactorily  explains  all  the 
known  laws  of  chemical  combination,  the  actual  existence  of 
atoms  is  far  from  being  thereby  positively  proved;  indeed, 
from  purely  chemical  considerations  it  appears  unlikely  that  the 
question  will  ever  be  solved.1  Nevertheless,  there  is  evidence, 
gradually  becoming  more  cogent,  connected  with  certain  physical 
phenomena,  which  compels  us  to  admit  a  limit  to  the  divisi- 
bility of  matter.  The  phenomena  in  question  belong  to  the  science 
of  molecular  physics,  and  have  reference  to  such  subjects  as  the 
capillary  attraction  of  liquids,  the  diffusion  of  gases,  and  the  pro- 
duction of  electricity  by  the  contact  of  metals.  Reasoning  from 
facts  observed  in  the  study  of  these  subjects,  physicists  have  not 
only  come  to  the  conclusion  that  matter  is  discontinuous,  and, 
therefore,  that  indivisible  particles  or  molecules  (molecula,  a  small 

1  Williamson,  "On  the  Atomic  Theory,"  Chem.  Soc.  Journ.,  22,  (1869) 328 and 
433. 


COMBINATION  BY  VOLUME  97 

mass)  exist,  but  they  have  even  gone  so  far  as  to  indicate  the  order 
of  magnitude  which  these  molecules  attain.  Thus  Sir  William 
Thomson  states  that  in  any  ordinary  liquid  or  transparent  or 
seemingly  opaque  solid,  the  mean  distance  between  the  centres 
of  contiguous  molecules  is  less  than  one  hundred-millionth,  and 
greater  than  the  two  thousand-millionth  of  a  centimetre.  Or,  in 
order  to  form  a  conception  of  this  coarse-grainedness,  we  may 
imagine  a  rain-drop  or  a  globe  of  glass  as  large  as  a  pea  to  be 
magnified  up  to  the  size  of  the  earth,  each  constituent  molecule 
being  magnified  in  the  same  proportion  ;  the  magnified  structure 
would  be  coarser-grained  than  a  heap  of  small  shot,  but  probably 
less  coarse-grained  than  a  heap  of  cricket-balls.1 

The  molecular  constitution  of  matter  is  likewise  an  essential 
condition  of  the  mechanical  theory  of  gases,  by  means  of  which 
nearly  every  known  mechanical  property  of  the  gases  can  be 
explained  on  dynamical  principles,  so  that  in  this  direction 
again,  we  have  a  confirmation  of  the  real  existence  of  molecules 
(p.  60). 

43  It  will  be  seen  that  the  atomic  theory  as  proposed  by  Dai- 
ton  does  not  provide  any  means  for  ascertaining  what  the  relative 
weights  of  the  atoms  really  are.  We  have  seen,  for  instance, 
that  7'94  parts  by  weight  of  oxygen  combine  with  one  part  of 
hydrogen  to  form  water,  but  we  cannot  draw  any  conclusion  from 
this  fact  as  to  the  atomic  weight  of  oxygen,  (that  of  hydrogen 
being  taken  as  equal  to  one),  until  we  know  how  many  atoms  of 
each  of  these  elements  have  taken  part  in  the  combination.  If 
the  combination  is  between  an  equal  number  of  atoms  of  each 
element,  then  the  relative  weights  of  their  atoms  must  be  as 
1  :  7*94,  whilst  if  two  atoms  of  hydrogen  have  combined  with  one 
of  oxygen  the  relation  will  be  as  1  :  15'88.  The  answer  to  this 
question — as  to  the  number  of  atoms  of  each  element  between 
which  combination  has  taken  place — has  been  supplied  by  a 
study  of  the  laws  of  combination  of  gaseous  substances. 


COMBINATION  BY  VOLUME. 

44  The  discovery  by  Gay-Lussac  and  Humboldt  in  1805  of 
the  simple  relation  existing  between  the  combining  volumes  of 
oxygen  and  hydrogen  gases,  followed  by  that  of  the  general  law 
of  gaseous  volumes  enunciated  in  1808  by  Gay-Lussac  alone, 

1  Nature,  March  31,  1870. 
8 


98  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

serves  as  a  powerful  argument  in  favour  of  Dalton's  Atomic 
Theory.  This  law  states  that  the  volumes  in  which  gaseous  sub- 
stances combine  bear  a  simple  relation  to  one  another  and  to  the 
volume  of  the  resulting  product.  This  is  true  both  for  elementary 
and  compound  gases,  the  simple  relations  which  exist  being 
illustrated  by  the  following  table  : 

1  vol.  of  chlorine  and  1  vol.  of  hydrogen  form  2  vols.  of  hydrochloric  acid  gas. 

1  vol.  of  oxygen  and  2  vols.  of  hydrogen  form  2  vols.  of  steam. 

1  vol.  of  nitrogen  and  3  vols.  of  hydrogen  form  2  vols.  of  ammonia. 

1  vol.  of  oxygen  and  2  vols.  of  carbonic  oxide  form  2  vols.  of  carbon  dioxide. 

According  to  the  atomic  theory,  however,  combination  takes 
place  between  the  atoms  of  which  substances  are  made  up,  and 
it  hence  follows,  if  we  accept  this  theory,  that  the  number  of 
atoms  which  is  contained  in  a  given  volume  of  any  gaseous  body, 
must  stand  in  a  simple  relation  to  that  contained  in  the  same 
volume  of  any  other  gas  (measured  under  equal  circumstances 
of  temperature  and  pressure).  The  simplest  as  well  as  the 
most  probable  supposition  respecting  this  question  is  that  put 
forward  by  Avogadro  in  181 1,1  who  assumed  that  equal  volumes 
of  all  the  different  gases,  both  elementary  and  compound,  contain 
the  same  number  of  particles  or  integrant  molecules,  and  this 
theory  is  now  generally  accepted  by  physicists,  who  have  arrived 
at  the  same  conclusion  as  the  chemists  have  reached  by  an  inde- 
pendent train  of  reasoning  (p.  61).  If  we  take  the  simplest  case 
of  volume  combination,  that  of  one  volume  (one  molecule)  of 
chlorine  and  one  volume  (one  molecule)  of  hydrogen  uniting  to 
form  two  volumes  (two  molecules)  of  hydrochloric  acid  gas,  it  is 
clear  that,  since  each  molecule  of  hydrochloric  acid  contains  at 
least  one  atom  of  chlorine  and  one  of  hydrogen,  there  are  at  least 
twice  as  many  atoms  as  molecules  of  these  elements  present. 
Hence,  to  conform  to  Avogadro' s  theory,  the  integrant  molecule 
of  free  chlorine  and  of  free  hydrogen  must  consist  of  at  least  two 
atoms  combined  together,  and  we  shall  represent  the  combination 
as  taking  place  between  one  volume  (one  molecule  of  two  atoms) 
of  chlorine,  and  one  volume  (one  molecule  of  two  atoms)  of  hydro- 
gen, forming  two  volumes  (two  molecules)  of  the  compound 
hydrochloric  acid  gas.  Again,  two  volumes  of  steam  are  formed 
from  two  volumes  of  hydrogen  and  one  volume  of  oxygen,  hence 
if  there  are  the  same  number  of  molecules  of  steam,  of  hydrogen 

1  Journ.  de  Phys.,  par  De  la  Metherie,  73,  Juillet,  1811,  58-76.  Ibid.,  Feb, 
18U. 


AVOGADRO'S  THEORY  99 

and  of  oxygen  in  the  same  volume  of  each  gas,  it  is  clear  that  in 
the  formation  of  water  from  its  elements,  each  molecule  of 
oxygen  must  be  split  up  into  two  similar  parts.  We  are  thus 
led  to  distinguish  between  the  atom  and  the  molecule,  the  latter 
term  being  applied  to  the  smallest  particle  of  an  element  or  com- 
pound ivhich  can  exist  in  the  free  state.  Avogadro's  theory  refers 
exclusively  to  these  molecules  and  states  that  equal  volumes  of 
all  gases,  measured  under  the  same  physical  conditions,  contain 
equal  numbers  of  molecules. 

An  immediate  consequence  of  this  theory  is  of  the  utmost 
importance.  If  we  weigh  equal  volumes  of  two  gases,  we  are 
obviously  weighing  equal  numbers  of  their  molecules,  and  the 
ratio  of  the  weights  of  the  gases  will  also  be  the  ratio  of  the 
weights  of  their  molecules,  so  that  we  are  thus  enabled  to  deter- 
mine the  relative  weights  of  the  molecules  of  all  gases,  by  simply 
finding  their  relative  densities.  Hydrogen  gas  is  taken  as  the 
standard  of  comparison  because  it  has  a  lower  density  than  any 
other  gas,  and,  since  it  has  been  shown  that  its  molecule  can  be 
divided  into  at  least  two  parts  (p.  98),  the  weight  of  its  molecule 
is  taken  as  equal  to  two.  The  molecular  weight  of  any  gas  is 
therefore  equal  to  twice  its  density  compared  with  hydrogen; 
nitrogen,  for  example,  is  about  14  times  as  heavy  as  hydrogen 
and  hence  its  molecular  weight  is  about  28,  whilst  carbon  dioxide 
has  a  density  of  22  and  therefore  a  molecular  weight  of  44. 

45  When  the  molecular  weight  of  a  gas  and  also  its  composi- 
tion, as  determined  by  analysis,  are  both  known,  it  is  possible  to 
calculate  what  proportion  of  each  of  the  component  elements  is 
present  in  the  molecule.  Water  for  instance  contains  88'81  per 
cent,  of  oxygen  and  1119  of  hydrogen,  whilst  the  density  of 
steam  is  about  8'94,  its  molecular  weight  being,  therefore,  equal 
to  17*88.  If  now  we  calculate  how  much  oxygen  and  hydrogen 
are  present  in  17'88  parts  of  water,  we  find  that  this  amount  is 
made  up  of  15 '88  of  oxygen  and  2  of  hydrogen.  Carbon  dioxide 
again  has  a  molecular  weight  of  43' 7,  and  contains 

Carbon  ....  27'27 
Oxygen      .    .    .  7273 

100-00 

Hence  437  parts  of  this  gas  contain  11-91   of  carbon  and  31'76 
of  oxygen. 


100  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

A  repetition  of  this  process  for  all  the  known  com  pounds  of  some 
particular  element  enables  us  to  ascertain  the  least  amount  of  that 
element  which  is  ever  found  in  a  molecule  of  one  of  its  compounds, 
and  to  this  amount  the  name  of  atom  is  given.  A  comparison 
of  all  the  compounds  of  oxygen,  for  example,  teaches  us  that  the 
least  amount  of  it  ever  found  in  a  molecule  is  15*88  parts, 
and  this  is,  therefore,  taken  as  the  atomic  weight  of  oxygen. 
This  having  been  ascertained,  we  are  in  a  position  to  say 
that  the  molecule  of  water  contains  one  atom  of  oxygen, 
whilst  that  of  carbon  dioxide  contains  two.  All  the  non-metallic 
elements  form  compounds  which  can  exist  in  the  state  of  gas, 
and  hence  the  atomic  weights  of  all  these  elements  have  been 
found  by  this  method.  Many  of  the  metals  on  the  other 
hand  do  not  form  volatile  compounds,  and  the  atomic  weights 
of  these  have,  therefore,  to  be  determined  by  different  methods. 
A  discussion  of  these  methods  will  be  found  in  a  later  volume 
(Vol  II.  Part  I.  p.  24).  In  any  case  it  must  be  remembered  that 
the  method  described  above  is  not  generally  capable  of  great 
accuracy  and  only  yields  an  approximate  number  for  the  atomic 
weight,  the  exact  value  being  found  by  determining  the  equiva- 
lent of  the  element  by  an  accurate  analysis  of  one  of  its 
compounds,  and  then  taking  as  the  exact  atomic  weight  the 
multiple  of  this  number  which  approaches  most  closely  to  the 
approximate  number  obtained  from  the  molecular  weights. 

46  It  will    be   seen  that  the    determination  of  the  atomic 
weight   is   quite  distinct  from  that  of   the  molecular  weight. 
This  is  well  shown  in  the  case  of  carbon ;  a  comparison  of  the 
gaseous  compounds  of  carbon  shows  that  the  atomic  weight  of 
this    element  is  about  12,  this  being  the  least  amount  of   it 
which  is  found  in  the  molecule  of  one  of  its  compounds ;  we  are, 
however,  quite  ignorant  of  the  molecular  iveight  of  carbon  itself, 
since  its  density  in  the  state  of  gas  has  never  been  determined. 

The  molecules  of  some  elements  contain  two  atoms,  this  being 
the  case  with  hydrogen,  oxygen,  nitrogen,  chlorine  and  others, 
whilst  the  molecules  of  mercury  vapour  and  of  the  vapours 
of  some  of  the  other  metals  consist  of  single  atoms,  the  molecular 
and  atomic  weights  being,  therefore,  identical. 

47  For  the  first  time  we  may  now  employ  chemical  symbols, 
a   kind  of  shorthand,  by  which  we  can  conveniently  express 
the   various  chemical  changes.     To   each  element  we   give   a 
symbol,  usually  the  first  letter  of  the  Latin,  which  is  generally 
also  that  of  the  English  name.     Thus  O   stands  for  oxygen ; 


EMPIRICAL  AND  MOLECULAR  FORMULAE          101 

H  for  hydrogen  ;  S  for  sulphur ;  Au  for  gold  (aumm) ;  Ag  for 
silver  (argentum).  These  letters,  however,  signify  more  than 
that  a  particular  substance  takes  part  in  the  reaction.  They 
serve  also  to  give  the  quantity  by  weight  in  which  it  is  present. 
Thus  O  does  not  stand  for  any  quantity,  but  for  15*88  parts  by 
weight  (the  atomic  weight)  of  oxygen;  H  always  stands  for 
one  part  by  weight  of  hydrogen ;  and  in  like  manner  S,  Au, 
and  Ag  stands  invariably  for  31'28,  195*7,  and  107*13  parts  by 
weight  of  the  several  elements  respectively.  By  placing 
symbols  of  any  elements  side  by  side,  a  combination  of  the 
elements  is  signified,  thus  : — 

HC1  Hydrochloric  acid  HI  Hydriodic  acid 

HBr  Hydrobromic  acid  HgO  Mercuric  oxide. 

If  the  molecule  contains  more  than  one  atom  of  any  element, 
this  is  indicated  by  placing  a  small  number  below  the  symbol 
of  the  atom  of  the  element,  thus  H2O  signifies  17*88  parts  by 
weight  of  a  compound  (water)  containing  two  atoms  or  2 
parts  by  weight  of  hydrogen  and  one  atom  or  15'88  parts  by 
weight  of  oxygen.  In  such  a  case  as  this,  where  the  molecular 
weight  and  the  number  of  atoms  in  the  molecule  are  known 
and  expressed  in  the  formula,  the  latter  is  said  to  be  a  molecular 
formula  and  consequently  represents  such  a  weight  of  the 
substance  as  will  in  the  state  of  gas  occupy  the  same  volume  as 
two  parts  by  weight  of  hydrogen.  When  the  molecular  weight 
of  the  compound  to  be  represented  by  a  formula  is  not  known, 
it  is  only  possible  to  express  the  relative  number  of  the  atoms 
of  the  constituent  elements  which  are  present.  A  formula  of 
this  kind  is  known  as  an  empirical  formula  and  may  be 
calculated  for  any  substance  of  which  the  composition  has  been 
determined  by  analysis. 

The  gas  known  as  ethylene  has  the  following  composition  as 
determined  by  analysis : 

Carbon   ....  85'62 
Hydrogen     .  .  14*38 

100*00 


In  order  to  find  the  empirical  formula  of  this  substance  it  is 
only  necessary  to  divide  the  percentage  of  each  element  by  the 


102  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

atomic  weight  of  the  element,  which  gives  us  the  ratio  of  the 
number  of  atoms  of  each  of  the  two  elements,  and  if  we  express 
this  ratio  in  the  smallest  possible  whole  numbers  we  have 
at  once  the  relative  numbers  of  atoms  present  in  the  molecule, 
without,  however,  having  any  information  as  to  the  absolute 
number. 

Percentage  Simplest 

Percentage.     Atomic  Weight.  Ratio. 

Carbon                        85*62               7*19  1 

Hydrogen                   14*38            14*38  2 

The  simplest  or  empirical  formula  of  ethylene  is  therefore 
CH2.  The  density  of  this  gas  however  is  found  to  be  equal  to 
13*91  and  its  molecular  weight  is  therefore  27*82,  its  molecular 
formula  being  consequently  C2H4. 

It  is  usual  to  represent  chemical  changes  in  the  form  of 
equations ;  the  materials  taking  part  in  the  change  being 
placed  on  one  side  and  the  products  formed,  which  are  always 
equal  to  them  in  weight  (p.  48),  being  placed  on  the  other. 
If  we  heat  potassium  chlorate,  a  substance  which  has  the 
empirical  formula  KC1O3,  it  is  decomposed  into  oxygen  and 
potassium  chloride  and  this  decomposition  is  represented  by  the 
equation 

2KC1O3  =  2KC1  +  302 

in  which  the  sign  -f  connects  the  two  products  and  signifies 
"  together  with."  This  equation  is  an  expression  of  the  fact, 
ascertained  by  experiment,  that  121*68  (38*85+35*19-i-3  X  15*88) 
parts  of  this  salt  by  weight  leave  behind  on  heating  74*04 
parts  (35*1 9+38*85)  of  potassium  chloride  and  liberate  47*64 
parts  of  oxygen.  Hence  it  is  clear  that  the  quantity  of  oxygen 
which  is  obtained  from  any  other  weight  of  the  salt  and  vice 
versd  can  be  found  by  a  simple  calculation  when  the  equation 
representing  the  chemical  change  is  known. 

To  take  a  more  complicated  case,  when  we  know  that  the 
equation  representing  the  change  which  occurs  when  we  heat 
potassium  ferrocyanide,  the  empirical  formula  of  which  is 
K4C6N6Fe,  with  strong  sulphuric  acid  H2SO4  and  water,  is  the 
following : — 

K4C6N6Fe+6H2S04+6H20-6CO-f2K2S044-3(NH4)2S04+FeS04 

yielding  carbon  monoxide  gas,  CO,  potassium  sulphate  K2S04. 
ammonium  sulphate  (NH4)2S04,  and  iron  sulphate  FeSO4,  we 


CALCULATION  OF  CHEMICAL  QUANTITIES  103 

can  easily  calculate  how  many  grams  of  carbon  monoxide  gas, 
CO,  can  be  obtained  from  any  given  weight  of  the  ferrocyanide, 
K4C6N6Fe,  inasmuch  as  analysis  proves  that  the  amounts 
represented  by  these  formulae  are  made  up  as  follows : 

Carbon  Monoxide.  Ferrocyanide  of  Potassium. 

Carbon  C  11-91  Potassium  K4   155'40 

Oxygen  O  15'88  Carbon       C6      71'46 

Nitrogen    N6     83'64 
27-79  Iron  Fe      55'58 


366-08 

The  foregoing  equation  then  shows  that  36 6 '08  parts  by 
weight  of  the  ferrocyanide  yields  166'74  parts  by  weight  of  car- 
bon monoxide,  and  hence  a  simple  proportion  gives  the  quantity 
yielded  by  any  other  weight.  The  illustration  is,  however,  not 
yet  complete  ;  commercial  potassium  ferrocyanide  contains,  as  do 
many  crystalline  compounds,  a  certain  quantity  of  water  cf 
crystallization,  which  is  given  off  when  the  salt  is  heated,  in 
consequence  of  which  the  crystals  fall  to  a  powder.  But,  as 
the  equation  shows,  a  certain  quantity  of  water  takes  part  in 
the  reaction,  and  it  is,  therefore,  unnecessary  to  dry  the  salt 
previously  if  only  we  know  how  much  water  of  crystallization 
it  contains.  Analysis  has  shown  that  the  commercial  salt 
has  the  composition  K4C6N6Fe-|-3H2O ;  hence  if  we  add 
3xl7'88,  the  weight  of  3  molecules  of  water,  to  366*08,  we 
obtain  the  number  419'72  as  the  weight  of  the  liydratcd  salt, 
which  must  be  taken  in  order  to  obtain  166'74  parts  by  weight 
of  carbon  monoxide. 

As,  however,  the  quantity  of  a  gas  is  almost  always  estimated 
by  measuring  its  volume,  and  from  this  volume  calculating  its 
weight,  it  becomes  of  the  greatest  importance  to  know  how  to 
calculate  the  volume  of  a  gas  from  its  weight,  or  vice  versa. 
This  can  readily  be  done,  for  we  know  that  the  density  of  every 
compound  in  the  gaseous  state  is  half  its  molecular  weight ;  or 
that  every  molecule  in  the  gaseous  state  occupies  the  volume  filled 
~by  two  parts  ly  weight  of  hydrogen.  One  litre  of  hydrogen  gas 
at  0°  and  under  760mm  of  mercury  weighs  0'089872  gram.; 
hence  the  weight  of  a  litre  of  any  other  gas  measured  under  the 
same  circumstances  of  temperature  and  pressure  is  obtained  by 
multiplying  the  density  of  the  gas,  or  half  its  molecular  weight, 


104  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

by  the  above  number,  or  by  0*0899,  when  absolute  accuracy 
is  not  required.     Thus  one  litre   of  carbon  monoxide  weighs 

27*79 

-  x  0*0899=1*249  grms.,  and  166*74  grms.  of  this    substance 

166*74 

occupy  a  volume  of litres  at  0°  and  760mm. 

1*249 

It  is  now  easy  to  calculate  what  volume  this  weight  will 
occupy  at  any  other  temperature  or  pressure,  for  we  know 
that  all  gases  expand  by  Y|^  of  their  volume  at  0°C.  when 
their  temperature  is  raised  1°C.  (at  constant  pressure,  Law 
of  Dalton),  and  that  their  volume  is  inversely  proportional 
to  the  pressure  to  which  they  are  subjected  (Law  of  Boyle). 
Hence  if  the  temperature  at  which  the  gas  was  collected 
were  17°C.,  and  if  the  barometer  then  stood  at  750mm 
the  volume  (v)  in  litres  of  the  carbon  monoxide  collected 
would  be 

166-74  x  (273  +  17)  x  760 


v  = 


1*249  x  273  x  750. 


EXPERIMENTAL   METHODS  FOR  THE   DETERMINA- 
TION OF  MOLECULAR  WEIGHTS. 

48  We  have  seen  above  (p.  99)  that  the  density  of  every  com- 
pound in  the  gaseous  state  is  half  its  molecular  weight,  from 
which  it  follows  that  in  order  to  determine  the  molecular  weight 
of  a  substance  it  is  simply  necessary  to  determine  its  density  in 
the  state  of  gas.  This  process  can  be  readily  applied  to  such 
substances  as  are  gases  under  ordinary  conditions  of  temperature 
and  pressure,  and  also  to  such  as  can  be  rendered  gaseous  by 
moderate  increase  of  temperature,  but  is  of  course  inapplicable 
to  bodies  which  decompose  on  heating  or  which  cannot  easily 
be  vapourised.  In  such  cases,  however,  several  methods  are 
available  which  depend  upon  the  properties  of  solutions,  and 
hence,  since  nearly  all  chemical  compounds  are  soluble  in 
some  liquid,  the  molecular  weight  of  a  body  can  almost  always 
be  determined. 

Special  experimental  methods  have  to  be  adopted  for  each  of 
these  classes  of  substances. 

o 


DETERMINATION  OF  MOLECULAR  WEIGHTS  105 


I.  DETERMINATION  OF  THE  MOLECULAR  WEIGHTS  OF 
PERMANENT  GASES. 

49  For  this  purpose  it  is  only  necessary  to  determine  the  specific 
gravity  of  the  gas,  air  being  usually  taken  as  the  practical  unit 
of  comparison.  A  large  glass  balloon  capable  of  holding  from  1  to 
10  litres  of  the  gas,  is  employed  and  is  first  of  all  freed  from  air 
as  far  as  possible  by  the  vacuum  pump,  and  weighed.  In 
weighing  a  body  of  such  large  volume  it  is  essential  to  make 
allowance  for  the  buoyancy  of  the  air,  since  the  body  to  be 
weighed  appears  to  be  lighter  than  it  really  is  by  an  amount 
equal  to  the  weight  of  the  air  which  it  displaces.  This  is  best 
accomplished  by  suspending  a  vessel  of  similar  size  and  shape  to 
the  other  arm  of  the  balance,  by  which  arrangement  the  effect 
of  buoyancy  is  neutralized,  each  of  the  vessels  being  affected  in 
the  same  manner,  and  at  the  same  time  the  uncertainties  of 
calculation,  due  to  the  varying  temperature,  pressure,  and 
moisture  of  the  atmosphere  are  avoided,  as  well  as  any  inaccuracy 
due  to  condensation  on  the  surface  of  the  glass.  As  soon  as  the 
weight  of  the  empty  vessel  has  been  ascertained,  it  is  removed 
from  the  balance  and  filled  with  the  pure  dry  gas,  the  tempera- 
ture and  pressure  being  carefully  observed.  The  globe  is  then 
reweighed  with  the  same  precautions  as  before.  One  additional 
correction  must  be  here  noticed  as  it  gives  rise  to  a  considerable 
error  especially  when  light  gases  are  being  weighed.  The 
capacity  of  a  vacuous  globe  of  glass  is  found  to  be  perceptibly 
less  than  that  of  the  same  globe  when  filled  with  gas  at  the 
pressure  of  the  atmosphere,  and  hence  the  globe  displaces  less 
air  in  the  former  condition  than  in  the  latter.  The  difference 
between  the  corrected  weights  of  the  globe  empty  and  filled 
with  the  gas  is  equal  to  the  weight  (W)  of  the  given  volume 
of  gas  at  the  observed  temperature  and  pressure.  The  same 
globe  is  then  filled  with  dry  air  freed  from  carbonic  acid  gas, 
or  else  with  pure  hydrogen,  and  the  weight  of  an  equal  volume 
of  the  latter  (A)  under  the  same  conditions  thus  ascertained. 
The  specific  gravity  of  the  gas  compared  with  air  is  then  equal 

W 
to  -j-.    Since  air  has  been  found  to  be  14*39  times  as  heavy  as 

hydrogen  and  the  molecular  weight  of  a  gas  is  twice  its  density 
with  respect  to  hydrogen,  it  is  only  necessary  to  multiply  the 


106  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

specific  gravity  by  14'89x2  to  obtain  the  molecular  weight  of  the 
gas  in  question. 

The  most  accurate  determinations  of  this  kind    have   been 
made  by  Regnault,  and  more  recently  by  Rayleigh,  Leduc  &c. 


II.    DETERMINATION    OF    THE    MOLECULAR  WEIGHTS    OF 
VOLATILE  LIQUIDS  AND  SOLIDS. 

50  A  detailed  account  of  the  various  methods  proposed  for  this 
purpose  is  to  be  found  in  a  later  volume  (Vol.  III.  Pt.  I. 
pp.  84-112)  ;  only  the  two  most  frequently  employed  will  be  here 
briefly  described. 

The  specific  gravity  of  the  vapour  of  a  liquid  or  solid  can  be 
determined  in  two  ways ;  either  by  weighing  the  vapour  which 
occupies  a  known  volume  under  given  conditions  of  temperature 
and  pressure  (Dumas'  method)  or  by  measuring  the  volume  occu- 
pied under  given  conditions  by  the  vapour  of  a  known  weight 
of  the  liquid  or  solid  (Gay-Lussac,  Hofmann,  Victor  Meyer). 


I.   METHOD   OF   DUMAS. 

51  For  this  purpose  a  thin  glass  globe  is  employed  of  150 — 200 
cubic  centimetres  in  capacity,  having  a  finely  drawn  out  neck  ; 
the  exact  weight  of  the  globe,  weighed  in  air  and  filled  with 
dry  air  at  a  certain  temperature  and  pressure,  having  been 
found,  a  small  portion  of  the  substance  of  which  the  vapour 
density  is  to  be  determined  is  brought  inside,  and  the  globe 
then  heated  by  plunging  it  into  a  water-  or  oil-bath  raised 
to  a  temperature  at  least  30°  above  the  boiling  point  of  the 
substance ;  as  soon  as  the  vapour  has  ceased  to  issue  from  the 
end  of  the  neck,  this  end  is  hermetically  sealed  before  a  blow- 
pipe, and  the  exact  temperature  of  the  bath  as  well  as  the 
barometric  pressure  observed.  The  bulb  thus  filled  with 
vapour  is  carefully  cleaned,  allowed  to  cool,  and  accurately 
weighed.  The  point  of  the  neck  is  next  broken  under  water, 
which  rushes  into  the  globe,  the  vapour  having  condensed,  and, 
if  the  experiment  has  been  well  conducted,  completely  fills  it. 
The  bulb  is  then  weighed  full  of  water,  and  its  capacity  calcu- 
lated from  the  weight  of  water  which  has  entered. 


DETEEMINATION  OF  VAPOUE  DENSITY 


107 


We  have  now  all  the  data  necessary  for  the  determination. 
In  the  first  place  we  have  to  find  the  weight  of  the  given 
volume  of  the  vapour  under  certain  circumstances  of  tempera- 


FIG.  21. 

ture  and  pressure,  and  we  then  have  to  compare  this  with  the 
weight  of  an  equal  volume  of  hydrogen  gas  measured  under 
the  same  circumstances.  The  following  example  of  the  deter- 
mination of  the  vapour  density  of  water  may  serve  to  illustrate 
the  method  : — 


FIG.  22. 


Weight  of  globe  filled  with  dry  air  at  15'5C 
Weight  of  globe  filled  with  vapour  at  140° 
Capacity  of  the  globe  . 


23*449  grams. 
23-326       „ 
I78cc. 


108  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

As  the  barometric  column  (760  mm.)  underwent  no  change 
from  the  beginning  to  the  end  of  the  experiment,  no  correction 
for  pressure  is  necessary.  In  order  to  get  at  the  weight  of 
the  vacuous  globe  the  weight  of  air  contained  must  be  deducted 
from  the  weight  of  the  globe  in  air. 

Now    1   cc.    of    air  at    0°  and   760    mm.  weighs    0*001293 

178  x  273 
grams,  and  178  cc.  of  air  at  15 '5°  would  occupy  -  -  _ 

288'5 

168-4  cc.  at  0°,  so  that  the  weight  of  this  air  is  168-4  x 
•001293  =  0'218  gram  ;  hence  the  weight  of  the  vacuous  globe 
is  23-231  (23-449  -  0'218),  and  the  weight  of  the  vapour 
23'326  -  23'231  =  0'095  gram.  We  must  now  find  what 
178  cc.  of  hydrogen  at  140°  will  weigh.  One  thousand  cc.  of 
hydrogen  at  0°  weigh  0'0899  gram. ;  178  cc.  at  140°  will  con- 

178  x  273  117-6  x   0-0899 

tract  to  -prs =  117'6  at  0  ,  which  weigh  1nrtA 

T<lo  1UUU 

=  0'0106  gram.  Hence  A.M-IQ^     =  8'96    *s  *ne  density  of  the 

vapour  as  found  by  experiment. 

The  molecular  weight  of  water  is  therefore  about  17'82.  In 
this  example  many  minor  corrections,  such  as  the  expansion  of 
the  glass  globe,  the  error  of  the  mercurial  thermometer,  &c.,  are 
not  considered,  but  the  above  method  carried  out  as  described 
gives  results  which  are  sufficiently  accurate  when  the  object, 
as  in  this  case,  is  to  control  the  molecular  weight  of  a 
compound. 


II.      METHOD   OF   VICTOR  MEYER. 

52  The  glass  vessel  (&)  filled  with  air  is  heated  by  the  vapour 
of  water  or  other  liquid  placed  in  the  bulb  tube  (c),  which  may 
if  necessary  be  replaced  by  an  air-bath,  until  no  more  air  is 
observed  to  pass  out  of  the  gas  delivery  tube  (a).  The  cork  (d) 
is  then  removed  and  the  weighed  quantity  of  the  substance  of 
which  the  vapour  density  is  required,  contained  in  a  small  glass 
bulb,  is  dropped  into  the  tube  (b)  and  the  cork  then  quickly 
inserted ;  the  substance  rapidly  evaporates  and  displaces  a 
portion  of  the  air  of  the  apparatus  which  is  collected  in  a 
graduated  tube  over  water  and  carefully  measured.  This 
method  has  the  great  advantage  of  dispensing  with  a  knowledge 
of  the  temperature  to  which  the  tube  is  heated.  It  is  to  be 


DISSOCIATION 


109 


borne  in  mind  that  what  we  require  to  know  is  the  weight  of 
air  (or  hydrogen)  equal  in  bulk  to  the  vapour.  Whether 
this  volume  of  air  be  measured  at  the  temperature  of  the 
vapour  or  at  that  of  the  atmosphere,  it  has  of  course  the  same 
weight. 

An  example  will  make  this  clear.  In  a  determination  of 
the  molecular  weight  of  chloroform,  CHC13, 
heated  by  water  vapour,  it  was  found  that 
0*1008  gram,  of  substance  displaced  20  cc.  of 
air,  measured  over  water  at  a  temperature 
of  15°C.  and  a  barometric  pressure  of  770 
mm.  The  corrected  volume  of  dry  air  is 
therefore  18-9  cc.,  the  weight  of  which  is 
18-9  X  0-01293  =  0-0244  gram.  The  va- 
pour density  of  chloroform  is  then  equal  to 

0*1008 

=  4"! 3  compared  with  air,  or  4*13  X 


14*39  =  59-4  compared  with  hydrogen,  the 
molecular  weight  being  accordingly  118*8,  which 
agrees  closely  with  118*5,  the  number  calculated 
from  the  formula,  CHC13. 


DISSOCIATION. 

53  In  many  cases  the  vapour  density  found 
by  experiment  alters  with  the  temperature  at 
which  the  deter  mi  nation  is  carried  out.  Iodine 
vapour,  for  example,  is  found  to  have  a  con- 
stant density  of  about  126  between  the  tem- 
peratures of  400 — 700°  C.,  and  its  molecular 
weight  then  corresponds  to  the  formula  I2. 
Above  this  temperature,  however,  the  density  is 
found  to  gradually  diminish,  until  at  about  1,500° 
it  again  becomes  constant  at  about  63,  almost  FlG-  23- 

exactly  half  of  its  previous  value.  We  must, 
therefore,  assume  that  the  molecule  of  iodine  (I2)  is  decomposed 
at  temperatures  above  700°,  a  gradually  increasing  number  of  its 
molecules  being  broken  up  as  the  temperature  rises,  until  finally 
at  1,500°  nearly  all  the  molecules  have  been  broken  up  into  free 
atoms,  each  of  which  must  now  be  considered  as  a  separate 
molecule,  the  molecular  weight  of  iodine  at  these  high 


110  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

temperatures  being  12ti,  and  its  formula  I,  the  change  being 
represented  by  the  equation  : 

L      =      I      +      I 
2  vols.     2  vols.     2  vols. 

A  decomposition  of  this  kind  is  known  as  dissociation.  It 
must  be  remembered  that  in  order  to  prove  that  a  substance 
exists  in  the  state  of  vapour  with  a  definite  molecular  weight, 
the  vapour  density  must  be  found  to  be  constant  throughout  a 
considerable  range  of  temperature.  Thus,  iodine  vapour  has  a 
constant  density  between  the  temperatures  400—700°,  and  only 
begins  to  dissociate  a.bove  the  latter  temperature.  Sulphur 
vapour  on  the  other  hand  at  a  temperature  near  its  boiling- 
point  has  a  density  of  96,  corresponding  to  the  formula  S6  ;  but 
this  density  is  not  constant  for  any  definite  range  of  temperature 
but  gradually  decreases  until  it  reaches  the  value  of  32  (S2)  at  a 
temperature  of  600°,  above  which  it  remains  constant.  In  the 
case  of  iodine  vapour,  therefore,  we  have  good  evidence  that 
molecules  of  the  formula  I2  exist,  whereas  this  is  wanting  for 
the  existence  of  sulphur  molecules  containing  six  atoms, 
although  it  was  formerly  believed  that  these  existed  at  low 
temperatures. 

54  Frequently  a  compound  which  exists  in  the  solid  or  Kquid 
state  cannot  be  converted  into  vapour  without  undergoing  dis- 
sociation. Thus  ammonia  and  hydrochloric  acid  unite  directly 
to  form  ammonium  chloride  : 

NH3  +  HC1  =  NH4C1. 

Phosphorus  trichloride  absorbs  two  atoms  of  chlorine,  and  is 
converted  into  the  pentachloride,  thus  :  — 


These  compounds,  however,  only  exist  in  the  solid  or  liquid 
state;  when  they  are  heated  they  decompose  into  the  two  mole- 
cules from  which  they  have  been  formed.  In  some  cases  this 
decomposition  can  be  readily  seen  ;  thus  antimony  pentachloride 
SbCl5,  decomposes  into  the  trichloride,  SbCl3,  and  free  chlorine. 
Other  compounds,  such  as  pentachloride  of  phosphorus,  PClft, 
appear  to  volatilize  without  decomposition,  but  in  this  case  it  can 
be  proved  that  the  vapour  is  a  mixture,  and  contains  the  mole- 
cules of  two  gases,  phosphorus  trichloride,  PC13,  and  free  chlorine. 


OSMOTIC  PKESSURE  111 


The  vapour  densities  of  these  bodies  accordingly  do  not  follow 
the  usual  law ;  thus  the  vapour  of  chloride  of  ammonium,  if 
it  consisted  of  similar  molecules,  must  possess  the  density  of 

35-19  +  13-94  +  4  =26'56.     In  fact,  however,  its  density  is  only 

2 

half  this  number,  for  four  volumes  contain  one  molecule  of 
ammonia  and  one  of  hydrochloric  acid  ;  hence  its  density  (or  the 
weight  of  one  volume)  is  half  the  above  or  13'28. 

In  the  same  way  iodine  forms  both  a  monochloride  IC1  and  a 
trichloride  IC13 ;  the  first  of  these  bodies  is  volatile  without 
decomposition,  the  second,  however,  decomposes  on  distillation 
into  the  molecules  IC1  and  CL. 


III.  DETERMINATION  OF  THE  MOLECULAR  WEIGHTS   OF 
SUBSTANCES  IN  SOLUTION. 

55  Until  recently  the  molecular  condition  of  substances  in  solu- 
tion was  quite  unknown,  but  much  light  has  been  thrown  upon 
the  subject  by  the  researches  of  Raoult,  Van't  Hoff,  Arrhenius, 
Ostwald,  and  others,1  who  have  shown  that  a  remarkable 
analogy  exists  between  the  properties  of  substances  in  dilute 
solution  and  those  of  gases.  This  conclusion  has  been  chiefly 
derived  from  the  study  of  the  interesting  phenomena  now  to 
be  described.  When  a  solution  of  a  crystalloid  substance  in 
water  is  placed  in  a  vessel  closed  by  a  porous  membrane, 
such  as  a  piece  of  parchment,  and  the  whole  immersed  in 
pure  water,  it  is  found  that  the  dissolved  substance  gradually 
passes  outwards  through  the  film  of  parchment,  whilst  water 
passes  inwards,  until,  after  a  sufficient  time  has  elapsed,  equi- 
librium is  established  and  the  liquid  has  the  same  composition 
both  inside  and  outside  the  membrane,  this  process  being  known 
as  osmosis. 

Diaphragms  of  other  substances  can,  however,  be  obtained 
which  allow  the  water  to  pass  freely  through  them  in  the  same 
way  as  the  parchment,  but  prevent  the  outward  passage  of  the 
dissolved  substance,  and  are  therefore  said  to  be  "semi- 
permeable."  Such  a  diaphragm  can  be  prepared  by  filling  an 
ordinary  porous  cell  with  a  dilute  solution  of  potassium  ferro- 

1  For  the  literature  of  this  subject  the  volumes  of  the  Zeitschrift  fur  PhysiJca- 
lische  Chemic  (Leipzig,  Engelmann),  edited  by  Van't  Hoff  and  Ostwald,  may  be 
consulted. 


112  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

cyanide  and  simultaneously  immersing  it  in  one  of  copper 
sulphate.  These  two  substances  gradually  diffuse  into  the 
porous  walls  of  the  cell  and  produce  an  insoluble  layer  of  copper 
ferrocyanide,  which  is  found  to  be  semi-permeable  for  solutions 
of  many  salts  and  other  bodies.  If  now  such  a  cell,  containing 
a  dilute  solution  of  sugar,  be  placed  in  a  vessel  containing  pure 
water,  the  latter  is  found  to  pass  inwards  through  the  film, 
whilst  the  sugar  does  not  pass  out,  and  consequently  the  level 
of  the  liquid  within  the  cell  rises.  If,  however,  the  cell  be  com- 
pletely filled  and  connected  with  an  arrangement  for  measuring 
the  pressure  it  will  be  found  that  the  latter  gradually  increases, 
but  after  a  time  becomes  constant.  The  pressure  thus  observed 
is  termed  the  osmotic  pressure  of  the  solution,  and  is  found 
to  depend  only  upon  (1)  the  nature  of  the  dissolved  substance, 
(2)  the  concentration  of  the  solution,  and  (3)  the  temperature. 

This  osmotic  pressure  plays  the  same  part  in  the  theory 
of  dilute  solutions  as  the  gaseous  pressure  in  that  of  gases, 
and  is  found  to  follow  the  same  laws.  Thus  when  the 
strength  of  the  solution  is  doubled  the  osmotic  pressure  be- 
comes twice  as  great ;  when  it  is  halved  the  pressure  falls  to 
one  half,  and  so  on.  Now  doubling  the  strength  of  a  solu- 
tion is  in  reality  halving  the  volume  occupied  by  the  unit 
weight  of  the  dissolved  substance,  so  that  the  law  that  the 
osmotic  pressure  of  a  dilute  solution  varies  directly  as  its 
concentration  corresponds  exactly  with  Boyle's  law  of  gases 
(p.  59).  This  is  well  seen  in  the  following  table,  which 
illustrates  the  variation  of  osmotic  pressure  with  the  concen- 
tration, for  solutions  of  cane  sugar  : l 

Strength  of  sugar  solution.         Osmotic  Pressure.        Pressure  per  cent,  of  sugar. 

1°/0  53*5  cm.  53*5  cm. 

2%  101'6  50-8 

4°/o  208-2  52-1 

6%  307-5  51-2 

The  osmotic  pressure  is  also  found  to  vary  with  the  tem- 
perature in  the  same  way  as  does  the  pressure  of  gases. 
Thus  a  solution  of  cane  sugar  was  observed  to  have  an  osmotic 
pressure  of  54'4  cm.  at  32°,  whilst  the  same  solution  at  14'1°  had 
a  pressure  of  51 '2  cm.  Calculating  the  pressure  at  the  lower 

1  See  Pfeffer,  "  Osmotische  Untersuchungen, "  71   quoted  in  Zcit.  Phys.  Chem.t 
1,  484. 


DEPKESSION  OF  THE  FREEZING-POINT  113 

temperature  from  that  at  the  higher  according  to  Dalton's  law 

54*4  x  287*1 

for  gases   (p.  60)  we  obtain  the  number  -        — —    -  =  5 TO, 

oU5 

or  almost  exactly  that  observed. 

56  Another  remarkable  fact  which  these  researches  have  estab- 
lished is  that  solutions  of  substances  which  contain  quantities 
of  the  compounds  proportional  to  their  molecular  weights  dis- 
solved in  equal  amounts  of  the  solvent  possess  the  same  osmotic 
pressure;  if  we  consider  cane   sugar   (molecular  weight  340) 
and  alcohol  (molecular  weight  46),  for  example,  we  find  that  a 
solution  of  sugar  in  water  containing  3'40  grams  in  100  cc.  has 
the  same  osmotic  pressure  as  one  of  0*46  grams  of  alcohol  in 
the  same  volume.     Moreover  it  appears  that  ?ne  osmotic  pres- 
sure of  a  solution  is  numerically  equal  to  the  pressure  of  a  gas 
containing  the  same  number  of  molecules  per  unit  of  volume 
as  the  solution  does  of  molecules  of  the  dissolved  substance.     A 
one  per  cent,  solution  of  sugar,  for  example,  has  at  15°  the 
specific  gravity  of  1*006,  so  that  1  gram  of  sugar  is  contained 
in   100'6   cc.   of  the  solution.      Now  the  molecular  weight  of 
sugar  is  340,  and  if  we  calculate  what  volume  of  solution  con- 
tains 340  grams  of  sugar,  we  obtain  the  number  34'2  litres. 
Now  we   know  that   two  grains  of  hydrogen    (the   molecular 
weight  of  which  is  2)  at  a  temperature  of  15°  occupy  a  volume  of 

99'9f)    x    288 

— =  23*53  litres  when  the  pressure  is  equal  to  one 

2/3 

atmosphere.  If  this  volume  of  the  gas  be  expanded  to  34*2 
litres  the  pressure  of  the  expanded  gas  will,  by  Boyle's  law,  be 

equal  to  -  -  that  is  to  say  52*3  cm.     The  gas  there- 

34'2 

fore  which  contains  the  same  number  of  molecules  per  litre  as 
there  are  sugar  molecules  in  a  one  per  cent,  solution  of  cane 
sugar,  would,  at  the  temperature  of  15°,  have  a  pressure  of 
52*3  cm.,  whilst  the  osmotic  pressure  of  the  solution  itself  has 
been  found  to  be  52*0  cm. 

As  a  consequence  of  these  remarkable  relations  it  will  be 
seen  that  the  molecular  weight  of  a  dissolved  substance  can  be 
determined  by  measuring  the  osmotic  pressure  of  the  solution, 
but  the  experimental  difficulties  are  so  great  as  to  prevent  the 
general  use  of  the  method  for  this  purpose. 

57  The  same  result  can,  however,  be  attained  by  several  allied 
methods,  the  chief  of  which  depends  upon  the  alteration  pro- 
duced in  the  freezing-point  of  a  liquid  by  dissolving  some  other 

9 


114  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

substance  in  it.  When  a  dilute  solution,  such  as  one  of  sugar 
in  water,  is  cooled,  the  solvent,  in  this  case  the  water,  begins  to 
separate  out,  and  if  the  solution  be  originally  sufficiently 
dilute,  the  solid  matter  which  deposits  is  quite  free  from  the 
dissolved  substance.  The  temperature  at  which  the  separation 
of  solid  matter  commences  is,  however,  lower  than  the  freezing- 
point  of  the  pure  solvent,  the  amount  of  the  depression  being 
proportional  to  the  concentration  of  the  solution  (Blagden).1 

Raoult  has  further  found  that  the  extent  of  the  depression 
depends  upon  the  molecular  weight  of  the  dissolved  substance, 
to  which  it  is  inversely  proportional,  or  in  other  words,  that  if 
equal  weights  of  a  series  of  compounds  be  each  dissolved 
in  a  liquid  so  as  to  produce  equal  volumes  of  solution,  the 
depressions  of  the  freezing-point  thus  caused  are  inversely  pro- 
portional to  the  molecular  weights  of  the  compounds.  If  these 
depressions  be  then  multiplied  by  the  molecular  weights  of  the 
compounds  a  constant  number  is  obtained,  which  is  known  as 
the  molecular  depression  for  the  solvent  in  question.  This  is 
illustrated  by  the  following  table  of  results  obtained  by  Raoult 2 
with  substances  dissolved  in  acetic  acid  : 

Substance  Formula          Molecular  Weight  D  MD 

Carbon  tetrachloride      CC14  153  0'252  38'6 

Carbon  disulphide           CS2  76  0*505  38'4 

Sulphur  chloride             S2C12  134  0'286  38'3 

Arsenious  chloride         AsCl3  180  0'234  421 

Stannic  chloride              SnCl4  258  0'159  41'0 

Sulphur  dioxide              S02  64  0'601  38'5 

Sulphuretted  hydrogen  H2S  34  1-047  35'6 

In  this  table  D  signifies  the  depression  in  degrees  Centigrade 
produced  by  1  grm.  of  substance  dissolved  in  100  grms.  of  glacial 
acetic  acid,  whilst  MD  is  the  product  of  this  number  with  the 
molecular  weight  of  the  compound.  It  will  be  seen  that 
the  molecular  depression  for  this  solvent  is  nearly  constant, 
the  numbers  varying  on  either  side  of  an  average  value  of 
about  39. 

Upon  these  facts  a  very  simple  method  for  determining  the 
molecular  weight  of  a  soluble  substance  has  been  based.  A 
weighed  quantity  of  a  suitable  solvent  is  placed  in  a  glass  tube, 

1  Phil.  Trans.,  1788,  78,  277. 

2  Ann.  Chim.  Phys.  [6],  2,  93. 


DEPRESSION  OF  THE  FREEZING-POINT  115 

the  latter  surrounded  by  a  freezing  mixture,  and  the  freezing- 
point  of  the  liquid  determined  by  means  of  an  accurate  thermo- 
meter, graduated  to  T^  of  a  degree.  An  exactly  weighed 
amount  of  the  substance  is  then  added,  allowed  to  dissolve 
completely,  and  the  freezing  point  of  the  solution  then  carefully 
ascertained  in  the  same  way  as  before.  The  difference  observed 
between  these  two  temperatures  (d)  is  the  depression  of  the 
freezing  point  produced  by  (b)  grms.  of  the  substance  dissolved 
in  (a)  grms.  of  the  solvent.  From  this  is  calculated  the  depression 
which  would  be  produced  by  1  grin,  of  the  substance  dissolved 

in  100  grms.  of  the  solvent  (D),  which  is  equal  to-^— — —  •    If 

b   x  100 

.now,  the  molecular  depression  is  MD,  it  follows  that  the  mole- 
cular weight  of  the  substance  in  question  (M)  is  given  by  the 
equation  : 

M     MD      MD  x  b  x  100 
~W  d     x  a' 

Thus  1'35  grm.  of  carbon  tetrachloride  dissolved  in  55  grms. 
of  acetic  acid  lowered  the  melting  point  of  the  latter  from 
16°*750  to  16°'132,  the  depression  being  therefore  equal  to 
0°'618,  Since  the  molecular  depression,  MD,  for  acetic  acid  is 
39,  it  follows  that  the  molecular  weight  of  carbon  tetrachloride 
must  be 

39  x  1-35  x  100 


55  x  0-618 


154-9. 


This  number  agrees  satisfactorily  with  that  obtained  by  the 
vapour-density  method,  viz.,  153. 

58  The  addition  of  a  soluble  substance  to  a  liquid  not  only 
lowers  the  freezing-point  of  the  latter,  but  also  diminishes  its 
vapour-pressure.  Like  the  depression  of  the  freezing-point, 
this  diminution  of  vapour-pressure  depends  upon  the  molecular 
weight  of  the  dissolved  substance  and  upon  the  strength  of  the 
solution.  Several  methods  of  determining  the  molecular  weight 
of  dissolved  substances"  have  been-  based  upon  these  facts,  for 
the  details  of  which  the  original  papers  must  be  consulted.1 

These  three  classes  of  phenomena  are  intimately  connected 
with  one  another,  inasmuch  as.  it  has  been  experimentally  found 
that  dilute  solutions,  the  solvent  being  the  same  in  all,  which 
have  the  same  osmotic'  pressure  have  also  the  same  freezing- 

1  Beckmann,  Zeit.  Phys.  Chem.  4,  532,  6,  437,  8,  223  ;  Will  und  Brediir,  Bet. 
22,  1084. 


116  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

point  and  the  same  vapour  pressure,  such  solutions  being  termed 
isotonic.  This  relation  has  also  been  deduced  theoretically,  so 
that  if  any  one  of  these  three  facts  be  known  about  a  solution 
the  other  two  can  be  calculated. 

It  is  important  to  remember  that  these  statements  only  hold 
good  in  the  case  of  dilute  solutions,  and  cannot  be  applied  to 
concentrated  solutions,  just  as  the  laws  of  Boyle  and  Dalton  do 
not  apply  without  modifications  to  gases  at  high  pressures  and 
low  temperatures. 


AQUEOUS  SOLUTIONS. 

59  The  behaviour  of  aqueous  solutions  when  examined  by  the 
methods  just  described  is  somewhat  anomalous,  but  an  explana- 
tion of  the  irregularities  observed  has  been  arrived  at  from  a 
study  of  the  phenomena  which  occur  when  an  electric  current 
is  passed  through  such  solutions.  Pure  water  is  practically  a 
non-conductor  of  electricity,  and  substances  which  are  soluble 
in  it  may  be  divided  into  two  classes  according  as  they  do  or  do 
not  produce  solutions  which  conduct  electricity.  Those  of  the 
former  class  are  termed  electrolytes,  and  the  passage  of  the 
current  through  their  solutions  is  accompanied  by  their 
decomposition,  whilst  those  of  the  second  class  are  known  as 
non-electrolytes.  The  laws  of  osmotic  pressure,  &c.,  as  stated 
above  are  true  of  all  non-electrolytes,  bat  require  some 
modification  before  they  can  be  applied  to  electrolytes,  a  class 
of  bodies  which  includes  acids,  alkalis,  and  almost  all  metallic 
salts.  When  the  molecular  weight  of  one  of  these  sub- 
stances is  determined  from  an  aqueous  solution  by  any  of  the 
methods  described,  the  number  obtained  is  found  to  be  some 
fraction  of  that  which  was  to  be  expected,  generally  about  J  or  J. 
This  result  is  comparable  with  that  obtained  by  the  vapour 
density  method  with  such  vapours  as  undergo  dissociation, 
and  it  seems  probable  that  something  of  an  analogous 
nature  also  occurs  in  dilute  aqueous  solutions  of  electro- 
lytes. When  a  current  of  electricity  is  passed  through  a 
solution  of  hydrochloric  acid,  HC1,  hydrogen  is  given  off  at 
the  negative  pole  and  chlorine  at  the  positive,  and  the  acid 
is  said  to  have  been  decomposed  into  the  two  "  ions  "  hydrogen 
and  chlorine.  Many  facts  make  it  appear  probable  that  this 
decomposition  is  not  actually  brought  about  by  the  electric 


ELECTKOLYTIC  DISSOCIATION  117 

current  but  that  the  ions  exist  already  separated  in  the  solution. 
According  to  this  view,  then,  a  dilute  solution  of  hydrochloric 
acid  does  not  only  contain  molecules  of  HC1  but  a  large  propor- 
tion of  separated  ions  H  and  Cl.  Each  one  of  these  ions  behaves 
like  a  molecule  of  a  non-electrolyte  and,  therefore,  produces  its 
own  effect  in  lowering  the  freezing  point,  etc.,  the  total  depres- 
sion being  due  to  the  sum  of  the  ions,  each  considered  as  an  in- 
dependent molecule,  together  with  the  unaltered  molecules. 
Since  the  depression  is  inversely  proportional  to  the  molecular 
weight,  the  result  obtained  by  this  method  seems  to  show  that 
hydrochloric  acid  has  about  one  half  of  the  molecular  weight 
corresponding  to  the  formula  HC1.  In  general,  salts,  such  as 
common  salt,  NaCl,  or  silver  nitrate,  AgNO3,  which  are  broken 
up  into  two  ions  (Na  and  Cl,  Ag  and  NO3),  appear  to  have  about 
half  the  calculated  molecular  weight,  whilst  the  salts  of  a  dibasic 
acid  or  a  divalent  metal  of  the  types  represented  by  sodium 
sulphate,  Na2SO4,  and  calcium  chloride,  CaCl2,  which  give  three 
ions  (Na,  Na  and  SO4 ;  Ca,  Cl  and  Cl)  appear  to  have  a 
molecular  weight  approaching  one  third  of  that  calculated. 

According  to  this  theory  of  "  electrolytic  dissociation  "  which 
is  due  to  the  Swedish  physicist  Arrhenius,  the  greater  number  of 
salt  molecules  in  a  strong  solution  of  an  electrolyte  are  unaltered, 
but  some  of  them  have  been  dissociated  into  their  ions ;  when 
the  solution  is  diluted  the  number  of  dissociated  molecules 
increases  rapidly,  and  in  a  very  dilute  solution  nearly  all  the 
salt  is  present  in  the  form  of  ions.  The  following  table  shows 
the  percentage  of  dissociation  which  would  account  for  the  results 
obtained  by  the  freezing  point  method  for  a  few  common  salts  i1 

Grins,  in  100  cc.  water.    Percentage  of  Molecules  dissociated. 

NaCl          0-682  87 

3-155  79        5-  into  2  ions. 

AgN03       2-381  85 

CaCl2         2-206  75 

K2S04        1-583  67 


j-  into  3  ions. 


Each  ion  is  supposed  to  bear  an  equal  charge  of  electricity, 
electropositive  ions  having  a  positive,  electronegative  a  neg- 
ative charge.  When,  therefore,  it  is  said  that  a  dilute  solution 
of  hydrochloric  acid  contains  free  ions  of  hydrogen  and  chlorine, 
it  must  not  be  supposed  that  what  we  are  familiar  with  as  free 
hydrogen  and  chlorine  are  meant ;  what  are  actually  present  are 

1  Arrhenius,  Zeit.  Phys.  Chem.  2,  491. 


118  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

positively  charged  atoms  of  hydrogen,  and  negatively  charged 
atoms  of  chlorine.  These  charged  atoms  cannot  escape  from  the 
solution  without  giving  up  their  charges  of  electricity  to  some 
oppositely  charged  body  and  this  is  what  happens  at  the  poles 
of  the  cell  during  electrolysis.  The  hydrogen  ions  give  up 
their  positive  charges  to  the  negatively  charged  pole  and  escape 
as  free  hydrogen,  the  atoms  uniting  to  form  molecules,  whilst  the 
chlorine  ions  behave  in  a  similar  manner  at  the  positively  charged 
pole.  In  many  cases  the  liberated  atoms  enter  into  reaction 
with  the  water  surrounding  the  pole  and  thus  give  rise  to 
secondary  products  which  either  escape  or  remain  in  the  solu- 
tion (p.  252).  Concerning  the  exact  nature  of  the  separation  of 
the  ions  our  knowledge  is  still  incomplete,  but  many  facts  in 
addition  to  those  already  adduced  (see  p.  283,  where  some 
of  these  are  discussed)  point  to  the  conclusion  that  such  a 
separation  does  actually  take  place.  The  fact  that  electrolytes 
in  aqueous  solution  show  this  anomalous  behaviour,  renders  it 
impossible  to  employ  the  methods  described  above  for  determin- 
ing their  molecular  weights,  unless  a  complete  study  of  the 
behaviour  of  each  substance  is  made.  Substances  which  give 
abnormal  numbers  when  examined  in  aqueous  solution,  usually 
yield  normal  results  when  their  solutions  in  other  solvents  are 
examined. 


CHEMICAL  NOMENCLATURE. 

60  Nomenclature  is  the  spoken  language  of  chemistry,  as  no- 
tation is  the  symbolic  written  language  of  the  science.  With 
the  progress  of  discovery  chemical  nomenclature  has  naturally 
undergone  great  and  frequent  changes.  The  ancients  were 
acquainted  with  only  seven  metals,  viz. : — gold,  silver,  copper, 
tin,  iron,  lead,  and  mercury.  Of  these  the  first  six  are  men- 
tioned by  Homer ;  mercury  was  not  known  in  his  time,  but 
mention  is  made  of  the  liquid  metal  by  authors  living  one  cen- 
tury before  Christ.  These  seven  metals  were  originally  supposed 
to  be  in  some  way  connected  with  the  seven  heavenly  bodies 
then  known  to  belong  to  our  system.  To  bright  yellow  gold 
the  name  of  Sol  was  given ;  whilst  white  silver  was  termed 
Luna;  copper,  which  had  chiefly  been  obtained  from  the 
island  of  Cyprus  and  received  its  common  name  (cuprum) 
from  this  source,  was  likewise  called  Venus,  after  the  pro- 


CHEMICAL  NOMENCLATURE  119 

tectress  of  the  island.  Tin  was  specially  dedicated  to  Jupiter ; 
iron  to  Mars,  the  god  of  war  ;  whilst  heavy  dull  lead  was  con- 
nected with  Saturn  ;  and  the  mobile  quicksilver  was  called 
Mercury,  after  the  active  messenger  of  the  gods.  The  alchemists 
not  only  invariably  used  these  names,  but  employed  the  signs 
of  the  heavenly  bodies  as  symbols  for  the  metals,  and  many 
remnants  of  this  practice  are  found  to  this  day  in  all 
languages.  Thus  we  still  speak  of  "  lunar  caustic "  for  silver 
nitrate,  "  saturnine  poisoning  "  for  poisoning  by  lead,  whilst  the 
name  mercury  has  become  the  common  one  of  the  metal.  To 
come  to  later  times  we  find  that  the  language  of  the  alche- 
mists was  always  and  designedly  obscure  and  enigmatical,  so 
that  their  names  for  chemical  compounds  were  not  based  on 
any  principle  but  even  chosen  for  the  sake  of  secrecy  or  de- 
ception, and,  therefore,  bore  no  relation  to  the  substances 
themselves.  From  these  fanciful  terms  the  progress  to  a 
better  state  of  things  has  been  slow,  and  the  changes  which 
the  names  have  undergone  have  been  numerous,  whilst  the 
same  substance  has  at  one  time  frequently  been  designated 
by  many  distinct  names,  several  of  which  are  still  in  use. 

Bodies  were  generally  named  and  classed  by  the  alchemists 
by  virtue  of  certain  real  or  fancied  resemblances  existing  be- 
tween their  physical  properties.  Thus,  bodies  which  can  be 
obtained  by  distillation  and  are,  therefore,  easily  volatile,  were 
all  termed  spirits,  so  that  alcohol  (spirits  of  wine)  was  classed 
together  with  hydrochloric  acid  (spirits  of  salt),  and  these 
again  with  spirits  of  turpentine,  although  these  three  sub- 
stances are  chemically  as  different  as  any  three  substances 
well  can  be.  In  the  same  way,  all  viscid,  thick  liquids  were 
termed  oils,  and  thus  sulphuric  acid,  or  oil  of  vitriol,  came  to 
be  placed  in  the  same  class  as  olive  oil;  whilst  semi-solid 
bodies,  such  as  antimony  trichloride,  were  termed  butters,  and 
considered  to  be  analogous  to  common  butter. 

As  soon  as  chemistry  became  a  science,  the  nomenclature 
assumed  a  more  scientific  character.  Some  of  the  terms  which 
came  into  use  during  the  growth  of  the  science  have  been 
mentioned  in  the  Historical  Introduction.  These  terms  have 
by  degrees  been  much  changed,  and,  such  revolutions  have 
accompanied  the  progress  of  the  science,  that  at  present  the 
same  compound  is  not  unfrequently  designated  by  different 
names.  Thus  it  is  clear  that  our  nomenclature  has  not  yet 
attained  a  permanent  form ;  the  names  of  chemical  substances 


120  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

are  not  identical  in  different  languages,  and  even  in  the  same 
language,  difference  of  practice  in  naming  compounds  is  found 
among  chemists.  Nevertheless,  we  are  guided  by  certain 
specific  rules,  and  the  science  no  longer  suffers  from  the 
arbitrary  nomenclature  which  the  descriptive  natural  sciences 
have  to  endure. 

6 1  The  foundation  of  the  modern  system  of  chemical  names 
was  laid  by  Lavoisier  and  his  colleagues,1  and  the  plan  proposed 
by  them  has  been  maintained,  with  slight  modifications, 
up  to  the  present  time.  The  principle  upon  which  our  system 
(for  Inorganic  Chemistry  at  least)  is  founded  is,  that  every 
compound  being  made  up  of  two  or  more  elementary  bodies 
united  in  different  proportions,  the  name  of  that  compound 
shall  signify  the  nature  of  its  elementary  constituents,  and  as 
nearly  as  possible  the  relative  proportions  in  which  they  are 
believed  to  be  present.  In  the  case  of  the  Carbon  compounds 
(Organic  Chemistry)  it  was  soon  found  impossible,  from  the 
large  number  of  closely-allied  substances,  uniformly  to  apply 
this  system,  and  names  suggested  by  the  origin  of  the  bodies 
have  been  in  many  cases  adopted. 

No  special  rule  has  been  applied  to  the  nomenclature  of  the 
elements.  The  old  common  names  of  those  which  have  long 
been  known  have  in  most  cases  been  retained,  and  when  new 
elements  have  been  discovered  they  have  been  named  according 
to  no  pre-arranged  plan.  Some  are  named  from  the  locality  in 
which  they  have  first  been  found ;  some  from  a  characteristic 
property  or  from  the  mode  of  their  discovery,  whilst  the  names 
of  others,  such  as  Gallium,  Scandium,  and  Germanium,  bear 
witness  to  the  patriotism  of  their  discoverers.  By  common 
consent  the  names  of  all  recently  discovered  metals  end 
in  " -ium,"  as  sodium,  barium,  vanadium.  The  names  of  a 
group  of  allied  non-metallic  elements  end  in  "  -ine"  thus 
we  have  fluorine,  chlorine,  bromine,  and  iodine ;  those  of 
another  group  of  somewhat  analogous  non-metallic  elements 
end  in  "  -on,"  as  boron,  carbon,  silicon,  whilst  those  of 
two  other  non-metals,  more  nearly  resembling  the  metals, 
end  like  the  latter  in  "-ium"  thus  we  have  selenium  and 
tellurium. 

Lavoisier  introduced  the  term  "  oxyde "  to  signify  the  com- 

1  Mtthode  de  Nomenclature  Chimique,  propose  par  MM.  de  Morveau,  Lavoisier, 
Berthollet,  et  de  Fourcroy.  Paris,  1787.  Translated  into  English  by  Pearson. 
Second  edition,  1799. 


CHEMICAL  NOMENCLATURE  121 

binations  of  oxygen  with  the  other  elements,  and  words  with  the 
same  ending  have  been  since  employed  to  denote  the  simple 
combinations  of  two  elements  or  groups  of  elements,  thus  : — 

The  compounds  of  form  such  as 

Hydrogen  Hydrides  Phosphorus  hydrides. 

Fluorine  Fluorides  Calcium  fluoride. 

Chlorine  Chlorides  Sodium  chloride. 

Bromine  Bromides  Magnesium  bromide. 

Iodine  Iodides  Lead  iodide. 

Oxygen  Oxides  Mercury  oxide. 

Sulphur  Sulphides  Zinc  sulphide. 

Selenium  Selenides  Potassium  selenide. 

Phosphorus  Phosphides  Calcium  phosphide. 

Carbon  Carbides  Iron  carbide. 

It  frequently  happens  that  a  metal  forms  several  distinct 
oxides  or  chlorides,  in  which  the  constituents  are  present  in 
simple  multiple  proportions  of  their  combining  weights.  In 
these  cases  it  is  usual  to  give  to  each  compound  a  name  indi- 
cating either  the  number  of  atoms  of  oxygen  which  we  believe 
to  be  combined  with  one  atom  of  metal,  or  the  simplest  relation 
which  we  suppose  it  possible  to  exist  between  the  number 
of  atoms  of  metal  and  oxygen  in  the  molecule  :  thus  the  oxide 
believed  to  contain  one  atom  of  oxygen  is  termed  the  monoxide  ; 
that  containing  two  atoms  is  the  dioxide ;  whilst  oxides  containing 
three,  four,  or  five  atoms  of  oxygen  are  called  trioxides.  tetrox- 
and  pentoxides  respectively.  Sometimes  the  first  oxide  is  termed 
the  protoxide  (TT/OWTO?,  first),  the  second  deutoxide  (Sevrepos, 
second),  the  third  trioxide  (T/HTO?,  third),  and  the  highest 
peroxide. 

When  the  relation  of  metal  to  oxygen  is  that  of  2  to  3,  as 
in  red  hematite,  Fe.2O3,  the  Latin  prefix  sesqui,  meaning  one  and 
a  half,  is  used,  and  the  oxide  is  termed  a  sesquioxide.  The 
same  mode  of  designation  applies  to  the  compounds  of  metal 
with  sulphur,  chlorine,  &c. :  thus  we  speak  of  iron  sesqui- 
sulphide,  or  if  we  please,  sesqui-sulphide  of  iron  Fe2S3,  of 
antimony  trichloride,  or,  if  we  prefer  it,  the  trichloride  of 
antimony,  SbCl3. 

In  the  case  of  metals,  such  for  instance,  as  iron  and  mercury 
which  form  two  distinct  series  of  compounds,  one  corresponding 
to  a  lower  oxide,  and  another  to  a  higher  one,  it  is  customary  to 


122  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

use  the  endings  "-ous"  and  "-ic"  (introduced  by  Berzelius1) 
to  denote  the  difference  between  the  two  sets  of  compounds. 
Thus  we  have  the  mercurows  and  the  mercuric  salts.  Among 
others,  mercimms  oxide,  Hg2O,  mercuro^s  chloride,  HgCl  (com- 
monly called  calomel),  mercurows  nitrate,  HgNO3  ;  and,  on  the 
other  hand,  mercuric  oxide,  HgO,  mercuric  chloride,  HgCl2  (com- 
monly called  corrosive  sublimate),  mercuric  nitrate,  Hg(NO3)2. 
In  the  same  way  we  have  the  ferrous  salts  (from  fer- 
rum,  iron)  corresponding  to  ferrows  oxide,  FeO  (also  termed 
the  monoxide),  and  the  ferric  salts  corresponding  to  ferric 
oxide,  Fe2O3. 

The  endings  -ous  and  -ic  are  applied  not  only  in  the  case 
of  oxides  and  chlorides  but  also  in  that  of  acids.  Thus 
sulphurous  acid,  H2SO3,  contains  less  oxygen  than  sulphuric 
acid,  H.,SO4;  nitrons  acid,  HNO2,  less  than  nitric  acid,  HNO3  ; 
and  carrying  this  distinction  still  further,  the  names  of  salts  of 
acids  ending  in  -ous  terminate  in  -ite,  whilst  those  derived  from 
acids  in  -ic  end  in  -ate  ;  thus  for  example— 

Nitrons  acid  forms  salts  termed  nitrides. 
Sulphurous  acid          „         „        sulphides. 
Nitric  acid  „         „        nitrates. 

Sulphuric  acid  „         „        sulphas. 

With  respect  to  the  nomenclature  of  acids  and  salts  some 
difference  of  opinion  has  been  expressed  by  chemists,  and 
hence  a  certain  amount  of  confusion  exists  in  chemical  writings. 
Lavoisier,  when  he  devised  the  present  scheme  of  chemical 
nomenclature,  believed  that  it  is  oxygen  (of  u?,  acid,  yevvdw,  I 
produce)  which  gives  to  the  bodies  formed  by  combustion  in  the 
gas  their  acid  characters,  and  hence  the  highest  oxides  of  the 
metals  and  non-metals  were  termed  acids,  thus  P2O5  was  called 
phosphoric  acid,  C02  carbonic  acid,  Cr03  chromic  acid,  &c- 
The  ordinary  well-known  substances  possessing  acid  properties, 
such  as  nitric  acid,  HNO3,  and  sulphuric  acid,  H2SO4,  were 
looked  upon  as  hydrates  of  the  anhydrous  oxides,  N2O5  and 
SO3,  from  which  they  may  be  obtained  by  the  action  of  water, 
thus  :  — 


N20,+H20  = 
and  S63  +  H2O=H2SO4. 

1  Journ.  dc  Physique,  Oct.  1811. 


NOMENCLATURE  OF  ACIDS  AND  SALTS  123 

Acid  bodies  were,  however,  next  discovered,  such  as  hydrochloric 
acid,  HC1,  hydrofluoric  acid,  HF,  and  hydrocyanic  acid,  HCN, 
which  contain  no  oxygen ;  and  thus  it  appears  that  Lavoisier's 
notion  that  the  presence  of  oxygen  is  alone  necessary  to  form  an 
acid  is  incomplete,  and  a  more  correct  definition  of  an  acid  is  that 
it  is  a  hydrogen  compound,  in  which  the  whole  or  a  part  of  the 
hydrogen  is  capable  of  being  replaced  by  a  metal ;  in  other  words 
an  acid  is  an  hydrogen  salt.  Nitric  acid,  therefore,  is  hydrogen 
nitrate  or  hydric  nitrate,  HNO3,  and  by  replacing  the  hydrogen 
by  the  metal  potassium  we  obtain  potassium  nitrate  or  nitrate 
of  potassium,  KN03.  Sulphuric  acid  is  hydrogen  sulphate  or 
hydric  sulphate,  H2SO4,  and  either  one  or  both  the  atoms  of 
hydrogen  can  be  replaced  by  potassium,  giving  rise,  in  the  first 
instance  to  a  salt  termed  hydrogen  potassium  sulphate  (or  com- 
monly bisulphate  of  potash)  HKSO4,  and  in  the  second  case  to 
potassium  sulphate  (commonly  termed  sulphate  of  potash), 
K2S04.  Acids  which  contain  one  atom  of  hydrogen  replaceable 
by  metals  are  called  monobasic,  those  containing  two,  dibasic  and 
those  containing  three  tribasic.  Nitric  acid,  HNO3,  is  a  mono- 
basic acid,  sulphuric  acid,  H2S04,  a  dibasic,  and  phosphoric  acid, 
H3PO4,  in  which  all  the  atoms  of  hydrogen  can  be  replaced,  a 
tribasic  acid. 

The  anhydrous  oxides  (such  as  N2O5  and  S03),  from  which 
the  acids  are  derived,  may  be  best  termed  anhydrides  or  acid- 
forming  oxides,  whilst  the  oxides  which  have  the  power  of  acting 
as  bases  and  of  forming  salts  when  brought  into  contact  with 
acids,  are  termed  basic  oxides. 

62  At  the  time  when  our  nomenclature  was  invented  all 
salts  were  supposed  to  be  compounds  of  an  acid  and  a  base  ; 
and  names  were  given  which  indicated  the  fact  that  when 
the  acid  and  the  base  are  brought  together  a  neutral  salt  is 
produced  ;  thus,  if  we  add  potash  (the  base)  to  sulphuric  acid 
(the  acid)  a  salt  is  formed  to  which  the  name  sulphate  of  potash 
was  given,  and  this  view  of  the  formation  of  salts  being  still 
held,  the  name  indicating  this  view  is  still  commonly  used. 
Where  the  acid  is  combined  with  a  heavy  metallic  oxide,  as  for 
instance  when  oxide  of  lead  is  dissolved  in  an  acid  such  as 
nitric  acid,  the  common  name  nitrate  of  lead,  or  more  simply 
lead  nitrate,  does  not  exhibit  the  analogy  between  this  salt  and 
that  obtained  by  adding  nitric  acid  to  potash  and  called  nitrate 
of  potash.  In  order  to  assimilate  these  names  some  chemists 
have  termed  the  first  nitrate  of  oxide  of  lead,  corresponding  to 


124  GENERAL  PRINCIPLES  OF  THE  SCIENCE 

nitrate  of  potash  (potash  being  oxide  of  potassium)  ;  whilst 
others,  to  avoid  the  recurrence  of  the  word  "  of,"  and  to  shorten 
the  names,  prefer  to  mention  in  the  name  of  the  salt  not  the 
base  but  the  metal  or  lasylous  group,  so  that  the  similar  names 
of  lead  nitrate  and  potassium  nitrate  become  the  designation  of 
both  compounds.  Other  chemists  prefer  to  modify  the  termina- 
tion of  the  name  of  the  metal,  making  it  an  adjective,  thus : — 
potassic  nitrate,  and,  as  the  common  word  lead  does  not  lend 
itself  to  such  adjective  forms,  we  are  compelled  to  use  the  Latin 
word  and  term  the  salt  plumbic  nitrate. 

In  this  work  no  special  system  of  nomenclature  will  be 
adopted  to  the  exclusion  of  every  other  system.  As  a  rule, 
however,  the  ordinary  name  of  the  metal  will  be  retained  for 
the  salts,  thus  : — lead  nitrate,  zinc  sulphate,  potassium  chloride. 
But  this  will  not  preclude  the  occasional  use  of  the  com- 
mon terms,  as  nitrate  or  carbonate  of  soda,  whilst  such  names 
as  ferrous-  and  ferric-,  mercurous-  and  mercuric-  salts,  will  of 
course  be  employed. 

We  define  an  acid  to  be  a  hydrogen  salt  and,  therefore,  HN03 
will  be,  as  a  rule,  termed  nitric  acid  :  the  names  hydrogen 
nitrate,  or  hydric  nitrate,  may  sometimes  be  used.  Bodies  such 
as  N205,  SO3,  CrO3,  will  not  be  termed  acids  but  are  referred  to 
as  anhydrides  or  acid-forming  oxides.  In  some  few  instances  the 
body  C02  may  be  mentioned  as  carbonic  acid  or  carbonic 
acid  gas,  owing  to  the  fact  that  it  has  for  a  long  time 
been  so  called  ;  but  the  systematic  name  by  which  it  will  be 
designated  in  these  pages  is  carbon  dioxide. 

The  following  comparison  of  some  of  the  older  and  common 
and  the  scientific  and  more  modern  names  of  important  acids 
and  salts  may  prove  useful. 

ACIDS. 

Older  and  Common  Name.  Formula.  Modern  and  Scientific  Name. 

Nitric  acid  HNO3  Hydrogen  nitrate. 

Nitrous  acid  HN02  Hydrogen  nitrite. 

Sulphuric  acid  H2SO4  Hydrogen  sulphate. 

Sulphurous  acid  H2SO3  Hydrogen  sulphite. 

Chloric  acid  HC103  Hydrogen  chlorate. 

Chlorous  acid  HC1O2  Hydrogen  chlorite. 

Hypochlorous  acid  HC1O  Hydrogen  hypochlorite. 


NOMENCLATURE  OF  ACIDS  AND  SALTS 


125 


Older  and  Common  Name. 
Nitrate  of  potash 
Nitrate  of  silver 
Sulphate  of  lime 
Sulphite  of  lead 
Chlorate  of  potash 
Chlorite  of  soda 
Hypochlorite  of  potash 
Protosulphate  of  iron 
Perchloride  of  iron 


SALTS. 

Formula. 

KN03 

AgN03 

CaS04 

PbS03 

KC1O3 

NaClO2 

KC10 

FeS04 

FeCL 


Modern  and  Scientific  Name. 
Potassium  nitrate. 
Silver  nitrate. 
Calcium  sulphate. 
Lead  sulphite. 
Potassium  chlorate. 
Sodium  chlorite. 
Potassium  hypochlorite. 
Ferrous  sulphate. 
Ferric  chloride. 


This  system  of  nomenclature  is,  however,  by  no  means 
perfect,  nor  is  it  universally  carried  out.  Were  we  to  do  so 
long  arid  inconvenient  names  would  have  to  be  used.  Thus 
instead  of  the  common  name  alum,  we  should  have  to  use  the 
words  potassium  aluminium  sulphate,  and  for  bitter-spar  the 
name  calcium  magnesium  carbonate.  Hence  we  shall  often  use 
the  common  instead  of  the  strictly  scientific  names,  as  common 
salt  for  sodium  chloride,  caustic  potash  for  potassium  hydroxide, 
sulphuric  acid  for  hydrogen  sulphate,  and  nitre  or  saltpetre  for 
potassium  nitrate. 


THE  NON-METALLIC  ELEMENTS. 


63  All  the  non-metallic  elements  form  volatile  compounds  with 
hydrogen,  and  if  these  be  compared  it  is  found  that  they  fall 
into  four  classes  ;  this  is  apparent  in  the  following  table,  which 
contains  the  molecular  formulae  of  these  compounds  : 


I.  Hydrogen.     Hydrofluoric     Hydrochloric   Hydrobromic 
acid.  acid.  acid. 


Hydriodic 
acid. 


HJ 

H)             H)             H  ) 
F  /              Cl  /             Br  } 

?} 

II.           Water. 

Sulphuretted          Seleniuretted 
Hydrogen.              Hydrogen. 

Telluretted 
Hydrogen. 

H}s         |}se 


III.     Ammonia. 


H 
H 


Phosphuretted 

Arseniuretted 

Hydride  of 

Hydrogen. 

Hydrogen. 

Boron. 

JJ\ 

H) 

H) 

H  VP 
HJ 

H  VAs 
HJ 

H  VB 
HJ 

IV. 


Marsh  Gas. 


Siliciurctted 
Hydrogen. 


It  appears  from  this  that  the  elements  differ  in  their  be- 
haviour, one  atom  of  some  of  them  combining  with  one  atom  of 
hydrogen,  whilst  an  atom  of  others  combines  with  two  atoms  of 
hydrogen,  and  others  again  with  three  and  four.  Each  element 
therefore  possesses  a  certain  combining  power  or  valency  as  it  is 
called. 


VALENCY.  127 


The  elements  of  the  first  group  in  the  preceding  table  are 
said  to  be  monovalent,  those  of  the  following  groups  being 
divalent,  trivalent  and  tetravalent.  This  is  also  sometimes 
expressed  by  saying  that  the  elements  of  the  first  group  are 
monads,  and  those  of  the  succeeding  ones  dyads,  triads  and 
tetrads.  So  long  as  we  only  examine  the  compounds  of  the  non- 
metals  with  hydrogen,  the  relations  are  simple  and  definite ;  but 
when  the  comparison  is  extended  to  their  compounds  with  other 
elements,  it  is  found  that  the  valency  does  not  possess  a  constant 
value.  Phosphorus,  for  example,  combines  with  chlorine  in  two 
different  proportions,  producing  the  compounds  phosphorus  tri- 
chloride, PC13,  and  phosphorus  pentachloride,  PC15,  in  the  first 
of  which  one  atom  of  phosphorus  is  united  with  three  atoms  of 
chlorine,  whilst  in  the  second  it  is  combined  with  five.  It  is  true 
that  when  the  latter  of  these  substances,  PC15,  is  heated,  it  is 
decomposed  into  the  simpler  molecules  PC13  and  C12,  and  hence 
the  conclusion  was  at  one  time  drawn  that  the  combination 
between  the  molecules  PC13  and  C12  to  form  PCL  differs  in 
some  way  from  that  between  the  atoms  of  phosphorus  and 
chlorine  to  form  PC13.  Substances  of  this  kind,  which  cannot 
be  vapourised  without  decomposing  into  simpler  molecules  were 
then  called  molecular  compounds,  and  the  valency  of  an  element 
measured  by  the  number  of  atoms  of  monad  elements  combining 
with  it  to  form  a  compound  vapourising  without  decomposition. 
The  whole  question  turns  upon  the  molecular  weights  of  the 
compounds  under  discussion,  and  although  phosphorus  penta- 
chloride cannot  be  vapourised  without  decomposition,  it  is  found 
that  in  solution  its  molecular  weight  does  correspond  to  the 
formula  PC15.  In  the  light  of  our  present  knowledge,  there- 
fore, these  distinctions  cannot  be  maintained,  and  the  valency 
of  the  elements  must  be  looked  upon  as  a  variable  quantity.  A 
more  definite  value  appears  to  attach  to  the  maximum  valency 
displayed  by  the  elements  in  particular  classes  of  compounds, 
which  can  be  ascertained  from  the  molecular  formula  of  their 
compounds,  but  even  this  is  subject  to  exceptions. 

Elements  which  are  of  equal  valency  combine  with  and  replace 
one  another  atom  for  atom,  whilst  one  atom  of  a  divalent  element 
can  replace  or  combine  with  two  monovalent  atoms,  and  a  trivalent 
atom  either  three  monovalent  or  one  mono-  and  one  divalent 
atom,  as  is  seen  in  the  following  equations : 


128  THE  NON-METALLIC  ELEMENTS 


(H 

c'H  +   2  °\ 

'In  +  2  of 
(H 

MO         2H)0 
CJO  +  2HJC 

ci      H, 

01  +  i}  [O     = 

01 

64  A  careful  study  of  the  chemical  properties  of  substances 
teaches  that  in  many  cases  special  relations  exist  between  the 
different  atoms  of  which  the  molecule  is  made  up,  and  these  may 
be  expressed  in  formulae  which  are  known  as  constitutional 
formulae.  The  properties  of  many  oxy-acids  for  instance  show 
that  their  hydrogen  atoms  stand  in  a  special  relation  to  some 
of  their  oxygen  atoms.  This  is  expressed  in  the  following 
constitutional  formulae  for  some  of  the  commoner  acids : 

Nitric  acid HO.NO2. 

Sulphuric  acid    ....  (HO)2.SO2. 
Phosphoric  acid  ....  (HO)3PO. 
lodic  acid HO.IO2. 

The  group  or  radical  (HO)  which  appears  in  all  these  formulae 
is  known  as  the  hydroxyl  group  and  behaves  as  a  monovalent 
radical,  since,  although  it  is  not  known  in  the  free  state,  it  is 
found  in  water  and  other  compounds  combined  with  or  re- 
placing a  monovalent  atom. 

It  is  only  after  the  constitution  of  a  compound  has  been 
ascertained  that  it  is  possible  to  determine  the  valency  of  the 
atoms  of  which  it  is  composed.  Thus  the  formula  of  iodic  acid, 
HIO3,  might  be  either  that  given  above  or 

H.O.O.O.I 

In  the  former  case,  which  seems  to  be  the  more  probable  (p.  333), 
the  atom  of  iodine,  being  directly  combined  with  two  divalent 
atoms  and  a  monovalent  group,  would  be  pentavalent,  whilst  in 
the  latter  it  would  be  monovalent. 


HYDROGEN  129 


HYDROGEN.     H  =  i. 

65  It  has  already  been   stated   (see   Historical  Introduction) 
that  water  was  long  supposed  to  be  an   elementary  or  simple 
substance,  and  it  was  not  until  the  year  1781  that  Cavendish 
proved  that  water  was  produced  by  the  union  of  oxygen  and 
hydrogen  gases,  whilst  Humboldt  and  Gay-Lussac  first  showed 
in  1805   that  these  gases  combine  by  volume  in  the  simple 
relation  of  one  to  two.     Turquet  de  Mayerne  at  the  commence- 
ment   of    the    seventeenth   century   had   indeed   obtained   an 
inflammable   gas   by    the    action    of    dilute   oil  of  vitriol   on 
iron,  but  the   true    nature    of  this   gas   was   first   ascertained 
by  Cavendish  in  1766,1  when  he  showed   that  hydrogen   was  a 
peculiar  gas  to  which  he  gave  the  name  of  "  inflammable  air." 

Hydrogen  occurs  almost  solely  in  a  state  of  combination  in 
nature,  although  it  has  been  found  to  exist  in  the  free  state 
mixed  in  small  quantities  with  other  gases  in  certain  volcanic 
emanations.2  It  has  also  been  found  by  Graham  as  occluded 
gas  in  the  meteoric  iron  from  Lenarto,3  and  by  Mallet  in  a 
meteorite  from  Virginia.4  It  is  produced  in  the  decay  and 
decomposition  of  various  organic  bodies,  being  found  in  the 
intestinal  gases  of  many  animals,  as  also,  according  to  Sadtler, 
in  the  gases  given  off  by  the  oil-wells  of  Pennsylvania. 

In  a  state  of  combination  hydrogen  occurs  in  water,  of  which 
it  constitutes  very  nearly  one  ninth  part  by  weight  (exactly 
11*18  per  cent.),  and  from  this  it  derives  its  name  (vbcop,  water ; 
and  yevvdo),  I  give  rise  to).  Hydrogen  likewise  occurs  in 
nature,  though  in  smaller  quantities,  combined  with  sulphur, 
phosphorus,  chlorine,  bromine,  iodine,  and  nitrogen,  whilst  it 
forms  an  essential  portion  of  nearly  all  organic  substances. 

66  Preparation. — (1)  Pure  hydrogen  is  best  prepared  by  the 
electrolysis  of  acidulated  water.     For  this  purpose  a  mixture 
of  one  part  by  weight  of  pure  sulphuric  acid  with  ten  parts 
of  water    is  placed  in  the  glass   decomposing  cell    (Fig.  24). 
The  positive  pole  consists  of  a  platinum  wire  (a)  melted  through 
the  glass  and  placed  in  contact  with  mercury  amalgamated  with 

1  "Experiments  on  Factitious  Air."     Phil.  Trans.  1766,  p.  144. 

2  Bunsen,  Fogg.  Ann.  83,  197.     Ch.  St.  Claire  Deville,  Compt.  Rend.  55,  75. 

3  Proc.  Roy.  Soc.  15,  502.  4  Proc.  Roy.  6'oc.  20,  365. 

10 


130 


THE  NON-METALLIC  ELEMENTS 


zinc  (6),  whilst  the  negative  pole  (c)  is  composed  of  a  platinum 
plate.  When  the  current  from  two  or  three  of  Bunsen's 
elements  is  passed  through  the  apparatus  a  constant  stream  of 
pure  hydrogen  is  evolved,  and  after  being  washed  by  the  small 
quantity  of  sulphuric  acid  contained  in  the  bulbs  (d),  the  gas 
may  be  collected  for  analytical  purposes.  The  oxygen  of  the 
water  is  all  absorbed  by  the  zinc  amalgam,  oxide  of  zinc  and 
ultimately  zinc  sulphate  being  formed,  whilst  the  whole  of  the 
hydrogen  is  evolved  in  the  pure  state.  Instead  of  dilute 


FIG.  24. 


sulphuric  acid,  dilute   solutions  of  caustic  potash  or  soda  are 
frequently  employed. 

(2)  By  acting  on  water  with  the  alkali  metals,  or  with  an 
amalgam  of  sodium  or  potassium.  In  this  case  the  metal  re- 
places an  equivalent  quantity  of  hydrogen  in  the  water,  hydrogen 
gas  and  the  soluble  hydroxide  of  the  metal  being  formed, 
thus  : 


When   a  small  piece   of  potassium  is  thrown  into  a  basin  of 
water,  it  swims  about  on  the  surface,  and  with  a  hissing  noise 


PREPARATION  OF  HYDROGEN  131 

bursts  into  flame ;  this  is  due  to  the  fact  that  the  metal  in 
uniting  with  the  oxygen  of  the  water  evolves  heat  enough  to 
melt  the  metal  and  to  ignite  the  liberated  hydrogen,  which 
then  burns  with  a  flame  coloured  violet  by  the  presence  of 
the  vapours  of  the  metal.  Sodium,  likewise,  decomposes  water, 
but  the  hydrogen  in  this  case  does  not  take  fire  spontaneously 
unless  the  water  be  hot,  or  the  motion  of  the  bead  of  metal 
be  stopped,  as  when  the  metal  is  thrown  on  to  a  viscid  starch- 
paste  or  on  to  a  moistened  sheet  of  blotting-paper,  in  which 
cases  the  globule  of  melted  metal  remaining  in  one  place  be* 
comes  hot  enough  to  cause  the  ignition  of  the  hydrogen,  which 
then  burns  with  the  yellow  flame  characteristic  of  the  sodium 


FIG.  25. 

compounds.  If  the  blotting-paper  be  previously  stretched  upon 
an  inclined  wooden  tray  and  moistened  with  a  red  solution 
of  litmus,  the  track  of  the  molten  potassium  or  sodium,  as  it 
runs  over  the  paper,  will  be  seen  by  a  blue  line  showing  the 
formation  of  an  alkaline  product.  In  order  to  collect  the 
hydrogen  thus  evolved,  the  small  clean  globule  of  sodium  may 
be  caught  and  depressed  below  the  surface  of  the  water  by 
means  of  a  little  sieve  of  wire-gauze  under  the  open  end  of  a 
cylinder  l ;  the  bubbles  of  gas  then  rise  and  may  be  collected,  as 
shown  in  Fig.  25. 

1  Explosions  may  ensue  if  the  sodium  adheres  to  the  glass. 


132  THE  NON-METALLIC  ELEMENTS 

(3)  By  passing  steam  over  red-hot  iron  wire  or  iron  borings 
placed  in  an  iron  tube  and  heated  in  a  furnace  as  shown  in 
Fig.  26,  (a)  being  a  retort  in  which  water  is  boiled.     The  iron 
is  converted  into  the  black  or  ferrosoferric  oxide    Fe3O4,    and 
hydrogen  is  evolved,  thus : 

3Fe  +  4H2O  =  Fe3O4  +  4H2. 

(4)  The  most  convenient  mode  of  preparing  hydrogen  gas  for 
ordinary  use  where  absolute  purity  is  not  requisite,  is  ,by  the 
action   of  sulphuric  acid,  diluted  with  six  to  eight  times  its 
weight  of  cold  water,  upon  metallic  zinc ;    the  water  must  be 
added    because    if  none   be   present   the  zinc  sulphate  ZnS04 
formed  in  the  reaction  coats  the  surface  of  the  metal,  which  is 
thus  protected  from  the  action  of  the  acid.     Hydrochloric  acid 


FIG.  26'. 

diluted  with  twice  its  weight  of  water  may  also  be  employed, 
and  poured  upon  clippings  of  metallic  zinc  contained  in  a  gas- 
generating  bottle.  Other  metals,  such  as  iron,  may  be  used 
instead  of  zinc,  and  magnesium  is  sometimes  employed  where 
a  very  pure  gas  is  required.  The  acid  is  gradually  poured 
upon  the  metal  by  means  of  the  tube  funnel,  and  the  evolved 
gas  can  be  collected  in  cylinders  over  the  pneumatic  trough  as 
shown  in  Fig.  27.  The  above  reactions  are  represented  as 
follows  : 

H2S04  +  Zn  =  ZnSO4  +  H2. 

2HCl+Zn  = 


Care  must  be  taken  that  all  the  air  is  expelled  from  the  flask 
before  the  gas  is  collected,  and  in  order  to  ensure  freedom  from 
air  the  gas  is  first  allowed  to  fill  an  inverted  test-tube,  which  is 


PREPARATION  OF  HYDROGEN 


133 


then  brought  mouth  downwards  to  a  flame  ;  if  the  hydrogen 
burns  quietly  all  air  has  been  expelled,  if  it  burns  with  a  slight 
explosion  the  evolution  must  be  allowed  to  continue  before  the 
gas  is  collected. 

Hydrogen  thus  prepared  always  contains  small  quantities  of 
impurities  derived  from  the  materials  used ;  these  can  be  got 
rid  of  by  passing  the  gas  though  various  absorbents,  Of  these 
impurities  the  most  common  are  arseniuretted  hydrogen,  when 
the  zinc,  iron,  or  acid  contains  arsenic ;  phosphuretted  hydrogen, 
when  they  contain  phosphorus ;  nitrous  fumes  when  the  acid 
contains  nitric  acid  or  nitrates ;  sulphur  dioxide  and  sulphuretted 


FIG.  27. 

hydrogen  when  these  gases  are  contained  in  the  acid  or  when 
hot,  even  diluted,  sulphuric  acid  is  allowed  to  come  in  contact 
with  the  metal. 

In  order  to  purify  the  gas,  the  best  method  is  to  pass  it 
through  two  U-tubes,  each  one  metre  in  length,  filled  with 
broken  glass  ;  in  the  first  tube  the  glass  is  moistened  with  an 
aqueous  solution  of  lead  nitrate,  which  absorbs  the  sulphuretted 
hydrogen ;  the  second  tube  contains  an  aqueous  solution  of 
silver  sulphate,  by  which  the  arseniuretted  and  phosphuretted 
hydrogen  gases  are  arrested.  After  this  the  gas  is  passed 
through  a  third  tube  containing  pumice  moistened  with  a  strong 


134  THE  NON-METALLIC  ELEMENTS 

solution  of  caustic  potash ;  then  through  two  others,  one  con- 
taining pumice  moistened  with  strong  sulphuric  acid,  and  the 
other  phosphorus  pentoxide,  by  means  of  which  the  gas  is 
thoroughly  dried.  When  absolute  purity  is  aimed  at  the  use 
of  sulphuric  acid  for  drying  the  gas  has  to  be  discontinued, 
since  sulphur  dioxide  is  formed  when  hydrogen  is  dried  in 
this  way. 

When  the  hydrogen  is  evolved  from  metallic  iron,  or  even 
from  impure  zinc,  the  gas  possesses  a  very  unpleasant  smell, 
due  to  the  presence  of  small  quantities  of  volatile  hydrocarbons 
derived  from  the  carbon  contained  in  the  metal.  The  best  way 
of  removing  this  odour  is  to  pass  the  hydrogen  through  a  tube 
filled  with  small  pieces  of  charcoal  which  absorbs  the  hydro- 
carbon. Another  impurity  which  it  is  much  more  difficult  to  re- 
move from  hydrogen  is  atmospheric  air.  This  is  partly  contained 
dissolved  in  the  liquids  used  in  the  preparation  of  the  gas,  but  its 
presence  may  also  be  due  to  the  high  diffusive  power  of  hydrogen, 
which  causes  it  to  escape  through  the  pores  of  the  cork  and 
caoutchouc,  whilst  at  the  same  time  a  certain  quantity  of  air 
diffuses  into  the  apparatus.  In  order  to  free  the  hydrogen  from 
traces  of  oxygen,  the  gas  must  be  passed  through  a  red-hot 
tube  filled  with  metallic  copper,  and  then  the  water,  produced 
by  the  combination  of  the  oxygen  and  hydrogen,  absorbed  by 
passing  the  gas  over  phosphorus  pentoxide.  The  nitrogen  of 
the  air  cannot  be  got  rid  of,  so  that  its  presence  must  be 
prevented  by  a  careful  air-tight  construction  of  the  apparatus. 

(5)  Strong  aqueous  solution  of  potash  dissolves  metallic  zinc 
in  presence  of  iron,  hydrogen  being  liberated,  and  a  compound 
of  zinc  oxide  and  potash  K2Zn02  being  formed.     This  process 
yields  an  inodorous  gas  : 

2KHO  +  Zn  =  H2  +  K2Zn02. 

Aluminium  may  also  be  used  in  place  of  zinc. 

(6)  When  zinc  is  immersed  in  an  aqueous  solution  of  any 
ammoniacal  salt  (except  the  nitrate),  such  as  sal-ammoniac,  at 
40°    in   contact  with  metallic  iron,  hydrogen    is    also    rapidly 
evolved.1 

67  Properties. — Hydrogen  is  a  colourless,  tasteless,  inodorous 
gas •  it  is  the  lightest  substance  known,  being  14*391  times  as 
light  as  atmospheric  air,  and  has  therefore  a  density  of  0'06949.  By 

1  Lorin,  Compt.  Rend.  60,  745. 


PROPERTIES  OF  HYDROGEN  135 

carefully  weighing  a  glass  globe,  first  empty,  and  then  filled  with 
air  and  hydrogen,  Regnault  found  that  1  litre  of  hydrogen 
at  0°  and  under  a  pressure  of  760  mm.  of  mercury,  weighs  at 
the  latitude  of  Paris,  0*089578  grams ;  he  omitted,  however,1  to 
make  a  correction  for  the  compression  of  the  glass  vessel  when 
vacuous  by  the  external  atmospheric  pressure,  and  if  this 
correction  be  introduced  his  results  show  that  the  litre  of 
hydrogen  weighs  0*89894  grams  under  the  above  conditions,  or 
that  1  gram  of  hydrogen  occupies  11*126  litres  at  the  normal 
temperature  and  pressure. 

Hydrogen  has  been  liquefied  by  Wroblewski 2  and  Olszewski 3 ; 
the  former  cooled  hydrogen  under  a  pressure  of  190  atmospheres 
by  means  of  boiling  nitrogen,  then  quickly  lowered  the  pressure 
to  1  atmosphere,  and  obtained  a  grey  foam-like  mass,  the  tem- 
perature being  — 208  —  211°.  Olszewski  compressed  the  gas  to 
180  atmospheres,  cooled  it  by  surrounding  it  with  liquid  air 
boiling  in  a  vacuum,  and  suddenly  reduced  the  pressure  to  40 
atmospheres  ;  it  then  forms  a  colourless  liquid  which  is  trans- 
parent, and  not  grey  as  stated  by  Wroblewski.  The  liquid 
obtained  in  a  similar  manner  from  electrolytic  gas  is  likewise 
colourless.  The  statement  of  Pictet,4  that  hydrogen  condenses 
to  a  steel-blue  liquid  which  by  rapid  evaporation  yields  solid 
particles  of  the  same  colour,  has  since  proved  to  be  erroneous. 

Hydrogen  is  an  inflammable  gas  taking  fire  when  brought 
in  contact  with  a  flame,  and  combining  with  the  oxygen  of  the 
air  to  form  water ;  it  does  not  support  ordinary  combustion  or 
animal  life  ;  when  pure  it  may  be  breathed  without  danger  for 
a  short  time,  but  it  produces  a  singular  effect  upon  the  voice, 
weakening  it  and  rendering  it  of  higher  pitch.  On  combining 
with  oxygen  to  form  water,  one  gram  of  hydrogen  evolves 
heat  sufficient  to  raise  34,462  grams  of  water  from  0°  to  1° 
Centigrade,  and  this  is  termed  the  calorific  power  of  hydrogen, 
which  is,  therefore,  equal  to  34,462  thermal  units  or  calories. 

Hydrogen  gas  is  very  slightly  soluble  in  water,  1  cc.  of  the 
latter  dissolving  only  0*021  cc.  at  0*5° ;  the  numbers  obtained 
by  Bunsen  appeared  to  show  that  the  solubility  of  hydrogen 
remained  constant  between  0°  and  20°  but  this  has  proved  on 
further  investigation  to  be  incorrect.  The  absorption  coefficient 

1  Rayleigh,  Proc.  Roy.  Soc.  43,  356  ;  Crafts,  Compt.  Rend.  106,  1662. 

2  Compt.  Rend.  100,  979. 

3  Compt.  Rend.  99,  133  ;  101,  238. 

4  Ann.  Chim.  Phys.  [5],  13,  145. 


136 


THE  NON-METALLIC  ELEMENTS 


between  0°  and    20°   is   given  by  the   following  interpolation 
formula : l 

0  =  0-02148  —  0'0002215t  +  0'00000285t2. 

Hydrogen  is  somewhat  more  soluble  in  alcohol  than  in  water, 
and    its    solubility    diminishes    with    the    temperature.      The 


FIG.  28. 

following  interpolation  formula  gives  the  absorption  coefficient 
(C)  in  alcohol  for  temperatures,  from  0°  to  25° : — 

C  =  0-06925  -  0'0001487t  +  O'OOOOOlt2. 

68  Absorption  of  Hydrogen  ly  Metals. — Graham  2  and  Deville 
and  Troost3  have  shown  that  hydrogen  gas  possesses  the 
peculiar  capability  of  diffusing  through  the  pores  of  certain 
red-hot  metals,  such  as  iron,  platinum,  or  palladium.  When 

1  Winkler,  Ber.  24,  98  ;  Timofejew,  Zeit.  Physik.  Chem.  6,  141. 

2  Graham,  Proc.  Roy.  Soc.  15,  223  ;  16,  422  ;  17,  212  and  500. 

3  Deville  and  Troost,  Compt.  Rend.  57,  894. 


ABSORPTION  OF  HYDROGEN  BY  METALS  137 

hydrogen  gas  is  passed  through,  a  red-hot  palladium  tube,  the 
rate  at  which  the  hydrogen  permeates  the  metal  is  such  that 
through  a  surface  of  one  square  metre,  3992'22  cc.  of  the  gas 
pass  each  minute,  whereas  the  rate  of  permeability  through  the 
same  surface  of  platinum  is  489'2  cc.  and  that  through 
a  sheet  of  caoutchouc  of  the  same  thickness  and  area  is 
represented  by  the  passage  of  127'2  cc.  of  gas  in  the  same 
time. 

The  power  of  hydrogen  to  pass  through  hot  iron,  palladium,  and 
platinum,  whilst  it  cannot  pass  through  when  the  metals  are  cold, 
probably  depends  on  the  fact  that  this  gas  is  absorbed  at  a  high 
temperature,  and  does  not  require  the  assumption  of  anything 
like  porosity  in  the  structure  of  the  metals.  This  property, 
which  has  been  termed  by  Graham  "occlusion"  can  be  ex- 
amined as  follows  :  A  known  weight  of  the  metal  palladium 
in  foil  or  wire  is  placed  in  a  small  porcelain  tube,  glazed  inside 
and  out,  and  connected  by  one  end  to  a  Sprengel's  mercury  pump, 
which  by  the  flow  of  mercury  down  the  long  tube  (Fig.  28) 
yields  an  almost  perfect  vacuum.  The  tube,  having  been  ex- 
hausted, is  now  heated  to  redness,  and  a  stream  of  hydrogen 
passed  over  the  red-hot  metal  for  some  time,  after  which  the 
tube  is  allowed  to  cool ;  the  current  of  gas  is  then  stopped, 
and  the  tube  again  rendered  vacuous.  Next,  the  tube  is  again 
heated,  and  the  gas,  which  is  thus  evolved  and  driven  out  by 
means  of  the  falling  mercury,  is  collected  and  measured  in  the 
divided  jar  placed  at  the  lower  end  of  the  barometric  tube 
of  the  pump. 

Of  all  metals  palladium  possesses  this  power  of  absorbing 
hydrogen  in  by  far  the  highest  degree.  A  palladium  wire  was 
found  by  Graham  to  absorb  at  a  red  heat  935  times  its  volume 
of  hydrogen  and  increased  in  length  from  609'14  mm.  to 
618*91  mm.,  or  1-6  per  cent.  In  another  experiment  the  metal 
showed  an  increase  in  bulk  of  9 '827  per  cent.  Even  at 
the  ordinary  temperature  palladium  absorbs  376  volumes  of 
the  gas. 

If  palladium  be  employed  as  negative  electrode  in  the  electro- 
lysis of  water,  it  likewise  very  readily  absorbs  hydrogen,  occluding 
935  times  its  volume  of  hydrogen,  the  expansion  being  pro- 
portional in  all  directions  to  the  amount  of  hydrogen  absorbed. 
If  the  electrolysis  be  continued  after  the  above  point  is  reached, 
the  palladium  becomes  supersaturated  with  hydrogen,  the  limit 
of  supersaturation  varying  with  the  strength  of  the  current; 


138  THE  NON-METALLIC  ELEMENTS 

the  excess  of  hydrogen  is,  however,  evolved  immediately  the 
current  ceases.1 

The  appearance  of  the  metal  does  not  undergo  any  change 
after  this  absorption  of  hydrogen,  but  its  specific  gravity  and  its 
conducting  power  for  heat  and  electricity  as  well  as  its  tenacity 
are  somewhat  diminished,  though  to  a  much  less  degree  than 
would  probably  be  the  case  by  the  similar  admixture  of  any 
non-metallic  substance.  For  various  reasons  Graham  concluded 
that  the  hydrogen  is  not  chemically  combined  with  the  palla- 
dium, but  rather  that  the  hydrogen  assumes  the  solid  form 
and  acts  as  a  quasi-metal,  giving  rise  to  a  kind  of  alloy,  such, 
for  instance,  as  is  obtained  when  sodium  and  mercury  are 
brought  together.  The  name  Hydrogenium  has  been  given 
to  this  absorbed  form  of  hydrogen,  and  its  specific  gravity  has 
been  calculated  from  the  expansion  of  alloys  of  palladium 
with  platinum,  gold,  and  silver,  when  charged  with  hydro- 
gen to  be  0'733,  whereas  the  number  obtained  from  experi- 
ments with  pure  palladium  (in  which  the  wire,  after  heating, 
does  not  return  exactly  to  its  original  volume)  is  0'863.  Of 
these  two  numbers,  the  former  is  probably  the  most  trust- 
worthy, but  subsequent  determinations  by  Dewar 2  give  a  specific 
gravity  of  0'620  to  hydrogenium,  which  is  equal  to  the  con- 
densation of  7  litres  of  gas  into  the  space  of  1  cc.  According 
to  Graham,  hydrogenium  is  distinctly  magnetic  (more  so  than 
palladium)  ;  and  has  an  electric  conductivity  of  5'99,  that  of 
palladium  being  8*10,  and  that  of  copper  100.  Troost  and 
Hautefeuille  3  have  shown  that  when  hydrogenized  palladium  is 
heated  in  vacuo,  the  tension  of  the  hydrogen  given  off  rapidly 
diminishes  until  the  mass  contains  600  vols.  of  H,  this  com- 
position corresponding  to  the  formula  Pd2H,  after  which  the 
tension  remains  constant  until  the  decomposition  is  complete. 
It  is  therefore  probable  that  a  definite  compound  of  this  formula 
exists,  which  has  the  power  of  absorbing  further  quantities  of 
hydrogen. 

Similar  compounds  of  hydrogen  with  the  alkaline  metals 
such  as  Na2H  and  K2H  have  been  prepared  by  the  last- 
mentioned  chemists,4  and  from  the  observed  densities  of  these 
compounds  as  compared  with  those  of  the  metals  themselves, 

1  Thoma,  Ze.it.  Physik.  Chem.  3,  69. 

2  Dewar,  Phil.  Mag.  [4],  47,  324  and  342. 

3  Compt.  Rend.  78,  686—690  ;  Journ.  Chem.  Soc.  27,  660. 

4  Compt.  Rend.  78,  968. 


EXPERIMENTS  WITH  HYDROGEN  139 

the  density  of  the  combined  hydrogen  has  been  calculated  to  be 
0*62,  a  number  exactly  agreeing  with  Dewar's  observations. 

Platinum  at  a  red  heat  absorbs  3'8  times,  and  at  100°  O76 
times,  its  volume  of  hydrogen ;  and  red-hot  iron  only  0*46  of 
its  volume. 

The  meteoric  iron  of  Lenarto,1  containing  90'88  per  cent,  of 
iron,  yields  when  heated  in  vacuo  2*85  times  its  volume  of  a 
gas  consisting  almost  entirely  (85*68  per  cent.)  of  hydrogen. 
This,  coupled  with  the  fact  that  under  the  ordinary  pressure 
iron  absorbs  only  about  half  its  volume  of  hydrogen,  would 
appear  to  show  that  the  Lenarto  meteorite  has  come  from  an 
atmosphere  containing  hydrogen  under  a  pressure  much  greater 
than  that  of  our  own  atmosphere,  and  thus  we  obtain  an  un- 
expected confirmation  of  the  conclusions  drawn  from  spectro- 
scopic  observations  by  Huggins,  Lockyer,  and  Secchi  respecting 
the  existence  of  dense  and  heated  hydrogen  atmospheres  in  the 
;sun  and  fixed  stars. 

The  spectrum  of  hydrogen  consists  essentially  of  four  bright 
lines — one  in  the  red,  identical  with  Fraunhofer's  dark  line  C, 
and  one  in  the  greenish  blue  coincident  with  the  dark  line  F. 
The  wave-lengths  of  these  four  lines,  according  to  Angstrom's 
measurements  are,  C  =  6562,  F  =  4861,  Blue  =  4340,  and  Indigo 
=  4101  (in  10  million ths  of  a  millimetre). 

68  Experiments  with  Hydrogen. — The  following  experiments 
show  that  hydrogen  is  a  very  inflammable  gas,  burning  with 
a  nearly  colourless  flame,  but  incapable  of  supporting  ordinary 
combustion. 

(1)  When  a  lighted  taper  is  brought  to  the  open  end  of  a 
cylinder   filled   with   hydrogen,  the  gas  will  burn  slowly  and 
quietly  if  the  open   end  be  held  downwards ;  but  quickly  and 
with  a  sudden  rush  of  flame  if  the  gas  be  allowed  to  escape  by 
holding  the  mouth  of  the  jar  upwards. 

(2)  That   hydrogen   does    not  support   the  combustion  of  a 
taper  may  be  shown  by  thrusting  a  burning  taper  into  a  jar  of 
hydrogen  held  with  its  mouth  downwards ;  the  gas  inflames  and 
burns  round  the  open  end  of  the  cylinder,  but  the  taper  goes 
out  and  may  be  rekindled  on  withdrawal  at  the  flame  of  burning 
hydrogen. 

(3)  Or  the  stream  of  gas  issuing  from  the  drawn-out  end  of 
a  tube  and  furnished  with  a  platinum  nozzle  attached  to  the 
generating  flask  may  be  ignited,  care  being  taken  that  all  the 

1  Graham,  Proc.  Roy.  Soc.  15,  502. 


140 


THE  NON-METALLIC  ELEMENTS 


air  has  previously  been  expelled,  when  the  flame  will  burn  with 
a  quiet  and  almost  colourless  flame. 

(4)  Owing  to  the  lightness  of  hydrogen  it  may  be  collected  by 
upward  displacement.  A  jar  filled  with  air  is  placed  over  the 
tube  by  which  the  gas  escapes  from  the  generating  flask;  in 
a  short  time  the  lighter  gas  will  have  displaced  (Fig.  29)  the 
heavier  air,  and  the  jar  is  found  to  be  full  of  hydrogen. 


FIG.  29. 


(5)  Another  striking  mode  of  showing  the  relative  weight  of 
air  and  hydrogen  has  already  been  described  in  Fig.  3,  page  43. 
The  suspended  beaker-glass  is  equipoised  by  weights  placed  in 
the  pan  at  the  other  end  of  the  beam  of  the  balance,  and  the  air 
is  then  displaced  by  pouring  upwards  the  hydrogen  contained 
in  a  large  cylinder.     The  beam  will  no  longer  be  horizontal, 
and  weights  must  be  placed  on  the  beaker-glass  to  restore  the 
equilibrium. 

(6)  Another  experiment   illustrating  the   same   property  of 


EXPERIMENTS  WITH  HYDROGEN  141 

hydrogen,  is  to  fill  a  cylinder  with  the  gas  and  to  bring  its  mouth 
downwards,  together  with  another  cylinder  filled  with  air,  also 
mouth  downwards  ;  by  gradually  lowering  the  end  of  the  hydro- 
gen cylinder  until  the  two  cylinders  come  mouth  to  mouth,  the 
hydrogen  will  be  found  in  the  upper  cylinder,  whilst  on  stand- 
ing for  a  moment  or  two  the  lower  one  will  be  found  to  be  full 
of  air. 


FIG.  30. 

(7)  Soap  bubbles  or  small  collodion  balloons  ascend  when 
filled  with  hydrogen  gas ;  the  caoutchouc  balloons,  now  so  com- 
mon, are  filled  and  expanded  by  forcing  hydrogen  in  with  a 
syringe,  as  seen  in  Fig.  30.  In  consequence  of  its  low  specific 
gravity,  hydrogen  used  to  be  frequently  employed  for  inflating 
balloons,  but  at  present  coal  gas  is  used  for  this  purpose. 


142  THE  NON-METALLIC  ELEMENTS 


THE  HALOGENS 
FLUORINE.     F  =  i8-g 

69.  FLUORINE  occurs  not  uncommonly  combined  with  calcium, 
forming  the  mineral  fluor-spar,  or  calcium  fluoride,  CaF2,  crys- 
tallizing in  cubes  and  octahedra,  and  found  in  Derbyshire,  the 
Harz,  Bohemia,  and  elsewhere.  It  is  likewise  contained  in 
other  minerals,  such  as  cryolite,  a  fluoride  of  aluminium  and 
sodium  (3NaF  + A1F3),  found  in  Greenland,  and  occurs  in  smaller 
quantities  in  fluor-apatite,  yttrocerite,  topaz,  lepidolite,  &c. 
Fluorine  has  been  detected  in  minute  traces  in  sea-water  and  in 
the  water  of  many  mineral  springs.  Nor  is  its  presence  con- 
fined to  the  mineral  kingdom,  for  it  has  been  found  in  the 
enamel  of  the  teeth  as  well  as  in  the  bones  of  mammalia,  both 
fossil  and  recent,  and  it  is  said  to  have  been  detected  in  the 
blood,  in  the  brain,  and  in  milk. 

The  fact  that  glass  can  be  etched  when  it  is  exposed  to  the 
fumes  arising  from  fluor-spar  heated  with  sulphuric  acid,  was 
known  towards  the  latter  part  of  the  seventeenth  century. 
Scheele  first  stated  that  fluor-spar  was  the  calcium  salt  of  a 
peculiar  acid,  which  he  obtained  in  an  impure  state  by  distilling 
a  mixture  of  sulphuric  acid  and  fluor-spar  in  a  tin  retort.  Scheele 
also  prepared  the  gaseous  tetrafluoride  of  silicon,  SiF4,  by  the 
action  of  the  acid  thus  produced  upon  silica.  It  is,  however,  to 
the  researches  of  Gay-Lussac,  and  Tbenard,1  that  we  are  indebted 
for  the  first  reliable  information  concerning  hydrofluoric  acid. 
The  views  then  held  concerning  this  compound  were  incorrect, 
inasmuch  as  it  was  supposed  to  contain  oxygen,  and  termed 
fluoric  acid,  until  Ampere  in  1810,  and  subsequently  Davy, 
showed  that  this  acid  is  analogous  to  hydrochloric  acid,  and 
that  fluor-spar,  formerly  termed  fluate  of  lime,  is,  in  fact,  a 
compound  analogous  to  calcium  chloride,  containing  the  metal 
calcium  combined  with  an  element  similar  to  chlorine,  termed 
fluorine  (from  fluo,  I  flow,  because  of  the  use  of  fluor-spar  as 
a  flux  in  smelting  operations).  Even  up  to  recent  years  the 
nature  and  constitution  of  the  fluorine  compounds  has  been 
discussed  ;  and  it  is  only  within  the  last  two  or  three  decades  that 

1  Ann.  Chim.  Phys.  [1],  69,  204. 


FLUORINE 


143 


)re's  researches  taken  together  with  the  preparation  of  organic 
luorides  have  definitely  proved  the  true  analogy  of  the  hydrogen 
mrpounds  of  fluorine  and  chlorine,  while  quite  recently  Moissan 
has  succeeded  in  isolating  fluorine,  and  thus  has  solved  one 
of  the  most  difficult  problems  of  modern  chemistry.  The 
reason  why  fluorine  has  for  so  long  resisted  the  innumerable 
attempts  which  have  been  made  to  isolate  it,  will  be  easily 
understood  from  its  properties. 

70  Moissan  obtained  fluorine  by  the  electrolysis  of  pure  anhydrous 
hydrofluoric  acid  in  which  some  potassium  hydrogen  fluoride  was 
dissolved  in  order  to  enable  the  liquid  to  conduct  the  electric 


FIG.  31. 

current  which  hydrofluoric  acid  by  itself  does  not  do.  The 
latest  form  of  apparatus  employed  by  Moissan  consists  of  a  U- 
shaped  tube  of  iridioplatinum  with  two  small  platinum  side 
tubes  attached,  and  possessing  a  capacity  of  about  160  cc.,  in 
which  a  mixture  of  about  100  grams  of  anhydrous  hydrofluoric 
acid  and  twenty  grams  of  potassium  hydrogen  fluoride  is  placed. 
The  construction  of  the  vessel  is  seen  in  Fig.  31 ;  the  open  ends 
are  closed  by  stoppers  (F)  of  fluor-spar,  ground  so  as  nearly  to 
fit  the  tube  and  wrapped  round  by  thin  platinum  foil.  The 


144 


THE  NON-METALLIC  ELEMENTS 


electrodes  of  iridio-platinum  (bb)  pass  through  the  stoppers 
which  are  held  in  position  by  brass  caps  and  screws  (E)  the  joints 
being  rendered  air-tight  by  placing  leaden  washers  at  p,  and 


coating  all  the  surfaces  with  shellac.  During  the  electrolysis, 
for  which  twenty-five  Bunsen  cells  arranged  in  series  are  required, 
the  platinum  U-tube  filled  with  the  mixture  of  hydrofluoric  acid 
and  potassium  fluoride  is  placed  in  a  glass  cylinder  as  shown  in 


FLUORINE 


145 


Fig.  32,  into  which  liquid  methyl  chloride  is  passed  from  the 
steel  cylinder.  This  liquid  at  once  boils  and  the  temperature  is 
reduced  to  —23°  at  which  the  electrolysis  is  carried  on.  A  second 
glass  cylinder  surrounds  that  in  which  the  methyl  chloride  is 
evaporating,  and  contains  fragments  of  calcium  chloride  to  dry 
the  air  and  thus  prevent  the  formation  of  hoar  frost  on  the  inner 
cold  cylinder.1 

Pure  hydrogen  is  evolved  from  the  negative  pole  and  is  carried 
off  by  the  platinum  exit-tube  to  the  left  of  the  figure.  Fluorine 
is  evolved  at  the  positive  pole  and  passes  from  the  U-tube  to  a 
spiral  tube  of  platinum  also  placed  in  a  glass  cylinder  containing 
rapidly  evaporating  methyl  chloride,  so  that  the  temperature  is 
kept  at  about— 50°.  This  serves  to  retain  hydrofluoric  acid 


FIG.  33. 


vapours  which  are  carried  over  with  the  gaseous  fluorine,  whilst 
the  latter  passes  on  through  two  platinum  tubes  containing 

I  lumps  of  sodium  fluoride  which  salt  absorbs  the  last  traces  of 
hydrofluoric  acid.  The  decomposition  is  not  merely  the  simple 
one  represented  by  the  equations 


(1) 

(2) 


2KF  =  2K  +  F2 
2K  +  2HF  =  2KF  + 


inasmuch  as  the  platinum  electrode  at  which  the  fluorine  is 
liberated  is  much  corroded  with  formation  of  a  certain  quantity 
of  a  black  powder  consisting  of  platinum  fluoride.  The  volume 


1  Ann.  Chim.  Phys.  [6],  12,  473  ;  [6],  24,  226. 


11 


146  THE  NON-METALLIC  ELEMENTS 

of  fluorine  obtained  with  this  apparatus  is  from  three  to  four 
litres  per  hour. 

71  Properties  of  Fluorine. — Fluorine  is  a  light  greenish-yellow 
gas,  paler  and  more  yellow  in  colour  than  chlorine,  possessing  a 
penetrating  odour  resembling  that  of  hypochlorous  acid.     It  has 
a  sp.gr.  (air=l)  of  1'265  and  does  not  liquefy  under  atmospheric 
pressure  at  a  temperature  of  —93°.     It  does  not  fume  in  dry  air 
but  in  presence  of   moisture  hydrofluoric  acid  is  formed  and 
ozone  set  free,  whilst  the  gas,  even  in  small  quantity  exerts  a 
most    irritating    effect   on   the   eyes   and    mucous   membrane. 
Fluorine  is  the  most  intensely  active  element  with  which  we  are 
acquainted ;  with  hydrogen  it  combines  explosively  even  in  the 
dark  and  at  temperatures  as  low  as  —  23°.     In  order  to  observe 
the  action  of  fluorine  on   hydrogen  and  other  gases  a  tube  of 
platinum  (Fig.  33)  closed  at  each  end  by  plates  of  transparent 
fluor-spar  is  employed.     The  fluorine  is  passed  into  the  observa- 
tion tube  by  one  of  the  thin  platinum  side  tubes,  the  other  gas 
by  the  second,  whilst  the  resultant  of  the  action  passes  out  by 
the  platinum  delivery  tube.     The  direct  combination  of  fluorine 
and  hydrogen  may  however  be  more  simply  shown  by  inverting 
a  jar  filled  with  hydrogen  over  the  positive  exit  tube  of  the 
electrolyte  apparatus.     As  soon  as  the  gas  comes  in  contact  with 
the  hydrogen  a  blue  red-bordered  flame  appears  at  the  end  of  the 
platinum  tube,  hydrofluoric  acid  being  formed,  which  slowly  at- 
tacks the  glass  jar. 

Fluorine  is  distinguished  from  all  the  other  elements  by  its 
inability  to  combine  with  oxygen,  no  combination  taking  place 
when  these  two  gases  are  brought  together  even  at  500°,  nor 
does  it  combine  directly  with  nitrogen. 

To  examine  the  action  of  fluorine  on  liquids  and  solids  it 
suffices  to  place  the  substance  to  be  examined  in  a  test  tube,  and 
to  allow  the  fluorine  to  pass  into  the  latter  from  the  electrolytic 
apparatus,  the  gas  having  no  action  on  dry  glass.  In  this  way  it 
can  be  shown  that  fluorine  at  once  liberates  chlorine  from  potas- 
sium chloride  and  carbon  tetrachloride  at  the  ordinary  tempera- 
ture ;  sulphur  and  selenium  at  once  melt  and  take  fire  in  the 
gas,  and  tellurium  like  the  former  elements  also  combines 
directly  with  formation  of  a  fluoride.  With  iodine,  bromine, 
phosphorus,  arsenic  and  antimony  it  combines  with  incandescence; 
crystallised  silicon,  amorphous  boron  and  finely  divided  carbon, 
such  as  lamp-black  and  charcoal,  when  thrown  into  fluorine  take 
fire  and  burn  with  formation  of  fluorides.  The  action  of  fluorine 


HYDROFLUORIC  ACID  147 

on  the  metals  is  even  more  decided  than  that  which  it  has  on 
the  non-metallic  elements ;  the  alkali-metals  and  those  of  the 
alkaline  earths  ignite  in  the  gas  ;  lead  is  slowly  transformed  into 
the  fluoride,  and  finely  divided  iron  becomes  red  hot  on  exposure 
to  the  gas.  Magnesium,  aluminum,  manganese,  nickel  and  silver 
when  slightly  warmed  burn  brightly  in  fluorine ;  gold  is  not  at- 
tacked at  the  ordinary  temperature,  but  between  300°  and  400° 
becomes  covered  with  a  yellow  coating  of  gold  fluoride,  and 
platinum  under  similar  conditions  yields  two  fluorides,  which 
like  the  gold  compound  readily  decompose  into  the  metal  and 
fluorine  at  a  dull  red  heat. 

In  order  to  prove  that  the  gas  evolved  by  electrolysis  is  in 
reality  fluorine  and  not  a  higher  hydrogen  compound  of  that 
element,  Moissan  attached  to  the  fluorine  delivery  tube  a  weighed 
platinum  tube  containing  iron  wire,  whilst  to  the  negative 
delivery  tube  an  arrangement  for  collecting  and  measuring  the 
hydrogen  was  attached.  On  starting  the  electrolysis,  and  heating 
the  platinum  tube  containing  the  iron  wire,  the  whole  of  the 
fluorine  was  absorbed  with  formation  of  iron  fluoride,  whilst  the 
hydrogen  simultaneously  evolved  was  collected  and  measured. 
As  the  mean  of  two  experiments  it  was  found  that  79  cc.  of 
hydrogen  weighing  (V00703  gram  were  obtained  ;  this  corre- 
sponds to  0'1335  of  fluorine  whereas  the  mean  increase  of  weight 
of  the  iron  was  0*135  gram. 

The  atomic  weight  of  fluorine  has  been  determined  by  several 
chemists  by  converting  either  calcium  fluoride,  potassium  fluoride, 
or  sodium  fluoride  into  the  corresponding  sulphate.  The  mean 
of  fairly  agreeing  experiments  gives  the  number  18'9. 


FLUORINE  AND  HYDROGEN. 

HYDROFLUORIC  ACID.  HF=19'9. 

72  Anhydrous  hydrofluoric  acid,  HF,  is  a  volatile  colourless 
liquid,  best  obtained,  according  to  Fremy1  and  Gore,2  by  heating 
to  redness  in  a  platinum  retort  the  double  fluoride  of  hydrogen 
and  potassium  HF  -f  KF,  which  has  been  previously  fused.  A 
description  of  the  process  employed  for  preparing  pure  hydro- 

Ann.  Chim.  Phys.  [3],  47,  5.  2  Phil.  Trans.  1869,  173. 


148  THE  NON-METALLIC  ELEMENTS 

fluoric  acid  may  give  an  idea  of  the  difficulty  and  danger  of 
chemical  investigations  on  fluorine  and  fluorides,  as  well  as  of 
the  precautions  which  must  be  taken. 

(1)  For  this  purpose  about  200  grammes  of  the  fused  salt  was 
placed  by  Gore  in  a  platinum  bottle,  or  retort  (a,  Fig.  34).  No 
vessels  of  glass,  porcelain,  or  other  substance  containing  silica 
can  be  used  in  the  preparation  of  this  acid,  as  the  silica  is  at 
once  attacked  by  hydrofluoric  acid  unless  it  is  absolutely  anhy- 
drous, a  volatile  tetrafluoride  of  silicon  and  water  being  formed, 
thus  : — 

4HF+SiO2=2H2O+SiF4. 

The  platinum  bottle  was  then  gently  heated  so  as  to  fuse  the 
salt,  and  thus  completely  drive  off  any  traces  of  water.  The 
long  platinum  tube  was  then  connected  by  means  of  a  lute  of 
fused  sulphur  to  the  neck  of  the  bottle,  the  condenser  surrounding 
this  tube  being  filled  with  a  freezing  mixture  poured  through 


FIG.  34. 

the  open  tube  &,  whilst  the  platinum  bottle  c,  immersed  in  a 
freezing  mixture,  was  employed  to  receive  the  distillate.  This 
bottle  was  provided  with  an  exit-tube  of  platinum,  upon  the 
upper  end  of  which  a  short  angle  tube  g  of  platinum,  turned 
downwards,  was  fixed  to  prevent  condensed  moisture  from  running 
down  into  the  bottle.  On  gradually  raising  the  temperature,  the 
fused  salt  begins  to  decompose,  hydrofluoric  acid  is  given  off 
as  a  gas,  which  condenses  in  the  platinum  tube  and  runs  into 
the  platinum  bottle.  Great  care  must  be  taken  to  have  all  the 
apparatus  free  from  moisture,  and  the  acid  must  be  re-distilled 
in  order  to  remove  traces  of  saline  matter  which  are  apt  to  be 


HYDROFLUORIC  ACID  149 

carried  over.  According  to  Moissan,  the  acid  thus  obtained  still 
contains  traces  of  moisture,  which  can  only  be  removed  by 
subjecting  the  liquid  to  electrolysis,  the  water  present  being 
then  decomposed  by  the  fluorine  evolved  with  formation  of 
hydrofluoric  acid  and  ozone. 

The  acid  thus  obtained  is  a  highly  dangerous  substance,  and 
requires  the  most  extreme  care  in  its  manipulation,  the  inhalation 
of  its  vapour  having  produced  fatal  effects.1  A  drop  on  the 
skin  gives  rise  to  blisters  and  sores  which  only  heal  after  a  very 
long  period.  From  its  great  volatility  the  anhydrous  acid  can 
only  be  safely  preserved  in  platinum  bottles  having  a  flanged 
mouth,  a  platinum  plate  coated  with  paraffin  being  tightly 
secured  to  the  flanged  mouth  by  clamp  screws.  The  acid  must 
be  kept  in  a  cool  place  not  above  a  temperature  of  15°,  other- 
wise it  is  very  likely  to  burst  the  bottle,  and  a  freezing  mixture 
should  always  be  at  hand  when  experimenting  with  it  (Gore.) 


FIGS.  35,  36. 

Anhydrous  hydrofluoric  acid  can  also  be  obtained  by  acting  on 
dry  silver  fluoride  with  hydrogen. 

(2)  If  the  hydrofluoric  acid  is  not  required  to  be  perfectly 
anhydrous  a  much  easier  process  than  the  foregoing  can  be 
adopted.  This  consists  in  the  decomposition  of  fluor-spar  by 
strong  sulphuric  acid,  when  hydrofluoric  acid  and  calcium 
sulphate  are  formed,  thus  : — 

CaF2  +  H2S04=2HF  +  CaSO4. 

For  this  preparation  vessels  of  platinum,  or,  on  the  large  scale, 
vessels  of  lead,  can  be  employed.  On  heating  the  mixture,  the 
nearly  anhydrous  acid  which  distils  over  can  either  be  condensed 

1  Professor  Nickles,  of  Nancy,  died  in  1869  from  accidentally  breathing  the 
vapour  of  this  acid  whilst  endeavouring  to  isolate  fluorine. 


150  THE  NON-METALLIC  ELEMENTS 

by  passing  through  a  tube  placed  in  a  freezing  mixture,  or  into 
a  small  quantity  of  water  contained  in  a  platinum  dish  if  a  dilute 
acid  be  needed.  The  dilute  acid  may  be  preserved  in  gutta- 
percha  bottles,  but  this  substance  is  at  once  acted  upon  by  the 
anhydrous  acid. 

One  form  of  platinum  apparatus  used  for  preparing  the  gas  is 
shown  in  Fig.  35.  The  U-tube  is  placed  in  a  freezing  mixture 
when  the  gas  has  to  be  condensed.  If  an  aqueous  solution  of 
the  acid  is  needed,  the  arrangement  shown  in  Fig.  36  may  be 
employed.  It  consists  of  a  leaden  retort,  a,  on  to  which  a  leaden 
head,  c,  can  be  cemented  at  IV.  The  neck  of  the  retort  fits  into 
a  leaden  receiver  at  e,  in  which  is  placed  a  platinum  basin  con- 
taining water.  The  acid  vapours  are  absorbed  by  the  water,  and 
thus  a  solution  of  the  acid  is  obtained  free  from  lead,  which 
would  not  be  the  case  if  the  water  had  been  simply  placed  in 
the  leaden  vessel.  The  tube  g  serves  to  allow  the  escape  of  air 
and  of  excess  of  hydrofluoric  acid  gas. 

73  Properties. — The  specific  gravity  of  liquid  anhydrous  hydro- 
fluoric acid  at  15°  is  G'9879  (Gore),  or  it  is  a  little  lighter  than 
water.  It  boils  at  19°'4,  and  solidifies  at  — 102°'5,  and  melts  again 
at  -  92°'3.  The  tension  of  its  vapour  at  15°  is  390mm.  If  it  is 
perfectly  dry  it  does  not  act  on  glass ;  the  slightest  trace  of 
moisture,  however,  renders  it  capable  of  doing  so.  The  acid 
scarcely  acts  upon  the  metalloids  or  on  the  noble  metals, 
and  the  other  metals  do  not  decompose  the  acid  below  20°. 
Potassium  and  sodium  dissolve  in  it  as  in  water  with 
evolution  of  hydrogen  and  formation  of  a  fluoride;  it  decom- 
poses the  carbonates  with  effervescence  and  with  formation  of 
fluorides. 

The  composition  by  volume  of  the  anhydrous  acid  was  ascer- 
tained by  Gore  by  measuring  the  volume  of  hydrogen  needed  to 
combine  with  the  fluorine  contained  in  a  given  weight  of  silver 
fluoride.  From  this  and  other  experiments  he  arrived  at  the 
conclusion  that  one  volume  of  hydrogen  necessarily  yields  two 
volumes  of  hydrofluoric  acid  gas,  and  that  this  contains  for  every 
one  part  by  weight  of  hydrogen  19*1  parts  by  weight  of 
fluorine. 

Thorpe  and  Hambly1  have  shown  that  the  vapour  density  of 
hydrofluoric  acid  varies  rapidly  with  variation  of  temperature 

1  Journ.  Chem.  Soc.  1889,  1,  163. 


HYDROFLUORIC  ACID  151 

and  pressure.     The  following  table  gives  the  results  of  their 
experiments  at  temperatures  between  26°"4  and  88°*1  : — 


Pressure 

Temp. 

of  Vapour 

V.D.  air  1 

Molecular 

in  mm. 

weight. 

26°'4 

745 

1-773 

51-18 

27°-8 

746 

1-712 

49-42 

29°-2 

750 

1-578 

45-54 

32°-0 

743 

1-377 

39-74 

33°-l 

750 

1-321 

38-12 

33°-8 

758 

1-270 

36-66 

36°-3 

739 

1-115 

32-20 

38°-7 

751 

1-021 

29-46 

39°'2 

743 

1-002 

28-94 

42°-8 

741 

0-910 

26-26 

47°-3 

745 

0-823 

23-76 

57°-5 

750 

0-737 

21-28 

69°-4 

746 

0-726 

20-96 

88°-l 

741 

0-713 

20-58 

These  numbers  show  that  the  process  of  dissociation  of  the 
vapour  of  hydrogen  fluoride  is  quite  continuous,  and  that  there- 
fore there  is  no  evidence  of  the  existence  of  molecules  H2F2, 
as  was  formerly  supposed.  It  was  also  found  that  the  vapour 
density  is  lowered  by  diminishing  the  pressure  of  the  gas  at 
a  constant  temperature  of  about  32°. 

Hydrofluoric  acid  is  very  soluble  in  water,  the  specific  gravity 
of  the  solution  rising  to  1'25.  The  concentrated  aqueous  acid 
becomes  weaker  on  boiling  until  at  120°  it  attains  a  constant 
composition  of  from  36  to  38  per  cent,  of  the  anhydrous  acid, 
but  it  does  not  thus  form  a  definite  hydrate ;  when  allowed  to 
evaporate  over  caustic  lime  in  the  air,  the  aqueous  acid  attains 
a  constant  composition  containing  32'6  per  cent,  of  the  anhydrous 
acid.1 

74  Qualitative  Detection  of  Fluorine. — In  order  to  test  for  the 
presence  of  hydrofluoric  acid,  its  power  of  etching  on  glass  is 
made  use  of.  For  this  purpose  a  small  flat  piece  of  glass  is 
covered  with  a  thin  and  even  film  of  melted  bees'-wax,  and, 
after  cooling,  some  lines  or  marks  are  made  by  removing  the  wax 
by  a  sharp  but  not  a  hard  point.  The  dry  substance  to  be  tested 

1  Roscoe,  Journ.  Chem.  Soc.  13,  162. 


152  THE  NON-METALLIC  ELEMENTS 

is  placed  in  a  platinum  crucible  or  small  leaden  cup,  and  covered 
with  strong  sulphuric  acid,  the  crucible  being  gently  warmed  ; 
after  the  lamp  has  been  removed,  the  slip  of  covered  glass  is 
placed  on  the  crucible  and  allowed  to  remain  for  ten  minutes. 
The  wax  can  then  be  re-melted  and  wiped  off  with  blotting 
paper,  when  the  etching,  indicating  the  presence  of  fluorine, 
will  be  seen.  In  performing  this  experiment  it  is  well  to  re- 
member, on  the  one  hand,  that  if  the  quantity  of  fluorine  present 
be  very  small,  the  etching  may  not  at  once  be  visible  but  may 
become  so  by  breathing  on  the  surface  of  the  glass,  whilst,  on 
the  other  hand, 'if  the  point  employed  to  remove  the  wax  be  a 
hard  one,  a  mark  or  scratch  may  sometimes  thus  be  seen  on  the 
glass  when  no  fluorine  is  present 

75  Fluorides. — The  compounds  of  fluorine  with  the  metals  are 
best  formed  by  acting  on  the  metal  or  on  its  oxide,  hydrate,  or 
carbonate,  with  hydrofluoric  acid.  The  fluorides  of  the  alkalis 
of  silver  as  well  as  those  of  most  heavy  metals  dissolve  in  water ; 
those  of  the  alkaline  earths  are  insoluble  ;  and  those  of  the 
earths,  with  the  exception  of  fluoride  of  yttrium,  are  soluble  in 
water.  Most  of  the  fluorides  unite  with  hydrofluoric  acid  to  form 
crystalline  compounds,  which  are  termed  the  acid  fluorides.  They 
also  have  a  remarkable  facility  of  union  among  themselves, 
giving  rise  to  double  salts,  which  frequently  crystallise  well. 
They  are  all  decomposed  by  treatment  with  sulphuric  acid, 
yielding  hydrofluoric  acid  and  a  sulphate,  whilst  some,  as  the 
silver  salt,  even  undergo  the  same  decomposition  in  presence  of 
hydrogen  alone. 

The  divisions  on  the  glass  of  eudiometers  and  thermometers 
are  etched  by  hydrofluoric  acid,  which  is  evolved  from  a  mixture 
of  fluor-spar  and  strong  sulphuric  acid  in  a  long  leaden  trough, 
over  which  are  placed  the  glass  tubes  covered  with  wax,  and 
having  the  divisions  marked  upon  them  by  scratching  off  the  wax 
The  etching  is  best  effected  in  the  cold,  and  with  anhydrous 
hydrofluoric  acid ;  the  tube  must  in  this  case  be  exposed  for 
some  hours  to  the  action  of  the  gas,  and  the  trough  covered  with 
several  folds  of  thick  paper. 


CHLORINE 


CHLORINE.  Cl.  =  35-19. 

76  Chlorine  gas  was  first  obtained  and  its  properties  first 
examined  by  Scheele  l  in  1774;  he  prepared  it  by  the  action 
of  hydrochloric  acid  on  manganese  ore,  and  termed  it  "  dephlo- 
gisticated  marine  acid  gas."  Berth  ollet,  in  1785,2  showed  that, 
according  to  the  then  prevailing  antiphlogistic  theory,  chlorine 
could  be  regarded  as  a  compound  of  hydrochloric  acid  gas 
with  oxygen,  and  this  view  of  its  constitution  was  held  until 
the  year  1810,  when  Davy3  satisfactorily  proved  the  elementary 
nature  of  the  gas  and  gave  it  the  name  which  it  now  bears 
(^\o)/)o9,  greenish-yellow),  Gay-Lussac  and  Thenard 4  having, 
in  the  year  1809  thrown  out  the  suggestion  that  it  might  be 
considered  to  be  a  simple  body. 

Chlorine  does  not  occur  in  the  free  state  in  nature,  but  is- 
found  in  large  quantities  combined  with  the  alkali  metals,, 
forming  the  chlorides  of  sodium,  potassium,  and  magnesium, 
which  constitute  the  largest  proportion  of  the  solid  components 
of  sea- water.  Sodium  chloride,  NaCl,  also  occurs  as  rock-salt, 
in  large  deposits  in  the  tertiary  formation  in  various  localities, 
whilst  the  chloride  of  potassium,  although  occurring  less 
frequently,  is  found  in  certain  localities,  as  in  the  salt-beds  of 
Stassfurt,  in  Germany,  both  in  the  pure  state,  as  sylvine  KC1,  and 
in  combination  with  chloride  of  magnesium  and  water,  as 
carnallite  KClMgCl2-t6H2O.  The  chlorides  and  oxychlorides  of 
several  other  metals  also  occur  in  nature,  although  in  small 
quantities;  thus  we  have  lead  oxychloride  PbCl2PbO,  known 
as  matlockite;  ferric  chloride  FeCl3,  found  in  the  craters 
of  volcanoes ;  silver  chloride,  or  horn  silver,  AgCl ;  copper 
>xychloride,  or  atacamite,  Cu2Cl(OH)3,  and  many  others. 

The  chlorides  of  the  alkalis  occur  in  the  bodies  of  plants  and 

limals,  and  play  an  essential  part  in  the  economy  of  the 
animal  and  vegetable  worlds.  Chlorine  likewise  occurs  combined 
with  hydrogen,  forming  hydrochloric  acid,  a  substance  which 

found  in  nature  in  small  quantities  in  certain  volcanic  gases. 

1  Opusc.  Tome  1,  247. 

2  Him.  de  I'Acad.  dcs  Science*,  Paris,  1785,  p.  276. 

3  Phil.  Trans.  1811,  pp.  1  and  32  ;  Bakerian  Lecture  for  1810,  read  Nov.  10th,. 
L810.  4  Memoires  d'Arcueil.     Tome  2,  357. 


154 


THE  NON-METALLIC  ELEMENTS 


77  Preparation. — (1)  Chlorine  gas  is  prepared  by  the  action 
of  the  black  oxide  of  manganese  or  manganese  dioxide  MnO2,  on 
strong  hydrochloric  acid,  HC1,  thus  :— 

4HC1+ MnO2=Cl2 + MnCl2+2H2O. 

This  reaction  consists  in  the  removal  of  two  atoms  of  hydrogen 
in  two  molecules  of  hydrochloric  acid  by  union  with  one  atom 
of  oxygen  of  the  manganese  dioxide  to  form  water,  the  two 
atoms  of  chlorine  being  set  free ;  whilst  the  manganese  mon- 
oxide MnO,  which  may  be  considered  as  also  being  formed, 
dissolves  in  the  two  remaining  molecules  of  hydrochloric  acid 
to  produce  manganese  chloride,  MnCl2,  and  another  molecule  of 
water,  H2O.  When  manganese  dioxide  and  cold  concentrated 


FIG.  37. 

hydrochloric  acid  are  brought  together  a  dark  brownish-green 
solution  is  formed,  and  this,  on  heating,  evolves  chlorine  gas, 
whilst  manganese  chloride,  MnCl2,  is  formed.  There  can  be 
little  doubt  that  this  dark  coloured  solution  contains  a  higher 
and  unstable  chloride  of  manganese,  probably  MnCl4,  corre- 
sponding to  MnO2,  which,  on  heating,  decomposes  into 
MnCl2  +  Cl2. 

For  this  preparation  the  oxide  of  manganese  should  be  used 
in  the  form  of  small  lumps  free  from  powder,  and  the  hydro- 
chloric acid  poured  on,  so  as  about  to  cover  the  solid ;  on  gently 
heating,  the  gas  is  copiously  evolved. 


PREPARATION  OF  CHLORINE  155 

(2)  It  is  often  more  convenient  for  laboratory  uses  to  evolve 
-the  hydrochloric  acid  in  the  same  vessel  in  which  it  is  acted 
upon  by  the  manganese  dioxide,  and  to  place  a  mixture  of  one 
part  of  this  substance  and  one  part  of  common  salt  in  a  large 
flask  (Fig.  37)  containing  a  cold  mixture  of  two  parts  of  strong 
sulphuric  acid  and  two  of  water ;  on  very  slightly  warming  the 
mixture,  a  regular  evolution  of  gas  takes  place.  The  change 
which  here  occurs  was  formerly  supposed  to  be  represented  by 
the  equation  :— 

2NaCl  +  3H2SO4-f  MnO2=Cl2+2NaHSO4  +  MnSO4  +  2H2O. 

according  to  which  the  whole  of  the  chlorine  is  evolved  in  the 
free  state.  Klason  has  shown,  however,1  that  this  is  not  the  case, 
the  correct  equation  being  as  follows  : — 

4NaCl+Mn02 I  2H2SO4-2NaHSO4+Na2SO4+MnCU2H2O+Cl2 

In  order  to  purify  and  dry  the  gas  prepared  by  either  of  the 
above  methods,  it  is  necessary  to  pass  it,  first  through  a  wash 
bottle  (&,  Fig.  38)  containing  water,  to  free  it  from  any  hydrochloric 
acid  gas  which  may  be  carried  over,  then  through  a  second  wash 
bottle  (a)  containing  strong  sulphuric  acid,  to  free  it  from  the 
larger  quantity  of  the  aqueous  vapour  which  it  takes  up  from 
the  water,  and  lastly  through  a  long  inclined  tube  (c)  containing 
pieces  of  pumice-stone,  moistened  with  strong  and  boiled  sul- 
phuric acid.     The  tube  (d)  which  dips  under  water  serves  as  a 
safety-tube  in  case  the  evolution  of  gas  becomes  too  rapid,  when 
bhe  excess  of  gas  can  thus  escape.     In  order  to  expel  the  air 
rhich  fills  the  apparatus,  the   evolution  of  the  chlorine  must 
allowed  to  go   on  until  the   gas  is  almost  entirely  absorbed 
)y  a  solution  of  caustic   soda.     As   the   crude   black  oxide  of 
langanese  frequently  contains  carbonate  of  lime,  the  presence 
)f  which  will  cause  the  admixture  of  small  quantities  of  carbon 
iioxide,  CO2  with  the  chlorine,  it  is  advisable  to  moisten   the 
>re  before  using  it  with  warm  dilute  nitric  acid,  which  will 
lissolve   out   the    carbonate    of  lime,   leaving   the   manganese 
Iioxide  unacted  upon  ;  after  well  washing,  the  latter  may  be 
ised  without  danger  of  this  impurity. 

(3.)  By  heating  a  mixture  of  bichromate  of  potash  (potassium 
lichromate)    and   hydrochloric   acid,   chlorine  gas  can  also  be 

1  Ber.  23,  334. 


156  THE  NON-  METALLIC  ELEMENTS 


obtained,   chromium   chloride    and   potassium    chloride    being 
formed  ;  thus  : — 

14HCl-|-K2Cr2O7=3Cl2-f2CrCl3+7H2O-h2KCl. 
Chlorine  gas  is  also  evolved  when   an  acid  is  added  to  an 


FIG.  38. 

alkaline  hypochlorite  or  to  bleaching  powder ;  to  prepare  it  from 
the  latter,  the  bleaching  powoler  is  made  coherent  by  mixing 
with  plaster  of  Paris  or  by  compressing  it  to  form  a  solid  cake 
which  is  afterwards  broken  into  lumps  of  suitable  size.1  As  the 

Winkler,  Ber.  20,  184  ;  Thiele,  Annalen,  253,  239. 


PREPARATION  OF  CHLORINE  157 

•evolution  of  the  gas  takes  place  at  the  ordinary  temperature,  the 
Kipp's  apparatus  shown  later  under  sulphuretted  hydrogen  may 
be  employed,  by  which  a  current  of  the  gas  can  be  obtained  as 
required. 

(4)  By  passing  a  mixture  of  air  and  hydrochloric  acid  over 
heated  bricks,  a  portion  of  the  hydrogen  of  the  hydrochloric 
acid  is  oxidised,  water  and  chlorine  gas  being  formed  (Oxland, 
1847).  If  the  mixture  of  gases  be  allowed  to  pass  over  a  heated 
surface  impregnated  with  certain  metallic  salts,  especially  sul- 
phate of  copper,  the  oxidation  of  the  hydrogen  of  the  hydro- 
chloric acid  goes  on  to  a  greater  extent,  and  by  the  absorption  of 
the  unaltered  hydrochloric  acid,  a  mixture  of  chlorine  and  nitrogen 
gases  can  be  obtained.  This  process,  patented  by  the  late 
Mr.  Henry  Deacon,  of  Widnes,  is  used  on  a  large  scale  for  the  eco- 
nomic production  of  chlorine  and  bleaching  powder.  The  singular 
.and  but  imperfectly  understood  decomposition  which  takes  place 
may  be  shown  on  a  small  scale  by  the  following  arrangement : — 

The  hydrochloric  acid  gas  is  evolved  from  the  common  salt  and 
sulphuric  acid  in  the  large  flask  a,  Fig.  39,  this  gas  passes  into 
the  tube,  b,  in  which  are  placed  pieces  of  tobacco-pipe  moistened 
with  a  saturated  solution  of  copper  sulphate,  the  tube  being 
exposed  to  a  gentle  heat.  As  the  hydrochloric  acid  enters 
"the  tube  containing  the  sulphate  of  copper  it  mixes  with 
atmospheric  air  which  is  driven  in  by  the  tube  c,  from  the  gas- 
holder. On  passing  over  the  heated  copper  sulphate,  the  hydro- 
chloric acid  and  the  oxygen  of  the  air  act  upon  one  another, 
water  and  chlorine  gas  being  formed  according  to  the  equation 

4HC1  +  02  =  2H20  +  2C12. 

During  the  process  the  sulphate  of  copper  remains  unchanged 
-and  may  be  used  for  a  great  length  of  time.  The  mixture  of 
•chlorine,  nitrogen,  steam,  and  any  undecomposed  hydrochloric 
acid  pass  by  a  bent  tube  into  a  bottle,  e,  containing  water,  by 
which  the  last-named  substance  is  arrested,  together  with  a 
portion  of  the  steam  which  is  condensed  ;  the  mixed  gases,  still 
containing  some  aqueous  vapour,  are  then  passed  through  a 
tube,  d,  containing  calcium  chloride,  by  which  the  gases  are 
completely  dried,  after  which  <the  chlorine  mixed  with  the 
nitrogen  may  be  collected  by  displacement  in  a  cylinder. 

(5.)  Chlorine  gas  is  prepared  for  manufacturing  purposes  on 
a  large  scale  by  means  of  reaction  (1) ;  the  mixture  of  black 
oxide  of  manganese  and  hydrochloric  acid  being  placed  in  large 


158 


THE  NON-METALLIC  ELEMENTS 


square  tanks,  made  of  Yorkshire  flags  clamped  together  by  iron 
rods,  and  the  joints  made  tight  by  a  rope  of  vulcanised  caout- 
chouc. On  heating  the  mixture  by  a  steam-pipe  the  chlorine 


PROPERTIES  OF  CHLORINE  159 

gas  is  evolved.     For  a  description  of  the  details  of  this  mode  of 
manufacture,  see  the  paragraph  on  Bleaching  Powder. 

(6.)  A  large  number  of  processes  have  been  patented  during 
the  past  few  years  for  preparing  chlorine  from  the  calcium  or 
magnesium   chlorides    obtained   in   large  quantities  as  a  bye- 
product  in  the  manufacture  of  soda  from  common  salt  by  the 
ammonia-soda  process  (Vol.  II.,  Part  1,  p.  153),  the  most  suc- 
cessful of  which  is  known  as  the  Weldon-Pechiney  process,  and 
is  worked  directly  in  connection  with   the  soda  manufacture. 
The  mother-liquors  containing  ammonium  chloride  are  heated 
with  magnesia  in  order  to  recover  the  ammonia,  and  the  residual 
solution  of  magnesium  chloride  evaporated  until  it  has  the  com- 
position MgCl2+  6H20,  and  then  thoroughly  mixed  with  1'5  parts 
of  magnesia  for  every  1  part  of  magnesium  chloride  present.    The 
mass  quickly  hardens  with  evolution  of  considerable  quantities 
of  heat,  and  is  broken  up  into  pieces  of  the  size  of  a  walnut,  the 
dust  formed  at  the  same  time  being  added  to  a  fresh  portion  of 
magnesium    chloride    solution.      The   magnesium   oxychloride 
Mg2OCl2  thus  formed  is  heated  in  air  first  to  250 — 300°,  whereby 
the  water  is  evolved,  together  with  at  most  8°/0  of  the  chlorine 
in  the  form  of  hydrochloric   acid.      The  temperature  is  then 
gradually  raised  until  it  reaches  more  than  1000°,  the  mixture 
of  chlorine,  hydrochloric  acid,  and  air  being  continuously  drawn 
off,  the  hydrochloric  acid  removed  by  washing  with  water  and 
the    chlorine   converted   into   chloride    of    lime    or    potassium 
chlorate.     45°/0  of  the  chlorine  is  evolved  in  the  free   state, 
40°/0  as  hydrochloric  acid,  and  15°/0  remains  in  the  residue.    The 
latter  is  sifted,  and  the  coarser  portions,  which  contain  all  the 
undecompdsed  chlorides,  again  heated,  whilst  the  finely  divided 
portion,  consisting  of  magnesia,  is  again  employed  for  decom- 
posing the  ammonium  chloride,  or   for    converting   the  mag- 
nesium chloride  into  the  oxychloride.1 

78  Properties. — Chlorine  at  the  ordinary  atmospheric  tempera- 
ture and  pressure  is  a  transparent  gas  of  a  greenish-yellow 
colour,  possessing  a  most  disagreeable  and  powerfully  suffocating 
smell,  which,  when  the  gas  is  present  in  small  quantities  only,  re- 
sembles that  of  seaweed,  but  when  it  is  present  in  large  quantities 
acts  as  a  violent  irritant,  producing  coughing,  inflammation  of  the 
mucous  membranes  of  the  throat  and  nose,  and  when  inhaled 
in  the  pure  state  even  causing  death. 

1  SeeDewar,  Journ.  Soc.  Chem.  2nd.  6,  775  ;  Kingzett,  Journ.  Soc.  Chem.  2nd. 
7,  286. 


160 


THE  NON-METALLIC  ELEMENTS 


The  density  of  chlorine  has  been  determined  by  Ludwig,1  who 
has  shown  that  at  the  ordinary  temperature  it  is  equal  to  2*48 
(air=l),  but  that  it  gradually  decreases  with  increase  of  tempera- 
ture till  at  200°  it  has  the  constant  value  of  2*45  (air  =  1)  or 
35*26  (H  =  1).  The  molecule  of  the  gas  therefore  consists  of 
two  atoms,  and  its  molecular  formula  is  C12,  the  theoretical 
density  being  35*19.  The  density  remains  constant  up  to  about 
1200°,  but  above  this  temperature  it  begins  to  decrease,  and  at 
1400°  is  only  2*02,  the  molecules  C12  being  partly  split  into  the 
atoms  at  this  temperature.2 

One  litre  of  the  gas  under  the  normal  conditions  weighs  3*208 
grams. 


FIG.  40. 

When  subjected  to  pressure  at  the  ordinary  temperature,  or  to 
a  temperature  below—  34°  under  atmospheric  pressure,  chlorine 
condenses  to  a  liquid  which  solidifies  at— 1020,3  boils  at  — 33*6°,4 
and  at  0°  has  a  sp.  gr.  of  1*4405  and  a  vapour  pressure  of  3*66 
atmospheres.  The  critical  point  of  the  liquid  is  1460.5 

Liquid  chlorine  has  a  yellow  colour,  with  a  tinge  of  orange  in 
thick  layers,  is  not  miscible  with  water,  does  not  conduct 

1  Ber.  1,  232. 

2  Langer  and  Meyer,  Pyrochem.  Untersiwh,  p.  46  (Vieweg,  1885). 

3  Olzewski,  Monatsh.  5,  127. 

4  Faraday,  Phil.  Tram.  1823,  160.        5  Knietsch,  Annalen,  253,  100. 


I    UNIVERSITY  j 


COMBUSTIONS  IN  CHLORINE 


161 


electricity,  and  has  a  refractive  index  lower  than  that  of 
water ;  it  is  now  prepared  commercially  and  brought  into  the 
market  in  cylinders  containing  5  kilos,  of  the  liquid. 

Chlorine  gas  dissolves  in  about  half  its  volume  of  cold  water, 
and  as  the  gas  instantly  attacks  mercury,  it  must  either  be  col- 
lected in  the  pneumatic  trough  over  hot  water,  or  by  displacing 
the  air  from  a  dry  cylinder,  as  shown  in  Fig.  38,  care  being 


FIG.  41. 


FIG.  42. 


taken  that  the  excess  of  chlorine  is  allowed  to  escape  into  a 
draught  cupboard,  as  represented  in  the  drawing. 

79  Combustions  in  Chlorine. — Chlorine  is  not  inflammable, 
and  does  not  directly  combine  with  oxygen  ;  it  unites,  however, 
with  great  energy  with  hydrogen,  forming  hydrochloric  acid 
HC1,  and  to  this  property  it  owes  its  peculiar  and  valuable 
bleaching  power.  It  also  combines  with  many  metals,  giving 
rise  to  a  class  of  compounds  termed  the  metallic  chlorides. 

In  each  case  of  combination  with  chlorine  a  definite  quantity 
of  heat  is  given  out,  whilst  sometimes  light  is  also  emitted,  so 

12 


162  THE  NON-METALLIC  ELEMENTS 

that  the  essential  phenomena  of  combustion  are  observed.  Thus 
if  we  plunge  a  jet  from  which  a  flame  of  hydrogen  burns  into 
a  cylinder  of  chlorine  gas  (Fig.  40),  the  hydrogen  continues  to- 
bum,  but  instead  of  water  being  produced,  hydrochloric  acid  is 
formed  by  the  combustion.  In  like  manner,  if  we  bring  a  light 
to  the  mouth  (held  downwards)  of  a  cylinder  of  hydrogen  and 
then  bring  this  over  a  jet  from  which  chlorine  gas  is  issuing 
(Fig.  41),  a  flame  of  chlorine  burning  in  hydrogen  will  be  seen. 

If  two  equal  sized  cylinders,  filled,  one  with  dry  chlorine 
and  the  other  with  dry  hydrogen,  are  brought  mouth  to  mouth, 
the  two  glass  plates  closing  them  withdrawn  and  the  gases 
allowed  to  mix,  and  if  then  a  flame  is  brought  near  the  mouths 
of  the  cylinders,  the  mixed  gases  combine  with  a  peculiar  noise, 
and  dense  fumes  of  hydrochloric  acid  gas  are  seen.  This  experi- 
ment must,  however,  be  made  in  a  room  partially  darkened,  or 
performed  by  gas-  or  candle-light,  as  the  two  gases  combine 
with  explosion  in  sunlight  or  strong  daylight. 

The  following  experiments  are  cited  as  showing  the  power 
with  which  chlorine  unites  with  hydrogen,  even  when  the  latter 
is  combined  with  some  other  element. 

(1.)  If  four  volumes  of  chlorine,  C14,  be  mixed  with  two 
volumes  of  olefiant  gas,  C2H4,  and  a  light  applied  to  the 
mixture,  the  chlorine  immediately  combines  with  the  hydrogen 
of  the  latter  to  form  hydrochloric  acid,  HC1,  while  the  carbon 
is  set  free  in  the  form  of  a  black  smoke  thus : — C2H4  +  2C12 
=  4HC1+  20. 

(2.)  If  a  piece  of  filter-paper  be  dipped  in  oil  of  turpentine, 
C10H16,  and  plunged  into  a  jar  of  chlorine  gas,  the  paper  bursts 
into  flame ;  the  chlorine  combining  with  the  hydrogen  of  the 
turpentine,  while  the  carbon  is  deposited. 

(3.)  When  sulphuretted  hydrogen  gas,  H2S,  is  passed  into 
chlorine  water,  hydrochloric  acid  is  formed  by  the  union  of  the 
hydrogen  of  the  former  with  the  chlorine  of  the  latter,  and  the 
sulphur  is  set  free  in  the  form  of  a  light  yellow  precipitate. 

(4.)  When  a  lighted  taper  is  plunged  into  a  jar  of  chlorine,  it 
continues  to  burn  with  a  dull  red  light,  and  dense  fumes  as  well 
as  a  cloud  of  black  smoke  are  emitted,  arising  from  the  com- 
bination of  the  hydrogen  of  the  wax  with  the  chlorine,  and  the 
liberation  of  the  carbon. 

In  order  to  exhibit  the  combination  of  certain  elements  with 
chlorine,  whereby  heat  and  light  are  evolved,  the  following 
experiments  may  be  made  : — 


EXPEKIMENTS  WITH  CHLORINE  163 

(1)  Place  some  leaves  of  Dutch  metal  (copper  in  thin 
leaves)  in  a  flask  provided  with  a  stopcock  (Fig.  42)  and  exhaust 
the  flask  with  the  air — or  water — pump  ;  attach  the  outer  end 
of  the  stopcock  to  the  neck  of  another  flask  containing  moist 
chlorine  gas.  On  opening  the  stopcock  the  chlorine  will  rush 
into  the  vacuous  flask,  and  the  copper  leaf  will  take  fire,  dense 
yellow  fumes  of  copper  chloride  being  formed.  (2.)  Finely 
powdered  metallic  antimony  thrown  into  a  jar  of  chlorine  gives 
rise  to  a  shower  of  brilliant  sparks,  chloride  of  antimony  being 
produced.  If  the  jar  be  placed  on  the  table  over  a  powerful 
down-draught,  all  risk  of  escaping  fumes  will  be  avoided.  (3.) 
A  small  piece  of  phosphorus  placed  in  a  deflagrating  spoon  and 
plunged  into  a  jar  of  chlorine  first  melts,  and,  after  a  few  minutes, 
bursts  into  flame  with  formation  of  the  chlorides  of  phosphorus. 
(4.)  Metallic  sodium  melted  in  a  spoon  also  takes  fire  OK  im- 
mersion in  the  moist  gas,  burning  brightly,  with  the  production 
of  common  salt — sodium  chloride ;  but  it  is  a  singular  fact,  first 
observed  by  Wanklyn1  that  sodium  may  be  melted  in  dry  chlorine 
without  any  combination  occurring,  the  surface  of  the  molten 
metal  remaining  bright  and  lustrous.  Cowper2  has  shown  that 
other  metals  are  also  not  attacked  by  dry  chlorine. 

80  Alleged  Allotropic  Condition  of  Chlorine. — It  has  been  stated 
by  Draper3  that  chlorine  which  has  been  exposed  to  light,  com- 
bines with  hydrogen  more  easily  than  that  which  has  been  kept  in 
the  dark,  and  the  conclusion  has  been  drawn  that  chlorine  exists 
in  two  allotropic  modifications.  Subsequent  careful  experiments 
made  on  this  subject  by  Bunsen  and  Roscoe,4  failed  to  detect  the 
slightest  difference  between  insolated  and  non-insolated  chlorine. 

According  to  the  observations  of  Budde  5  dry  chlorine  gas, 
when  exposed  to  the  chemically  active  rays  of  the  sun  increases 
in  volume,  and  the  expansion  is  not  due  to  any  action  of  the 
heating  rays.  This  subject  has  been  recently  investigated  by 
Richardson,6  who  has  confirmed  Budde's  results,  and  has 
further  shown  that  the  expansion  is  proportional  to  the  actinic 
intensity  of  the  light.  If  the  light  be  previously  passed  through 
cobalt  glass,  expansion  still  takes  place,  though  to  a  lesser 
extent,  but  if  faintly  yellow  or  ruby  glass  be  used  no  expansion 
is  observed. 

1  Journ.  Chem.  Soc.  1883,  i.  153.  2  Chem.  News,  20,  271. 

3  Phil.  Mag.  for  1845,  27,  327  ;  ditto  for  1857,  14,  161. 

4  Thil.  Trans.  1857,  Part  ii.  p.  378. 

5  Phil.  Mag.  1871  [4]  42,  290.  6  Phil.  Mag.  1891  [5]  32,  277. 


164  THE  NON-METALLIC  ELEMENTS 


Richardson  has  availed  himself  of  this  property  of  chlorine 
to  construct  a  continuous  recording  actinometer.  The  apparatus 
consists  of  two  bulbs  connected  by  a  narrow  tube,  one  of  which  is 
filled  with  dry  air  and  the  other  with  dry  chlorine,  sulphuric  acid 
being  employed  to  separate  the  two  gases.  The  bulbs  are  fixed  on 
the  beam  of  a  balance  in  such  a  manner  that  the  flow  of  acid  from 
one  arm  to  the  other  produces  a  movement  of  the  beam,  which  is 
communicated  by  means  of  a  lever  to  a  pen  and  is  recorded  on  a 
rotating  drum.  By  means  of  an  ingenious  compensating  arrange- 
ment the  expansion  caused  by  the  heat  rays  is  eliminated.1 

The  cause  of  the  expansion  which  occurs  when  chlorine  is 
acted  upon  by  the  actinic  rays  is  as  yet  unknown.  It  may 
possibly  be  due  to  the  fact  that  under  these  conditions  some  of 
the  diatomic  molecules  C1.2  are  dissociated  into  monatomic  mole- 
cules, the  change  being  similar  to  that  which  takes  place  when 
chlorine  is  exposed  to  temperatures  above  1200°,  It  has  also 
been  suggested  by  Tyndall 1  that  the  expansion  is  due  to  the 
great  absorptive  power  of  chlorine  for  the  more  refrangible  rays, 
and  its  low  emission  power  for  the  less  refrangible  rays. 

8 1  Bleaching  Poiver  of  Chlorine. — The  characteristic  bleaching 
action  which  chlorine  exerts  upon  organic  colouring  matters, 
and  which  has  become  of  such  enormous  importance  in  the 
cotton  and  paper  trades,  depends  upon  its  power  of  combining 
with  hydrogen.  This  bleaching  action  takes  place  only  in 
presence  of  water,  the  colouring  matter  being  oxidised  and 
destroyed  by  the  liberated  oxygen  of  the  water,  whilst  the 
hydrogen  and  chlorine  combine  together. 

That  dry  chlorine  does  not  act  upon  colouring  matters  may  be 
readily  shown  by  immersing  a  piece  of  litmus  paper,  or,  better 
still,  a  small  piece  of  turkey-red  cloth,  previously  well  dried, 
in  a  jar  of  dry  chlorine,  when  the  colour  will  remain  for  hours 
unaltered,  whereas  the  addition  of  a  small  quantity  of  water 
causes  its  immediate  disappearance. 

Chlorine  cannot  as  a  rule  destroy  mineral  colours,  nor  can  it 
bleach  black  tints  produced  by  carbon ;  this  is  well  shown  by 
rendering  illegible  the  ordinary  print  (printer's  ink  is  made  with 
lamp-black  or  carbon)  on  a  piece  of  card  or  paper,  by  covering 
the  whole  with  common  writing  ink  (which  generally  consists 
of  the  iron  salts  of  organic  acids).  On  immersing  the  blackened 
card  in  moist  chlorine  gas,  or  in  a  solution  of  chlorine  water,  the 
printed  letters  will  gradually  make  their  appearance. 

1  Phil.  Mag.  loc.  cit.  p.  283.  2  Pogg.  Ann.  144,  213. 


BLEACHING  ACTION  OF  CHLORINE 


165 


Chlorine  also  possesses  powerful  disinfecting  properties,  and 
the  gas  is  largely  used  for  the  destruction  of  bad  odours  and  of 
the  poisonous  germs  of  infectious  disease  floating  either  in  the 
tir  or  in  water.  It  is  probable  that  this  valuable  property  also 
depends  upon  the  oxidation,  and  consequently  the  destruction  of 
these  poisonous  emanations  and  miasmata. 

82  Chlorine  and  Water.     Hydrate  of  Chlorine,  Cl  -{-  5H2O.— 
When  chlorine  gas  is  passed  into  water  a  few  degrees  above 


FIG.  43. 


the  freezing-point,  a  solid  crystalline  compound  of  the  gas  and 
water,  termed  chlorine  hydrate,  is  formed.  By  quickly  pressing 
the  crystals  between  blotting-paper,  they  may  be  freed  from 
adhering  water  and  analysed.  Faraday  found  that  they  con- 


FIG.  44. 

tained  27'70  per  cent,  of  chlorine,  showing  that  they  are 
composed  of  one  atom  of  chlorine  to  five  molecules  of  water,  the 
hydrate  having,  therefore,  the  composition  Cl4-oH20.  Accord- 
ing to  Bakhuis  Roozeboom  they  have  the  composition  C1  +  4H2O 
or  C12  +  SH^O.1  This  hydrate  forms  beautiful,  apparently  regular 

1  Rec.  Trav.  Chim.  3,  59. 


166  THE  NON-METALLIC  ELEMENTS 

octohedra,  and  decomposes  readily,  the  critical  point  of  decom- 
position being  9°'6  in  open  and  28°'7  in  closed  vessels  ;  in  the 
latter  case  it  forms  two  layers,  one  of  liquid  chlorine,  and  the 
other  of  the  aqueous  solution  of  the  gas.  This  decomposition 
of  the  hydrate  may  be  made  use  of  as  the  most  ready  way 
of  preparing  liquid  chlorine.  For  this  purpose  the  dried  hydrate 
is  placed  in  the  limb  (ab)  of  a  strong  bent  glass  tube  (Fig.  43). 
The  open  limb  is  then  sealed  whilst  the  hydrate  is  kept  cool 
by  dipping  the  other  limb  into  a  freezing  mixture  ;  after  the  tube 
is  closed,  the  Hmb  (a  b)  is  placed  in  a  vessel  of  lukewarm  water 
and  the  crystals  then  resolve  themselves  into  two  distinct  layers 
of  yellow  liquid,  the  lower  of  which  is  liquid  chlorine.  By  placing 
the  limb  (be)  in  a  freezing  mixture  the  liquid  chlorine  distils  over, 
leaving  the  less  volatile  aqueous  solution  of  chlorine  behind. 


FIG.  45 

Aqueous  solution  of  Chlorine  possesses  a  greenish-yellow 
colour  and  smells  strongly  of  the  gas.  Chlorine  is  most  soluble 
in  water  at  10°,  as  below  this  temperature  the  formation  of  the 
hydrate  commences,  and  as  the  temperature  increases  above  10° 
the  solubility  diminishes,  until  at  100°  no  gas  dissolves.  It  is 
prepared  by  passing  washed  chlorine  gas  through  water  as  shown 
in  Fig.  44,  chlorine  being  evolved  in  the  flask  A,  and  the 
solution  of  the  gas  obtained  in  the  bottles  c,  D,  and  E. 

If  we  wish  to  absorb  in  water  the  whole  of  a  small  quantity 
of  chlorine  gas  evolved  in  a  given  reaction,  the  apparatus 
represented  in  Fig.  45  may  be  used.  Chlorine  is  led  by  a  gas 
delivery-tube  into  an  inverted  retort  having  a  wide  neck  and 
filled  with  water  ;  the  gas  displaces  some  of  the  water,  collects  in 
the  upper  portion  of  the  retort,  and  may  there  be  absorbed  and 
its  quantity  estimated. 


CHLORINE  AND  WATER 


167 


83  The  absorption  co-efficient  of  chlorine  in  water  between 
10°  and  41°"5,  is  given  by  the  equation 

C  =  3-0361  -  0-046196t  +  O'OOOllOTt2 

from  which  the  following  values  are  obtained. 

One  volume  of  water  absorbs  the  following  volumes  of  chlorine 
gas  calculated  at  0°  and  760  mm.1 


Temperatures 

Centigrade.  Coefficient. 

10  .  .  2-5852 

11  .  .  2-5413 

12  .  .  2-4977 

13  .  .  2-4543 

14  .  .  2-4111 

15  .  .  2-3681 

16  .  .  2-3253 

17  .  .  2-2828 

18  .  .  2-2405 

19  .  .  2-1984 

20  .  .  2-1565 

21  .  .  2-1148 

22  .  .  2-0734 

23  ..  2-0322 

24  .  .  1-9912 

25  1-9504 


Temperatures 

Difference. 

Centigrade. 

Coefficient. 

26    .    . 

1-9099    . 

0-0439 

27    .    . 

1-8695    . 

0-0436 

28    .    . 

1-8295     . 

0-0434 

29    .    . 

1-7895    . 

0-0432 

30    .    . 

1-7499    . 

0-0430 

31    .    . 

1-7104    . 

0-0428 

32    .    . 

1-6712    . 

0-0425 

33    .    . 

1-6322    . 

0-0423 

34    .    . 

1-5934    . 

0-0421 

35    .    . 

1-5550    . 

0-0419 

36    .    . 

1-5166    . 

00417 

37    .    . 

1-4785    . 

0-0414 

38    .    . 

1-4406    . 

0-0412 

39    .    . 

1-4029    . 

0-0410 

40    .    . 

1-3655    . 

0-0408 

• 

Difference. 

0-0405 
0-0404 
0-0400 
0-0400 
0-0396 
0-0395 
0-0392 
0-0390 
0-0388 
0-0384 
0-0384 
0-0381 
0-0379 
0-0377 
0-0374 


If  chlorine  mixed  with  another  gas,  such  as  hydrogen  or 
carbon  dioxide  C02,  be  passed  into  water  at  temperatures 
between  11°  and  38°,  the  volume  of  absorbed  chlorine  is  found 
to  be  greater  2  than  that  calculated  from  the  law  of  Dalton  and 
Henry  for  partial  pressures,  and  this  excess  of  dissolved  chlorine 
varies  in  amount  with  the  temperature  of  the  water,  and 
with  the  nature  of  the  other  gas  present. 

Saturated  chlorine  water  gives  off  chlorine  freely  on  exposure 
to  the  air ;  it  bleaches  organic  colouring  matters,  and  if  free 
from  hydrochloric  acid,  it  does  not  redden  a  piece  of  blue 
litmus  paper  before  it  bleaches  it.  When  chlorine  water  is 
exposed  to  direct  sunlight,  it  undergoes  decomposition  if  suf- 
ficiently dilute,  with  formation  of  hydrochloric  acid  and  oxygen  : 

2H2O  +  2C12  =  4HC1  4-  02. 

1  Schonfeld,  Annalen,  93,  26  ;  96,  8.        2  Roscoe,  Journ.  Chem.  Soc.  1856,  14. 


168 


THE  NOX- METALLIC  ELEMENTS 


It  has  been  proposed  to  employ  this  reaction  in  measuring  the 
chemical  action  of  light,  brut  the  decomposition  is  not  sufficiently 
regular  for  this  purpose ;  thus  Pedler l  has  shown  that  a  solution 
containing  1  molecule  of  chlorine  to  64  of  water  undergoes  no 
appreciable  alteration  during  two  months'  exposure  to  tropical 
sunlight,  whilst  more  dilute  solutions  undergo  more  or  less 
decomposition,  as  shown  in  the  following  table : 

Mols.  H20  for  Percentage  of  Cl 

1  mol.  Cl.  acting  on  water. 

64 no  action 

88 29 

130 46 

140 29 

412    .    '. 78 

In  the  case  of  more  dilute  solutions,  the  reaction  appears  to 
take  place  almost  completely  in  accordance  with  the  above 
equation,  except  in  so  far  as  small  quantities  of  chloric  acid  are 
formed.  In  diffused  daylight,  however,  a  considerable  quantity 
of  hypochlorous  acid  is  formed,  which  is  in  turn  decomposed  by 
light  with  formation  of  chloric  acid,  so  that  in  this  case  the 
reactions  are  probably  those  put  forward  by  Popper.2 

4C19  +  4H.70  =  4HC1  +  4HC10 
8HC10  =  2HC103  +  6HC1  +  O, 

Under  certain  conditions,  however,  sunlight  brings  about  the 
reverse  change,  causing  the  formation  of  free  chlorine  from  a 
mixture  of  hydrogen  chloride  and  oxygen  (see  p.  175). 


CHLORINE    AND    HYDROGEN. 

HYDROCHLORIC  ACID,  CHLORHYDRIC  ACID,  HYDROGEN 
CHLORIDE,  OR  MURIATIC  ACID.    HC1  =  36-19. 

84  The  Arabian  alchemists  were  acquainted  with  hydro- 
chloric acid  in  its  mixture  with  nitric  acid  to  form  aqua  regia, 
which  they  obtained  by  distilling  nitre,  sal-ammoniac,  and  vitriol 
together,  but  the  first  mention  of  the  pure  acid  under  the 

1  Journ.  Chem.  Soc.  1890,  i.,  613  2  Annalen,  227,  161. 


CHLORINE  AND  HYDROGEN  169 

name  of  "  spiritus  salis,"  prepared  from  "  guter  vitriol "  and  "  sal 
commune,"  occurs  in  the  supposed  works  of  Basil  Valentine. 
Glauber  first  obtained  this  acid  by  the  action  of  sulphuric  acid 
on  common  salt  about  the  year  1648,  and  Stephen  Hales,  in 
his  work  on  Vegetable  Staticks,  published  in  1727,  observed  that 
a  large  quantity  of  a  gas  which  was  soluble  in  water  was  evolved 
when  sal  ammoniac  and  oil  of  vitriol  were  heated  together.  It 
was  not,  however,  until  Priestley l  collected  the  gas  thus  evolved 
over  mercury,  by  using  this  metal  instead  of  water  in  a  pneu- 
matic trough,  that  the  gaseous  hydrochloric  acid  was  first  pre- 
pared, and  to  this  gas  Priestley  gave  the  name  of  marine-acid 
air,  as  calling  attention  to  its  production  from  sea-salt.  Lastly, 
Davy  in  1810  proved  that  the  gas,  which  had  been  considered 
to  be  an  oxygen  compound,  was  entirely  composed  of  chlorine 
and  hydrogen. 

Hydrochloric  acid  gas,  the  only  known  compound  of  chlorine 
and  hydrogen,  occurs  in  the  exhalations  from  active  volcanoes,2 
especially  in  Vesuvius,3  and  in  the  fumeroles  on  Hecla.4  In 
aqueous  solution,  the  acid  has  been  found  in  the  waters  of 
several  of  the  South  American  rivers  rising  in  the  volcanic 
districts  of  the  Andes.  It  is  also  found  in  small  quantities  in 
the  gastric  juice  of  man  and  other  animals. 

85  Hydrochloric  acid  can  be  formed  by  the  direct  union  of  its 
constituent  elements.  If  equal  volumes  of  chlorine  and  hydro- 
gen be  mixed  together,  no  combination  occurs  so  long  as  the 
mixture  remains  in  the  dark  and  at  the  ordinary  atmospheric 
temperature  ;  but  if  the  mixed  gases  be  exposed  to  a  strong 
light,  or  if  a  flame  be  brought  to  the  mouth  of  the  jar,  or  an 
electric  spark  passed  through  the  gases,  a  sudden  combination 
takes  place,  the  heat  suddenly  evolved  by  the  union  of  the 
chlorine  and  hydrogen  being  sufficient  to  produce  a  violent 

xplosion.  In  order  to  exhibit  this  singular  action  of  light, 
inducing  the  combination  of  chlorine  and  hydrogen,  a  small  thin 
flask  may  be  filled,  in  a  darkened  room,  half  with  chlorine  gas 

by  displacement  over  hot  water)  and  half  with  hydrogen. 
The  flask,  corked  and  covered  up,  may  then  be  exposed 
either  to  sunlight,  or  to  the  bright  light  of  burning  magnesium 

ibbon,    when    a    sharp    explosion    will     instantly    occur,    the 

1  Observations  on  Different  Kinds  of  Air,  1772,  vol.  iii.  208. 

-  Pseudo-  Volcanic  Phenomena  of  Iceland,  Cav.  Soc.  Mem.  p.  327. 

3  Palmieri,  The  Late  Eruption  of  Vesuvius,  1872,  p.  136. 

4  Bun  sen.  Anna  If  n,  62,  1- 


170  THE  NON-METALLIC  ELEMENTS 

flask  will   be   shattered,  and  fumes   of  hydrochloric   acid    will 
be  seen. 

A  better  method  of  showing  this  combination  is  to  obtain  a 
mixture  of  exactly  equivalent  volumes  of  chlorine  and  hydrogen 
by  the  electrolysis  of  aqueous  hydrochloric  acid  itself.  For  this 
purpose  an  apparatus  shown  in  Fig.  46  is  employed  ;  this  con- 
sists of  an  upright  glass  tube  filled  with  about  120  cc.  of  pure 
fuming  aqueous  hydrochloric  acid,  containing  about  30  per 
cent,  of  HC1.  Two  poles  of  dense  carbon,  as  used  for  the  electric 
lamp,  pass  through  tubulures  in  the  sides  of  the  glass,  being 
fastened  in  their  place  by  means  of  caoutchouc  stoppers.  The 


FIG.  46. 

apparatus  having  been  brought  into  a  room  lighted  only  by  a 
candle  or  small  gas  flame,  the  carbon  poles  are  connected  with 
three  or  four  Bunsen's  elements,  the  current  from  which  is 
allowed  to  pass  through  the  liquid.  At  first  gas  is  evolved  from 
the  negative  pole  only,  and  this  consists  of  hydrogen,  whilst 
all  the  chlorine,  which  is  evolved  at  the  positive  pole,  is  ab- 
sorbed by  the  liquid.  After  the  evolution  has  gone  on  for  two  or 
three  hours  the  liquid  becomes  saturated  with  chlorine,  and  the 
gases  are  given  off  at  each  pole  in  exactly  equivalent  volumes, 
and  consist  of  hydrogen  and  chlorine,  uncontaminated  with  oxy- 
gen, or  oxides  of  chlorine.1  The  gaseous  mixture  thus  obtained 
is  washed  by  passing  through  a  few  drops  of  water  contained 

1  Rosooe.  Jonrn.  Chem.  Soc.  1856,  16. 


HYDROCHLORIC  ACID 


171 


in  the  bulb-tube  ground  into  the  neck  of  the  evolution  vessel, 
and  then  passes  into  a  thin  glass  bulb  of  about  the  size  of  a  hen's 
egg  blown  on  a  piece  of  easily  fusible  tubing.  At  each  end  the 
tube  is  drawn  out  so  as  to  be  very  thin  in  the  glass,  and  to  have 
the  internal  diameter  not  greater  than  1  mm.,  whilst  at  the 
extremities  the  tube  is  wider,  so  as  to  fit  ordinary  caoutchouc 
joinings.  In  order  to  absorb  the  excess  of  chlorine  the  further 
end  of  the  bulb  is  placed  in  connection  with  a  condenser 
containing  slaked  lime  and  charcoal  placed  in  alternate  layers. 

When  the  gas  has  passed  through  the  bulb-tube  (at  the  rate  of 
about  two  bubbles  every  second)  for  about  ten  minutes,  the  join- 
ings are  loosened  and  each  end  stopped  by  a  piece  of  glass  rod. 
In  order  to  preserve  the  gaseous  mixture,  which  is  unalterable  in 
the  dark  for  any  length  of  time,  the  bulbs  are  hermetically  sealed. 
For  this  purpose  the  thinnest  part  of  the  tube  is  brought  some  little 


FIG.  47. 

listance  above  a  very  small  flame  from  a  Bunsen's  gas-burner : 
te  glass  softens  below  a  red  heat,  and  the  ends  may  be  drawn 
mt  and  sealed  with  safety.  It  is,  however,  advisable  to  hold  the 
>ulb  in  a  cloth  during  the  operation  of  sealing,  as  not  unfre- 
[uently  the  gas  explodes.  As  soon  as  one  bulb  is  removed  a 
second  is  introduced,  and  placed  in  connection  with  the  evolution 
lask,  and  after  ten  minutes  sealed  as  described.  The  bulbs 
rims  obtained  should  be  numbered,  and  the  first  and  last  tested 
>y  exposing  them  to  a  strong  light,  and  if  these  explode,  all  the 
itermediate  bulbs  may  be  considered  good.  Sixty  such  bulbs 


172  THE  NON-METALLIC  ELEMENTS 

may  be  prepared  with  the  above  quantity  of  acid,  and  may  be 
kept  in  the  dark  for  an  unlimited  time  without  change.1  On 
exposing  one  of  these  bulbs  to  the  light  emitted  by  burning 
magnesium  ribbon,  or  to  bright  daylight,  a  sharp  explosion  occurs 
and  hydrochloric  acid  is  formed  (Fig.  47). 

When  the  mixture  of  hydrogen  and  chlorine  is  exposed  to 
diffused  daylight  combination  gradually  takes  place,  the  rate  of 
formation  of  hydrochloric  acid  increasing  with  an  increasing 
proportion  of  the  more  refrangible  rays  of  light.  Bunsen  and 
Roscoe 2  found,  however,  that  the  action  of  light  is  at  first  very 
slow  and  only  attains  its  full  activity  after  a  certain  length  of 
time.  Thus,  for  example,  when  a  mixture  of  the  gases  was 
exposed  to  the  light  of  a  small  petroleum  lamp  burning  at  a 
constant  rate  it  was  found  that  the  amount  of  hydrochloric  acid 
formed  in  each  minute  increased  for  the  first  nine  minutes  and 
then  became  constant.  To  this  phenomenon  they  have  given 
the  name  "  photo-chemical  induction."  From  the  recent  in- 
vestigations of  Pringsheim  3  it  seems  probable  that  the  phe- 
nomenon is  due  to  the  formation  of  an  intermediate  product, 
for,  just  as  in  the  case  of  carbon  monoxide  and  oxygen  the 
mixture  of  gases  undergoes  combination  far  more  easily  when 
moist  than  dry,  even  a  very  bright  light  bringing  about  an 
explosion  in  the  latter  case  only  with  great  difficulty,  and 
hence  it  appears  unlikely  that  in  the  combination  we  have  a 
simple  reaction  between  hydrogen  and  chlorine,  but  that  inter- 
mediate compounds  are  formed  by  the  combined  influence  of 
light  and  moisture.  The  amount  of  hydrochloric  acid  formed 
during  the  first  period  of  the  action  of  light  would  then  be  too 
small,  because  the  formation  of  the  acid  could  only  take  place 
after  a  certain  quantity  of  the  intermediate  compound  had  been 
formed  ;  after  a  time,  however,  the  quantity  of  the  intermediate 
compound  formed  would  be  equal  to  the  quantity  decomposed, 
and  the  amount  of  the  latter  in  the  mixture  and  of  the  hydro- 
chloric acid  formed  in  a  given  time  would  remain  constant.  The 
first  period  corresponds  to  the  period  during  which  "  induction  " 
takes  place,  and  the  second  to  that  in  which  the  light  brings 
about  the  formation  of  the  product  of  the  reaction  at  a 
constant  rate. 

Concerning  the  nature  of  this  intermediate  compound,  which 
is  probably  present  in  only  extremely  minute  quantities,  nothing 

1  Roscoe,  Proc.  Manch.  Lit.  and  Phil.  Soc.  Feb.  1865. 

2  Phil.  Trans.  1857.  3  Ann.  Phys.  Chem.  [2],  32,  384. 


PREPARATION  OF  HYDROCHLORIC  ACID  173 

can  be  definitely  said ;  Pringsheim  has,  however,  suggested  that 
the  two  reactions  may  be  represented  by  the  following  equations  : 

(1)  H20  +  C12  -  C190  +  H2 

(2)  2H2  +  C12O  =  H2O  +  2HC1. 

Combination  of  hydrogen  and  chlorine  also  takes  place  when 
the  mixed  gases  are  heated  to  a  sufficiently  high  temperature  : 
in  closed  vessels  explosion  occurs  between  240°  and  270°,  whereas 
if  the  mixture  is  passed  in  a  stream  through  the  heated  vessel 
the  explosion  does  not  take  place  till  the  temperature  reaches 
430°  to  4400.1 

86  Hydrochloric  acid  is  also  formed  by  the  action  of  chlorine 
upon  almost  all  hydrogen  compounds;  which  are  decomposed  by 
it  either  in  the  dark  or  in  presence  of  light ;  thus  sulphuretted 
hydrogen,  olefiant   gas,  turpentine  (see  p.  162),  and  water,  are 
all  decomposed   by  chlorine,  hydrochloric   acid   being  formed. 
When  hydrogen  is  passed  over  many  metallic  chlorides,  such  as 
silver   chloride,  hydrochloric   acid    is    evolved,  and    the    metal 
produced,  thus : — 

2AgCl  +  H2  =  2Ag  +  2HC1. 

By  these  and  other  reactions  hydrochloric  acid  is  frequently 
formed ;  but  none  of  them  serve  for  the  preparation  of  the  gas 
on  a  large  scale. 

87  Preparation. — For  this  purpose  six  parts  by  weight  of  com- 
mon salt  are  introduced  into  a  capacious  flask,  and  eleven  parts 
of  strong  sulphuric  acid  slowly  poured  on  it  through  a  bent  tube- 
funnel  ;    the  gas,  which  is  at  once  rapidly  evolved,  is  purified 
from  any  sulphuric  acid  or  salt  which  may  be  carried  over,  by 
passing  through  a  small  quantity  of  water  contained  in  a  wash 
bottle,  and  it  may  then  either  be  collected  by  displacement  (like 
chlorine),  or  over  mercury,  or  passed  into  water,  as  shown  in  Fig. 
53,  if  an  aqueous  solution  of  the  acid  is  needed. 

The  reaction  which  here  occurs  is  represented  by  the  equation  : 

NaCl  +  H2SO4  =  HC1  +  NaHSO4. 

Hydrochloric  acid  comes  off,  and  a  readily  soluble  acid  sulphate 
of  soda,  or  hydrogen  sodium  sulphate,  NaHSO4,  is  left.  If  two 
molecules  of  salt  be  taken  to  one  of  sulphuric  acid,  a  less  easily 
soluble  salt,  normal  sodium  sulphate  Na2SO4,  is  formed,  a  greater 

1  Freyer  and  V.  Meyer,  Zeit.  Phys.  Chem.  H,  28. 


174 


THE  NON-METALLIC  ELEMENTS 


heat  being  needed  to  complete  the  decomposition  than  when  an 
excess  of  acid  is  employed,  thus  : — 

NaCl  +  NaHS04  =  Na2S04  +  HC1. 

The  pure  aqueous  acid  is  best  prepared  for  laboratory 
from  pure  salt  and  pure  sulphuric  acid. 

88  Properties. — Hydrochloric  acid  is  a  colourless  gas,  which 
was  first  liquefied  by  Davy  and  Faraday,1  who  estimated  the 
tension  of  the  gas  to  be  20  atmospheres  at  —  16°,  25  atmos- 
pheres at  --  4°,  and  40  atmospheres  at  +  10°.  The  method 
adopted  by  Davy  for  the  liquefaction  of  the  gas  is  shown  in  Fig.  48. 


FIG.  48. 

Into  a  tube,  three  times  bent  at  a  right  angle  and  closed  at  one 
end,  are  placed,  at  a,  a  few  small  pieces  of  sal-ammoniac  (a  com- 
pound of  hydrochloric  acid  and  ammonia) ;  some  strong  sulphuric 
acid  is  then  poured  by  means  of  a  bent  tube-funnel  (d),  into  the 
second  bend  at  I.  The  open  end  of  the  tube  is  next  carefully 
drawn  out,  thickened,  and  closed  before  the  blow-pipe,  and  when 
the  tube  is  cold,  it  is  so  inclined  as  to  allow  the  acid  to  flow  on  to 
the  sal-ammoniac.  Hydrochloric  acid  gas  is  at  once  disengaged 
according  to  the  equation  : — 

2NH4C1  +  H2S04  =  2  HC1  +  (NH4)2SO4, 

and  after  a  time  the  pressure  becomes  sufficiently  great  to 
liquefy  the  further  portions  of  the  gas  which  are  evolved,  and 

1  Davy  and  Faraday,  Phil.  Trans.  1823,  p.  164. 


PROPERTIES  OF  HYDROCHLORIC  ACID  175 

by  gentle  heat  the  liquid  may  be  distilled  over  into  the  empty 
limb  of  the  tube.  It  is  a  colourless  liquid,  has  a  specific  gravity 
of  0'854  at  —  105°,  and  solidifies  at  --  115°  to  a  crystalline 
mass  which  melts  at  —  112°'5  (Olszewski),  and  does  not  conduct 
electricity.  The  action  of  liquid  hydrochloric  acid  upon  various 
substances  has  been  carefully  examined  by  Gore.1  These  experi- 
ments show  that  the  liquid  acid  has  but  a  feeble  solvent  power 
for  bodies  in  general,  and,  with  the  exception  of  aluminium,  the 
metals  are  not  attacked  by  it. 

Hydrochloric  acid  gas  is  heavier  than  air.  Its  specific  gravity, 
according  to  the  most  accurate  experiments  of  Biot  and  Gay- 
Lussac,  is  1'278  (air  =  1)  whilst  according  to  Leduc 2  it  is  1*2696 
or  its  density  is  18'39  (H  =  1),  the  calculated  density  being 
18*095.  At  1500°  the  gas  appears  to  have  undergone  no  change, 
but  at  1700°  a  considerable  amount  of  dissociation  has  taken 
place.3  The  gas  fumes  strongly  in  the  air,  uniting  with  atmo- 
spheric moisture,  and  it  is  instantly  absorbed  by  water  or  ice, 
yielding  the  aqueous  acid.  It  possesses  a  strongly  acid  reaction 
and  suffocating  odour,  and  is  not  inflammable.  A  burning  candle 
is  extinguished  when  plunged  into  the  gas,  the  outer  mantle  of 
the  flame,  before  extinction,  exhibiting  a  characteristic  green 
coloration. 

Direct  sunlight  has  no  action  on  a  dry  mixture  of  hydrogen 
chloride  and  oxygen,  but  if  more  moisture  than  is  necessary  for 
the  complete  saturation  of  the  gas  be  present,  decomposition 
gradually  takes  place,  free  chlorine  and  water  being  formed. 
The  amount  of  chlorine  liberated  depends  upon  the  proportion 
of  oxygen  present ;  thus  a  mixture  of  4  volumes  HC1  and 
1  volume  O  only  gave  0'34  per  cent,  of  free  chlorine  after  24 
days'  exposure,  whilst  with  8  volumes  of  oxygen  73'81  per  cent, 
of  the  chlorine  was  liberated.  It  appears  that  in  some  cases 
hypochlorous  acid  is  also  formed.4 

89  The  composition  of  hydrochloric  acid  gas  can  be  best 
ascertained  as  follows : — 

Metallic  sodium  decomposes  the  gas  into  chlorine,  which 
combines  with  the  metal  to  form  sodium  chloride,  and  into 
hydrogen,  which  is  liberated.  If  a  small  piece  of  sodium  be 
heated  in  a  deflagrating  spoon  until  it  begins  to  burn,  and 
then  plunged  into  a  jar  of  hydrochloric  acid  gas,  the  combus- 

1  Proc.  Roy.  Soc.  14,  204.  3  Comptes  Rend.  116,  968. 

3  Langer  and  V.  Meyer,  Pyrochem.  Unters.  (Vieweg),  p.  67. 

4  McLeod,  Journ.    Chem.  Soc.  1886,  i.  591  ;  Richardson,  Journ.  Chcm.  Soc. 
1887,  i.  802. 


176 


THE  NON-METALLIC  ELEMENTS 


tion  of  the  metal  (union  with  chlorine)  will  go  in  the  gas. 
In  order  to  show  what  volume  of  hydrogen  is  evolved  fro: 
a  given  volume  of  hydrochloric  acid  gas  by  this  reactio 
the  following  experiment  may  be  made  with  the  eudiome 
tube,  the  construction  of  which  is  clearly  seen  in  Fig.  49.  T 
begin  with,  both  limbs  are  filled  completely  with  dry  mercury, 
then  the  end  of  the  tube  carrying  the  stop-cock  is  connected  by 
a  piece  of  caoutchouc  tubing  with  an  evolution  flask,  from  which 
pure  hydrochloric  acid  gas  is  being  slowly  evolved  from  a  mix- 
ture of  dry  salt  and  strong  sulphuric  acid,  care  being  taken  that 
the  air  has  been  driven  out.  On  turning  the  stop-cock  at  the  top 
of  the  tube,  and  opening  the  screw-tap  on  the  caoutchouc  in  the 
U-tube,  the  mercury  will  run  out,  and  dry  hydrochloric  acid  gas 


FIG.  49. 

will  enter  the  one  limb,  whilst  air  fills  the  other  to  the  same 
level.  As  soon  as  the  gas  reaches  a  mark  on  the  tube  indicating 
that  it  is  two-thirds  full  of  gas  the  stopcock  is  closed.  A 
small  quantity  of  sodium  amalgam  is  now  prepared  by  pressing 
six  or  eight  small  pieces  of  clean  cut  sodium,  one  by  one, 
under  the  surface  of  a  few  ounces  of  mercury  contained  in  a 
porcelain  mortar.  The  amalgam  is  then  poured  into  the  open 
limb  of  the  U-tube  so  as  to  fill  it,  and  the  end  firmly  closed  with 
the  thumb  ;  the  hydrochloric  acid  gas  is  now  transferred  to  the 
limb  containing  the  amalgam,  and  well  shaken  so  as  to  bring 
the  gas  and  amalgam  into  contact.  The  gas  is  next  passed  back 
into  the  closed  limb,  and  the  pressure  equalised  by  bringing  the 
mercury  in  both  limbs  to  the  same  level,  which  is  easily  done 
by  allowing  some  mercury  to  flow  out  by  loosening  the  screw-tap 


COMPOSITION  OF  HYDROCHLORIC  ACID  177 

at  the  bottom  of  the  U-tube.  The  hydrochloric  acid  gas  will 
be  completely  decomposed  by  contact  with  sodium  amalgam, 
chloride  of  sodium  being  formed,  whilst  the  hydrogen  is  left 
in  the  ga,seous  state.  This  will  be  found  to  occupy  exactly 
half  the  volume  of  the  original  gas,  the  level  of  the  mercury 
having  risen  to  a  mark  previously  made  and  indicating  exactly 
one-third  of  the  capacity  of  the  tube.  As  the  closed  limb  is 
provided  with  a  stopcock,  the  residual  gas  may  be  inflamed, 
and  thus  shown  to  be  hydrogen. 

90  It  still,  however,  remains  to  ascertain  the  volume  of  the  chlo- 
rine which  has  disappeared.  This  is  done  as  follows  : — Two  glass 
tubes  about  50  cm.  long  and  1'5  cm.  in  diameter,  drawn  out  at  each 
end  to  fine  threads,  are  filled  with  the  gaseous  mixture  evolved 
by  the  electrolysis  of  the  aqueous  acid  (see  p.  170).  The  process 
is  conducted  exactly  as  if  a  bulb  were  being  filled,  and  the  tubes 
are  then  sealed  up  and  kept  in  the  dark.  When  it  is  desired  to 
exhibit  the  composition  of  the  gas,  one  of  the  tubes  thus  filled 
is  brought  into  a  dimly-lighted  room,  and  one  of  the  drawn- 
out  ends  broken  under  mercury.  No  alteration  in  the  bulk  of 
the  gas  will  be  noticed.  The  mercury  in  which  the  tube  dips  is 
now  replaced  by  a  colourless  solution  of  iodide  of  potassium, 
and  by  giving  the  tube  a  slight  longitudinal  shaking,  a  little 
of  this  solution  is  brought  in  contact  with  the  gas.  No  sooner 
does  the  liquid  enter  the  tube  than  it  becomes  of  a  dark  brown 
colour,  due  to  the  liberation  of  the  iodine,  the  chlorine  uniting 
with  the  potassium  to  form  the  chloride  of  that  metal.  A 
consequent  diminution  of  bulk  occurs  which  corresponds 
precisely  to  the  volume  of  chlorine  contained  in  the  tube, 
and  in  a  few  moments  the  column  of  liquid  fills  half  the 
tube,  proving  that  half  the  volume  of  the  mixed  gas  con- 
sists of  chlorine,  and  the  other  half  of  hydrogen.  We  have, 
however,  learnt  from  the  previous  experiment  that  hydrochloric 
acid  gas  contains  half  its  own  volume  of  hydrogen,  so  that 
we  have  now  ascertained  (1)  that  hydrochloric  acid  gas  is 
entirely  made  up  of  equal  volumes  of  chlorine  and  hydrogen, 
and  (2)  that  these  elementary  components  combine  together 
without  change  of  volume  to  produce  the  compound  hydrochloric 
acid  gas. 

This  fact  may  be  further  illustrated   by  exposing  a  second 

sealed-up  tube,  containing  the  electrolytic  gas,  for  a  few  minutes, 

first  to  a  dim,  and  then  to  a  stronger  daylight.     The  greenish 

colour  of  the  chlorine  will  soon  disappear,  a  gradual  combination 

13 


178 


THE  NON-METALLIC  ELEMENTS 


of  the  gases  having  occurred.  On  breaking  one  end  of  the  tube 
under  mercury,  no  alteration  of  bulk  will  be  observed,  whilst  on 
raising  the  open  end  into  some  water  poured  on  the  top  of  the 


FIG.  50. 


mercury,  an  immediate  and  complete  absorption  will  be  noticed, 
and  the  tube  will  become  filled  with  water. 

In  order  to  determine  with  a  greater  degree  of  exactitude 
than  is  possible  by  the  above  methods,  the  relation  existing 
between  the  two  gases,  a  quantitative  analysis  of  the  chlorine 
contained  in  a  given  volume  of  the  electrolytic  gas  must  be 


MANUFACTURE  OF  HYDROCHLORIC  ACID  179 

made.     Two  experiments  thus   conducted   gave  the  following 
results : — 

I.  II.  Calculated. 

Chlorine     ...     49*85     ...     50*02     ...     50*00  volumes. 
Hydrogen  ...     50*15     ...     49*98     ...     50*00 


100-00  100-00  100-00 


Showing  that  the  gas  obtained  by  decomposing  aqueous  hydro- 
chloric acid  consists  exactly  of  equal  volumes  of  chlorine  and 
hydrogen,  or 

£  vol.   of   chlorine  weighing  1— —  =  17*595 
J       „         hydrogen        „  1     =     0'500 


1  vol.  of  hydrochloric  acid  weighing  ...  18*095 

91  Hydrochloric  acid  gas  is  very  soluble  in  water,  and  the  solu- 
tion is  largely  used  for  laboratory  and  for  commercial  purposes, 
and  frequently  termed  muriatic  acid.     In  order  to  exhibit  the 
solubility  of  the  gas  in  water,  a  large  glass  globe  (Fig.  50)  placed 
on  a  stand   is   filled,    by   displacement,  with  the  gas  ;  a  tube, 
reaching  to  the  centre  of  the  globe  and  dipping  to  the  bottom  of 
an  equal-sized  globe  placed  beneath,  being  fixed  in  a  caoutchouc 
stopper  placed  into  the  neck  of  the  upper  globe.   Between  the  two 
globes  the  tube  is  joined  by  a  piece  of  caoutchouc  tubing,  closed 
by  a  screw-tap.     When  it  is  desired  to  show  the  absorption,  the 
lower  globe  is  filled  with  water   coloured  blue  by  infusion  of 
litmus,  the  screw-top  is  opened  and  a  little  of  the  water  forced 
into  the  upper  globe  (so  as  to  begin  the  absorption)  by  blowing 
through  the  side  tube  into  the  space  above  the  surface  of  liquid 
in  the  lower  globe.     As  soon  as  the  water  makes  its  appearance 
at  the  top  of  the  tube,  a  rapid  absorption  occurs,  the  liquid  rushes 
up  in  a  fountain,  and  at  the  same  time  becomes  coloured  red. 

92  Manufacture  of  Hydrochloric  Acid. — This  acid  is  obtained  on 
the  large  scale  as  a  bye-product  in  the  manufacture  of  soda-ash, 
(Cf.  Vol.  II.  Part  I.   p.  132.)     In  the  alkali  works  10  cwt.  of 
salt  is  introduced  into  a  large  hemispherical  iron  pan,  9  feet 
in   diameter,    heated  by  a   fireplace   underneath,    and  covered 
by  a   brickwork   dome ;  upon  this   mass  of  salt  the  requisite 


180 


THE  NON-METALLIC  ELEMENTS 


quantity  (10  cwt.)  of   sulphuric  acid   (sp.  gr.    I1 7)    is  allowed 
to  run  from  a  leaden  cistern  placed  above  the    decomposing 


FIG.  51. 


pan.  Torrents  of  hydrochloric  acid  gas  are  evolved,  which 
collect  in  the  space  between  the  pan  and  the  brickwork  dome, 
whence  they  pass  by  a  brickwork  or  earthenware  flue  into 


MANUFACTURE  OF  HYDROCHLORIC  ACID 


181 


upright  towers  or  condensers,  built  of  bricks  soaked  in  tar,  or  of 
Yorkshire  flags  fitted  and  clamped  together.  These  towers,  shown 
in  vertical  section  in  Fig.  51,  and  in  ground  plan  in  Fig.  52,  are 
filled  with  bricks  or  coke,  down  which  a  small  stream  of  water, 
from  a  reservoir  at  the  top  of  the  tower,  is  allowed  to  trickle. 
The  gas  passing  upwards,  as  shown  in  the  figures  by  the  arrow, 
meets  the  water  and  is  dissolved  by  it ;  and  as  the  acid -liquor 
approaches  the  bottom  of  the  tower  it  becomes  more  and  more 
nearly  saturated  with  the  gas. 

The   aqueous   commercial   acid   thus   obtained  from  impure 
materials  is  generally  far  from  pure  ;  it  is  usually  of  a  yellow 


—  i {-r/ ('-', --j 

*  i    'i          \\  ,\  I    '/ 

V1    v  '    /'/  \\    V  /   // 

\\  ^L.S//       /S>J-,''  // 
^J_-^''       ,'VVVJ  -->' 

- -  Ax    S     S    .'* ' 


0  5  10  15  20  25  SOPeot 


colour,  due  to  organic  matter,  and  may  also  contain  sulphur 
dioxide,  sulphuric  acid,  chlorine,  iron,  and  arsenic :  this  last  is 
often  present  in  large  quantities,  being  derived  from  the  pyrites 
used  in  making  the  sulphuric  acid. 

The  presence  of  arsenic  may  be  detected  by  Marsh's  reaction ; 
or  by  the  addition  of  stannous  chloride,  which  produces  a  brown 
precipitate  of  impure  arsenic.  To  remove  traces  of  arsenic, 
solution  of  stannous  chloride  may  be  added,  the  precipitate 
allowed  to  settle,  and  the  clear  liquid  re-distilled.  Chlorine  may 
be  detected  by  the  addition  to  the  diluted  acid  of  pure  iodide 
of  potassium  and  starch  solution,  when  if  chlorine  be  present 
the  blue  iodide  of  starch  will  be  formed.  The  presence  of 


182 


THE  NON-METALLIC  ELEMENTS 


sulphuric  acid  can  be  easily  ascertained  by  adding  chloride  of 
barium  solution  to  the  diluted  acid,  whilst  that  of  sulphurous 
acid  may  be  shown  by  adding  zinc  to  the  diluted  acid,  when 
sulphuretted  hydrogen  will  be  given  off  and  its  presence  readily 


FIG.  53. 


ascertained  by  its  blackening  action  on  lead  paper.  It  is, 
however,  not  easy  to  separate  these  substances  so  as  to  obtain 
a  strong  pure  acid  from  one  originally  impure,  and  by  far  the 
simplest  plan  is  to  exclude  the  foreign  matters  by  employing 
pure  materials  to  begin  with. 


PROPERTIES  OF  HYDROCHLORIC  ACID  183 

93  The  pure  saturated  aqueous  acid  is  a  colourless  liquid 
fuming  strongly  in  the  air,  and  freezing  when  cooled  below 
-  40°  to  a  butter-like  mass  having  the  composition  HC1  -f  2H9O. 
It  is  prepared  for  laboratory  use  by  means  of  the  apparatus 
shown  in  Fig.  53.  One  volume  of  water  at  0°  absorbs  503 
times  its  volume  of  hydrochloric  acid  gas.  The  weight  and 
volume  of  the  gas  absorbed  under  the  pressure  of  760  mm.  by 
one  gram  of  water  at  different  temperatures  is  given  in  the 
following  table.1 

Temp.                      Grms.  HC1.  Temp.                       Grms.  HC1. 

0°  ....  0-825  32°  ....  0-665 

4  ....    0-804  36  ....  0-649 

8  ....  0-783  40  ...    .  0633 

12  ....  0762  44  ....  0-618 

16  ....  0-742  48  ....  0-603 

20  ....  0-721  52  ....  0-589 

24  ....  0-700  56  ....  0575 

28  ....  0-682  60  ....  0'561. 

The  weight  of  gas  dissolved  under  changing  pressure  (the  tempe^- 
rature  remaining  constant)  does  not  vary  proportionally  to  the 
pressure,  and,  therefore,  this  gas  does  not  follow  Dalton  and 
Henry's  law.  Thus,  for  instance,  under  the  pressure  of  1  metre 
of  mercury  1  grm.  of  water  dissolves  0'856  grm.  of  the  gas ; 
according  to  Dalton  and  Henry's  law  the  weight  of  gas  absorbed 
under  a  pressure  of  1  decimetre  of  mercury  should  be  0'0856 
.grm.,  whereas  it  is  found  to  be  0'657  grm. 

On  heating  a  saturated  solution  of  the  gas  in  water  having  a 
specific  gravity  of  T22,  hydrochloric  acid  gas  is  given  off,  and  the 
liquid  becomes  weaker.  On  the  other  hand,  a  weak  acid  on  being 
boiled  loses  water  and  becomes  stronger,  so  that  at  last  both 
the  strong  acid  and  the  weak  acid  reach  the  same  strength,  and 
both  when  boiled  distil  over  unchanged,  provided  the  pressure 
does  not  vary.  The  aqueous  acid,  which  boils  unchanged  at 
110°  under  the  normal  pressure  contains  20*24  per  cent,  of 
hydrochloric  acid  HC1.2  If  the  distillation  proceeds  under  a 
greater  or  less  pressure  than  the  normal,  distillates  of  constant 
composition  are  obtained,  but  each  one  contains  a  different 
quantity  of  hydrochloric  acid.  This  is  clearly  seen  from  the 
following  table. 

1  Roscoe  and  Dittmar,  Quart.  Journ.  Chem.  1860,  128. 

2  Roscoe  and  Dittmar  .  loc.  cit. 


184  THn  NON-METALLIC  ELEMENTS 

Column  I.  gives  the  pressure  in  metres  of  mercury  under 
which  distillation  was  conducted ;  Column  II.  the  percentage 
of  hydrochloric  acid  (HC1)  found  in  the  residual  acid. 

I.  II.  I.  II.  I.  II. 

0-05  23-2  0-8  20-2  17  18'8 

O'l  22-0  0-9  19-9  1-8  187 

0-2  22  3  1-0  197  1'9  18'6 

0-3  21-8  1-1  19'5  2-0  18'5 

0-4  21-4  1-2  19-4  21  18-4 

0-5  21-1  1-3  19-3  2-2  18'3 

0-6  207  1-4  191  2-3  18'2 

07  20'4  1-5  19-0  2-4  181 

076  20-24  1-6  18-9  2'5  18'0 

Here  the  percentage  of  the  acid  and  the  constant  composition 
obtained  by  distillation  under  a  pressure  of  0'5  decimetre  is  seen 
to  be  23'2  HC1 ;  whereas  when  the  pressure  is  increased  to  2'5 
metres  the  percentage  of  the  acid  of  constant  boiling  point  is  18'0 
HC1.  From  this  it  is  clear  that  definite  hydrates  of  hydro- 
chloric acid  (i.e.,  compounds  of  hydrochloric  acid  and  water  in 
simple  atomic  proportions)  are  not  formed  on  distillation,  al- 
though it  happens  that  by  chance  the  liquid  distilling  under  a 
pressure  of  760  mm.  corresponds  to  HC1  +  8H2O.  In  the  same 
way,  if  dry  air  is  passed  through  aqueous  hydrochloric  acid  a 
part  of  the  acid  is  vaporized  and  a  residue  is  obtained  which 
for  each  given  temperature  remains  of  constant  composition. 
An  acid  weaker  or  stronger  than  this  ultimately  attains  this 
composition. 

The  following  table  shows  the  composition  of  the  constant 
aqueous  hydrochloric  acids  obtained  by  leading  air  at  given 
temperatures  through  the  liquid. 

Temp.  Per  Cent,  of  HC1.        Temp.  Per  Cent,  of  HC1. 

0° 25-0  60°  .....  23-0 

10 247  70 22-6 

20 24-4  80 22-0 

30 241  90 21-4 

40 23-8  100 207 

50  .    .    .    .    .  23-4 

Hence  it  is  seen  that  an  aqueous  acid  which  boils  unaltered 
under  a  given  pressure,  and,  therefore,  at  a  constant  tempera- 
ture, contains  the  same  percentage  of  HC1  as  the  constant  acid 


THE  CHLORIDES  185 


obtained  by  passing  dry  air  through  the  aqueous  acid.  Thus 
the  boiling  point  of  the  acid  under  0*1  metre  of  pressure,  con- 
taining 22-9  per  cent,  of  HC1,  is  from  61°  to  62° ;  and  if  dry  air 
be  passed  through  an  aqueous  acid  at  62°  the  constant  point  is 
attained  when  the  liquid  contains  22*9  per  cent,  of  HC1. 

94  The  following  table  gives  the  specific  gravity  of  solutions 
of  aqueous  hydrochloric  acids  of  varying  strengths,  according 
to  the  experiments  of  Kolb.1 

100  of  Aqueous  Specific  Gravity  at 

Acid  contain 
HC1. 

2*22 

3-80 

6-26 
11-02 
15-20 
18-67 
20-91 
2372 
25-96 
29-72 
31-50 
34-24 
36-63 
38-67 
40-51 
41-72 
43-09 

From  these  numbers  the  percentage  of  any  acid  of  known 
specific  gravity  can  easily  be  found  by  interpolation. 

95  The  Chlorides. — The  compounds  of  chlorine  with  the  metals 
are  formed  either  by  the  direct  union  of  chlorine  with  a  metal 
or  by  the  replacement  of  the  hydrogen  in  hydrochloric  acid  by 
a  metal.     Certain  metals  enter  very  readily  into  combination 
with  chlorine,  heat  being  always  evolved,  and  the  phenomena 
of  combustion  frequently   observed.      Other  metals    again   do 
not  combine  so  easily.      Most   of  the   metallic   chlorides   are 
soluble  in  water  ;  amongst  those  insoluble  are  silver  chloride 
AgCl,  mercurous  chloride  (calomel)  HgCl,  and  cuprous  chloride 

1  Compt.  Rend.  74,  737. 


0° 

15° 

1-0116 

1-0103 

1-0202 

1-0189 

1-0335 

1-0310 

1-0581 

1-0557 

1-0802 

1-0751 

1-0988 

1-0942 

1-1101 

1-1048 

1-1258 

1-1196 

1-1370 

1-1308 

1-1569 

1-1504 

1-1666 

1-1588 

1-1806 

1-1730 

1-1931 

1-1844 

1-2026 

1-1938 

1-2110 

1-2021 

1-2165 

1-2074 

1-2216 

1-2124 

186  THE  NON-METALLIC  ELEMENTS 

Cu2Cl2.   Many  metals  combine  in  more  than  one  proportion  with 
chlorine,  thus  we  find  : 

Cuprous  Chloride,  Cu2Cl2,  Cupric  Chloride,  CuCl2. 

Mercurous  Chloride,  HgCl,  Mercuric  Chloride,  HgCl2. 

Tin  Bichloride,  SnCl2,  Tin  Tetrachloride,  SnCl4. 

Platinum  Bichloride,  PtCl2,  Platinum  Tetrachloride,  PtCl4. 

Ferrous  Chloride,  FeCl2,  Ferric  Chloride,  FeCl3. 

The  chlorides  of  the  metals  are  usually  prepared  by  one  of 
the  following  processes.  (1)  By  acting  on  the  metal  with 
chlorine  gas,  especially  when  the  anhydrous  chloride  is  required. 
(2)  By  the  action  of  chlorine  upon  metallic  oxides,  when  it 
drives  off  the  oxygen  and  unites  with  the  metal  to  form  a 
chloride.  (3)  By  acting  on  the  metal  with  hydrochloric  acid. 
(4)  By  dissolving  the  oxide,  hydrate,  or  carbonate  of  the  metal 
in  hydrochloric  acid.  (5)  In  certain  cases,  by  adding  a  soluble 
chloride  to  a  solution  of  a  salt  of  the  metal,  when  the  metallic 
chloride  is  obtained  as  an  insoluble  precipitate. 

Chlorine  also  unites  with  all  the  non-metallic  elements,  and 
with  certain  groups  of  atoms  termed  radicals,  to  form  chlorides 
of  these  elements  and  radicals  respectively,  some  examples  of 
which  are  as  follows  : — 

Non-Metallic  Chlorides. 

Hydrochloric  Acid HC1. 

Chloride  of  Sulphur SC12. 

Trichloride  of  Boron  .    , BC13. 

Tetrachloride  of  Silicon SiCl4. 

Pentachloride  of  Phosphorus PC15. 

Chlorides  of  Inorganic  Radicals. 

Chloride  of  Sulphuryl S02C12. 

Chloride  of  Phosphoryl POC13. 

Chlorides  of  Organic  Radicals. 

Chloride  of  Ethyl C2H5C1. 

Chloride  of  Ethylene C2H4C12. 

Chloride  of  Acetyl C2H3OC1. 

Chloride  of  Cyanogen CNC1. 

96  Detection  and  Estimation  of  Chlorine. — In  the  free  state 
chlorine  gas  is  recognized  by  its  peculiar  colour,  its  suffocating 


DETECTION  OF  CHLORINE  187 

smell,  and  by  its  bleaching  action  on  organic  colouring  matters. 
When  present  in  smaller  quantities,  its  presence  may  be  de- 
tected by  the  blue  colour  which  it  causes  on  a  paper  moistened 
with  a  solution  of  iodide  of  potassium,  KI,  and  starch  paste, 
owing  to  the  fact  that  chlorine  liberates  iodine  from  its  com- 
pound with  potassium,  combining  with  the  metal  to  form  the 
chloride,  KC1,  whilst  the  liberated  iodine  forms  a  deep  blue 
compound  with  starch.  This  reaction  is  very  delicate,  but  it 
must  be  remembered  that  an  excess  of  chlorine  again  removes 
the  blue  colour,  and  also  that  the  same  effect  is  produced  by 
bromine,  nitrous  fumes,  ozone,  and  other  oxidising  substances. 

When  combined  with  metals  to  form  chlorides  soluble  in 
water,  the  element  is  usually  detected  by  the  formation  of  the 
curdy  white  precipitate  of  silver  chloride  AgCl,  on  addition  of 
a  solution  of  silver  nitrate,  AgNO3,  to  that  of  a  soluble  chloride, 
such  as  KC1,  thus  : — 

KC1  +  AgNO3  =  AgCl  +  KN03. 

One  part  of  chlorine  in  one  million  parts  of  water  can  thus  be 
detected — a  faint  opalescence  occurring.  The  precipitated  silver 
chloride  becomes  violet-coloured  on  exposure  to  light,  and  is  in- 
soluble in  water  and  dilute  acids,  especially  nitric  acid,  but 
readily  soluble  in  ammonia  and  in  solutions  of  potassium  cyanide, 
and  sodium  thiosulphate  (the  so-called  hyposulphite  of  soda). 
Mercurous  nitrate  likewise  produces  in  solutions  of  a  chloride  a 
white  precipitate  of  mercurous  chloride  (calomel),  which  does 
not  dissolve,  but  turns  black  on  addition  of  ammonia. 

In  order  to  detect  a  chloride  in  presence  of  an  iodide  and 
bromide,  the  dried  salt  is  distilled  with  potassium  chromate  and 
strong  sulphuric  acid,  when  chromium  oxychloride,  CrO2Cl.2, 
distils  over  as  a  dark  red  liquid,  decomposed  by  addition  of 
water  or  ammonia,  and  yielding  a  yellow  solution  which,  on 
addition  of  hot  acetic  acid  and  a  soluble  lead  salt,  gives  a 
yellow  precipitate  of  lead  chromate ;  neither  bromine  nor  iodine 
forms  a  similar  compound  with  chromium.  Chlorine,  when 
combined  to  form  a  chloride,  is  always  estimated  as  silver 
chloride,  AgCl,  and  according  to  Stas  142 '32  of  silver  chloride 
contain  35 '19  of  chlorine.  If  the  chlorine  is  present  in  the  free 
state  it  can  be  determined  by  volumetric  analysis  (see  Chlorimetry, 
under  Bleaching  Powder),  or  it  may  be  reduced  by  sulphur  di- 
oxide, SO2  to  hydrochloric  acid,  and  then  precipitated  as  silver 
chloride  and  weighed. 


188  THE  NON-METALLIC  ELEMENTS 

The  atomic  weight  of  chlorine  was  first  determined  by 
Berzelius,1  together  with  that  of  silver  and  potassium.  Penny  2 
and  Marignac3  have  also  made  similar  determinations;  the 
latter  obtaining  the  numbers  35*372  and  35*462  by  a  method 
similar  to  that  employed  by  Berzelius.  It  is  to  Stas  4  that  we 
owe  the  most  exact  determinations.  He  converted  pure  silver  into 
silver  chloride  by  four  different  methods,  and  found  as  the  mean 
of  closely  agreeing  results  that  the  relation  of  the  weight  of 
silver  to  that  of  the  silver  chloride  it  yields  is  1  :  T3285.  It 
is  further  known  that  the  relation  of  the  atomic  weights  of 
silver  and  oxygen  is  6 '745 6  :  1,  this  ratio  having  been  also 
obtained  from  a  large  number  of  experiments,  such  for  example, 
as  the  ratios  of  equivalent  quantities  of  the  following : — 

Ag2S  :  Ag2S04  and  AgCl :  AgClO3. 

Hence  if  the  atomic  weight  of  oxygen  is  taken  as  15*88  that  of 
chlorine  is  3519. 


BROMINE.    Br  =  79-36 

97  BROMINE  does  not  occur  in  the  free  state  in  nature ;  it 
was  discovered  in  the  year  1826  by  Balard,5  who  prepared  it 
from  the  liquor  called  bittern,  remaining  after  the  common 
salt  has  crystallised  out  from  concentrated  sea-water,  in  which 
it  occurs,  combined  with  metals  to  form  bromides ;  he  gave  it 
the  name  from  /fyw/i-o?,  a  bad  smell. 

Bromine  occurs  in  combination  with  silver  in  certain  ores, 
from  Mexico,  Chili,  and  Bretagne ;  but  is  found  in  large 
quantities  (combined  with  sodium,  potassium,  magnesium,  or 
calcium,  forming  bromides)  in  the  water  of  many  mineral  springs, 
some  of  which  contain  enough  to  serve  as  a  source  of  this 
element.  It  is  also  found,  though  in  very  small  quantity,  in 
all  sea-water,6  and  has  been  detected  in  sea- weed  from  many 

1  Pogg.  Ann.  8,  1.  2  Phil.  Trans.  1839,  p.  20. 

3  Annalen,  44,  11  ;  Bill.  Univ.  de  Geneve,  43,  350. 

4  Recherches  sur  Us  Lois  des  Proportions  Chimiques,  Bmxelles,  1865. 

5  Ann.  Chim.  Phys.  [2],  32,  337. 

6  The  water  of  the  Dead  Sea  is  said  to  contain  large  quantities,  of  no  less  than 
0  -42  gram,  in  the  litre  (Lartet). 


BROMINE  189 


localities,  and  even  in  certain  marine  animals,  as  well  as  in 
English  rock-salt.  The  mineral  springs  at  Kreuznach,  Kis- 
singen,  and  Schonebeck,  and  the  potash  beds  of  Stassfurt,  as 
well  as  certain  American  springs  in  Ohio  and  elsewhere,  contain 
considerable  quantities  of  bromine,  the  commercial  article  being 
obtained  almost  entirely  from  the  two  last-named  sources. 

Preparation. — In  order  to  detect  bromine  in  a  mineral  water, 
or  to  prepare  it  in  small  quantities,  the  following  method  is 
employed.  The  mother-liquor  remaining  after  the  brine  from 
any  of  the  above  sources  has  been  well  crystallised  is  treated 
with  a  stream  of  chlorine  gas,  so  long  as  the  yellow  colour  of 
the  liquid  continues  to  increase  in  depth.  Chlorine  has  the 
power  of  liberating  bromine  from  bromides,  itself  uniting  with 
the  metal,  and  the  bromine  being  set  free,  thus  : — 

MgBr2  +  Cla  =  MgCl2  +  Br2. 

The  addition  of  excess  of  chlorine  is  to  be  avoided,  as  a  com- 
pound of  chlorine  and  bromine  is  then  formed.  The  yellow 
liquid  is  then  well  shaken  with  chloroform,  which  dissolves  the 
bromine,  forming,  on  standing,  a  brown  solution  below  the 
aqueous  liquid.  On  adding  caustic  potash  to  this  solution  the 
colour  at  once  disappears,  the  bromine  combining  to  form  the 
bromide  KBr,  and  bromate  of  potassium,  KBr03  thus  : — 

3Br2  +  6KHO  =  KBrO3  +  5KBr  +  3H2O. 

On  concentrating  the  solution  a  mixture  of  these  salts 
remains,  and  from  these  the  bromine  is  again  liberated  by  dis- 
tilling the  liquid  with  black  oxide  of  manganese  and  sulphuric 
acid  in  a  tubulated  retort.  The  decomposition  which  here 
occurs  is  similar  to  that  which  takes  place  in  the  preparation  of 
chlorine,  thus : — 

2KBr  +  3H2S04  +  Mn02  =  Br2  +  2KHSO4  +  MnSO4  +  2H2O. 

Dark  red  fumes  of  bromine  are  liberated,  and  a  black  liquid 
condenses  in  the  well-cooled  receiver. 

If  the  bromine  is  required  to  be  anhydrous  it  must  be 
redistilled  over  concentrated  sulphuric  acid ;  and  if  iodine  is 
present  this  must  be  got  rid  of  previously  by  precipitation  as 
subiodide  of  copper. 

By  far  the  greater    quantity  of  the  bromine  brought  into 


190 


THE  NON-METALLIC  ELEMENTS 


commerce  is  now  manufactured  at  Stassfurt  from  the  mother- 
liquor  remaining  after  the  separation  of  the  potassium  salts 
contained  in  the  salt  deposits,  the  process  adopted  consisting  in 
the  treatment  of  the  liquors  with  chlorine  under  suitable  con- 
ditions. The  most  recent  apparatus  for  this  purpose  is  shown 
in  Fig.  54,  and  is  so  arranged  that  the  process  is  continuous. 
The  mother-liquor  enters  the  apparatus  by  the  hydraulically 
sealed  pipe  a,  and  by  means  of  the  sandstone  drum  b,  and 
perforated  plate  e,  is  distributed  equally  over  the  whole  area  of 


FIG.   54. 


the  tower  A.  The  latter  is  filled  with  balls  over  which  the 
liquor  flows,  thus  exposing  a  large  surface  to  the  action  of 
the  chlorine  gas  passing  through  the  tower  in  the  opposite 
direction ;  the  waste  liquor  passes  away  by  the  pipe  d,  which  is 
sufficiently  large  to  allow  of  the  simultaneous  passage  of  the 
chlorine  gas  to  the  tower  from  the  generator  D.  In  order  to 
free  the  waste  liquor  completely  from  all  traces  of  chlorine  and 
bromine  it  is  run  into  the  vessel  B,  which  is  kept  full  to  the 
bottom  of  the  pipe  d  ;  to  pass  away  from  B  the  liquor  must  pass 
over  the  sandstone  shelves,  in  the  direction  shown  by  the 


PROPERTIES  OF  BROMINE  191 

arrows,  in  doing  which  it  is  subjected  to  the  action  of  a  current 
of  high  pressure  steam  which  is  introduced  into  the  apparatus 
by  the  pipe  g.  All  the  chlorine  and  bromine  are  thus  driven 
out  of  the  liquor  and  rise  to  the  top  of  B,  where  they  mingle 
with  the  chlorine  gas  passing  from  the  generator  to  the  tower, 
whilst  the  now  quite  innocuous  liquor  passes  away  by  the  pipe  i. 

The  bromine  vapour  and  excess  of  chlorine  pass  out  from  the 
top  of  the  tower  by  the  pipe  o,  through  the  condenser  p,  where 
most  of  the  bromine  condenses,  the  last  traces  of  the  bromine 
and  chlorine  being  removed  by  the  vessel  c,  which  contains  iron 
filings  kept  moist  by  a  small  stream  of  water. 

The  bromine  thus  obtained  is  purified  by  redistillation,  the 
small  quantities  of  chlorine  present  being  removed  by  the 
addition  of  potassium-  or  ferrous-bromide,  or  by  collecting 
separately  the  more  volatile  portions  of  the  distillate,  which 
contain  all  the  chlorine  in  the  form  of  chloride  of  bromine. 

The  manufacture  of  bromine  was  commenced  in  Stassfurt  in 
1865,  in  which  year  the  quantity  obtained  only  amounted  to 
25  cwt. ;  in  1885  the  amount  had  risen  to  260  tons,  whilst  the 
quantity  manufactured  in  America  during  the  same  year  was 
estimated  at  120  tons,  and  elsewhere  20  tons,  giving  a  total 
production  of  about  400  tons. 

98  Properties. — Bromine  is  a  heavy  mobile  liquid,  so  dark 
as  to  be  opaque  except  in  thin  layers.  It  is  the  only  liquid 
element  at  the  ordinary  temperature  except  mercury.  Its 
specific  gravity  at  0°  is  3'1883  :  it  freezes  at  —7°  to  a  dark  brown 
solid,  evaporates  quickly  in  the  air,  and  boils  at  59°  (Thorpe). 
Bromine  possesses  a  very  strong  unpleasant  smell,  the  vapours 
when  inhaled  produce  great  irritation,  and  affect  the  eyes  very 
painfully.  When  swallowed,  it  acts  as  an  irritant  poison,  and 
when  dropped  on  the  skin  it  produces  a  corrosive  sore,  which  is 
very  difficult  to  heal. 

In  its  general  properties,  as  well  as  in  those  of  its  compounds, 
bromine  closely  resembles  chlorine,  although  they  are  not  so 
strongly  marked.  Thus  it  bleaches  organic  colouring  matters,  but 
much  less  quickly  than  chlorine  does,  and  it  combines  directly 
with  metals  to  form  bromides,  though  its  action  is  less  energetic 
than  that  of  chlorine.  It  does  not  combine  at  all  at  ordinary 
temperatures  with  metallic  sodium ;  indeed  these  two  substances 
may  be  heated  together  to  200°  before  any  perceptible  action 
commences,  whereas  bromine  and  potassium  cannot  be  brought 
together  without  combination  occurring,  sometimes  with  almost 


192  THE  NON-METALLIC  ELEMENTS 

explosive  violence. l  The  addition,  however,  of  a  drop  of  water 
to  bromine  and  clear  sodium  sets  up  a  lively  reaction.  If 
brought  into  contact  with  free  bromine,  starch-paste  is  coloured 
orange  yellow.  The  vapour  density  of  bromine  at  temperatures 
slightly  above  its  boiling  point  is  somewhat  higher  than  the 
normal — namely,  5*8691  (air  =  1),  but  at  228°  the  density  is 
5'5247,  showing  that  at  that  temperature  the  bromine  molecules 
contain  2  atoms. 2  Moreover,  if  bromine  vapour  be  mixed  with 
10  volumes  of  air,  its  vapour  density,  calculated  from  that  of 
the  mixture,  corresponds  to  the  formula  Br2  even  at  the  ordinary 
temperature.  3  At  about  1570°  the  observed  density  is  about 
I  of  that  at  228°,  so  that,  as  in  the  case  of  chlorine,  the 
molecules  Br2  are  at  this  temperature  partially  dissociated  into 
molecules  consisting  of  simple  atoms.4 

Bromine  is  largely  employed  in  the  colour  industry  and  also 
in  medicine,  whilst  smaller  quantities  are  used  in  analytical  and 
synthetical  chemistry,  and  as  a  disinfectant.  To  employ  it  for 
the  latter  purpose,  advantage  is  taken  of  the  fact  that  the 
siliceous  earth  known  as  "  kieselguhr "  absorbs  as  much  as 
75  per  cent,  of  its  weight  of  bromine,  and  still  retains  its 
solid  form ;  the  product  is  sold  under  the  name  of  "  bromum 
solidificatum." 

Bromine  and  Water. — A  definite  crystalline  compound  of 
bromine  and  water  is  obtained  by  exposing  a  mixture  of  the 
two  substances  to  a  temperature  near  the  freezing  point.  This 
hydrate  consists  of  Br  +  5H0O,  and  undergoes  decomposition 
into  bromine  and  water  at  15°.  The  analysis  of  the  compound 
gave  the  following  results : — 

Calculated.  Found  (Lowig). 

Bromine.     .     .     .     79*36  47'00  45'5 

5H2O      ....     89-40  53-00  54'5 


168-76  100-00  100-0 


From  the  dissociation  pressure,  Roozeboom  5  believes  that  it 
has  the  composition  Br  4-  4H2O  or  Br2  +  8H2O. 

The    solubility    of    bromine    in    water    between    the    tem- 

1  Merz  und  Weith,  Per.  6,  1518.  3  Jahn,  Ber.  15,  1238. 

3  Langer  and  v.  Meyer,  Ber.  15,  2773. 

4  V.  Meyer  and  Ziiblin,  Ber.  13,  405  ;  Crafts,  Compt.  Rend.  90,  183. 

5  Bee.  Trav.  CMm.  3,  73. 


BROMINE  AND  WATER  193 

peratures  of  5°  and  300,1  is  shown  in  the  following  table,  100 
grams  of  saturated  bromine  water  containing  by  weight — 

At    5°  .        .  3*600  grams  of  bromine. 

„  10  ...  3-327       „ 

„  15  ...  3-226       „ 

„  20  ...  3-208       „ 

„  25  ...  3-167       „ 

„  30  ...  3126       „ 


The  solution  of  bromine  in  water  has  an  orange  red  colour ; 
it  soon  loses  bromine  in  contact  with  the  air,  and  bleaches 
organic  colouring  matter.  Bromine  water  is  permanent  in  the 
dark,  but  on  exposure  to  sunlight  it  becomes  acid  from  the  form- 
ation of  hydrobromic  acid  and  evolution  of  oxygen.  Bromine 
also  dissolves  readily  in  chloroform,  carbon  disulphide,  alcohol, 
ether,  and  acetic  acid. 

The  atomic  weight  of  bromine  has  been  determined  by 
Marignac  and  by  Stas ;  the  latter  obtained  as  a  mean  of  a 
large  number  of  experiments  79*36  as  the  atomic  weight  of 
bromine  when  that  of  oxygen  is  15*88. 


BROMINE    AND    HYDROGEN. 

HYDROBROMIC  ACID.    HBr  =  80-36. 

99  Bromine,  like  chlorine,  forms  only  one  compound  with 
hydrogen,  containing  one  atom  of  bromine  and  one  of  hydrogen, 
but,  unlike  chlorine,  these  two  bodies  do  not  unite  to  form 
hydrobromic  acid  when  brought  together  in  sunlight.  If, 
however,  hydrogen  and  the  vapour  of  bromine  are  passed 
through  a  red-hot  tube  containing  finely-divided  metallic  plati- 
num or  pieces  of  charcoal,2  a  combination  occurs  of  equal 
volumes  of  bromine  and  hydrogen  and  formation  of  hydro- 
bromic acid  gas.  The  combination  of  these  two  bodies  may  be 
easily  shown  by  passing  hydrogen  over  bromine  vapour  and 
lighting  the  escaping  gas,  when  dense  fumes  of  hydrobromic 
acid  will  be  noticed. 

Preparation. — Hydrobromic  acid  gas  cannot  well  be  prepared 
by  the  action  of  the  ordinary  acids  on  the  bromides,  as  in  the 

1  Dancer,  Journ.  Ghem.  Soc.  1862,  ii,  477. 

2  Merz  and  Holzman,  Bcr.  22,  867. 
14 


194  THE  NON-METALLIC  ELEMENTS 

case  of  hydrochloric  acid,  owing  to  the  facility  with  which 
hydrobromic  acid  splits  up,  with  formation  of  free  bromine.  If, 
however,  phosphoric  acid  be  used,  free  hydrobromic  acid  is 
obtained. 

Sulphuric  acid  of  sp.  gr.  1*41  liberates  hydrobromic  acid  from 
potassium  bromide  without  simultaneous  formation  of  bromine, 
and  in  this  manner  a  dilute  solution  of  the  acid  may  be  obtained, 
which  may  be  concentrated  by  distillation.1 

One  of  the  best  methods  of  preparing  hydrogen  bromide  is 
to  bring  bromine  and  phosphorus  together  in  presence  of  a  little 
water,  when  a  violent  action  occurs,  hydrobromic  acid  gas  and 
phosphoric  acid  being  formed  : — 

p  +  5Br  +  4H20  =  5HBr  +  H3P04. 


FIG.  55. 

In  order  to  prepare  the  gas  a  flask  provided  with  a  doubly 
bored  caoutchouc  cork,  Fig.  55,  is  made  use  of;  through  one  of  the 
holes  a  gas  delivery-tube  is  fixed,  whilst  through  the  other  a  stop- 
pered funnel  tube  is  passed.  A  mixture  of  one  part  by  weight 
of  amorphous  phosphorus  and  two  parts  of  water  is  introduced 
into  the  flask,  and  ten  parts  of  bromine  are  allowed  to  fall  drop 
by  drop  through  the  stoppered  funnel-tube  on  to  the  mixture 
in  the  flask.  As  each  drop  falls  in  a  sudden  evolution  of  gas 
occurs,  accompanied  in  the  first  part  of  the  operation  by  a  flash 
of  light,  and  as  soon  as  a  certain  amount  of  hydrobromic  acid 
has  been  formed  the  bromine  dissolves  quietly,  and  on  gently 
warming  the  flask  hydrobromic  acid  gas  is  given  off.  This  is 
then  allowed  to  pass  through  a  U-tube  containing  amorphous 

i  Feit  and  Kubierschky,  J.  Pharm.  [5],  24,  159. 


HYDROBROMIC  ACID  195 


phosphorus,  to  free  it  from  any  vapour  of  bromine,  and  may 
be  collected  in  dry  stoppered  cylinders  by  displacement  or  over 
mercury.  The  same  apparatus  serves  for  preparing  a  saturated 
aqueous  solution  of  the  gas  ;  for  this  purpose  the  gas  delivery- 
tube  is  removed  and  a  short  tube  substituted  for  it.  This  passes 
through  a  cork  fitting  in  the  tubulus  of  a  retort  placed  in  the 
position  shown  in  Fig.  56 ;  the  neck  of  the  retort  dips  under 
water,  and  the  retort  itself  serves  as  a  safety  tube  in  case  the 
gas  be  absorbed  so  quickly  that  the  liquid  rushes  back,  as  then 
the  solution  is  not  sucked  back  into  the  flask,  but  simply  rushes 
into  the  upper  part  of  the  retort. 

A  second  method  is  to  pass  a  current  of  hydrogen  sulphide 
evolved  from  one  of  the  continuous  apparatus  through  a  layer  of 
bromine  contained  in  a  tall  cylindrical  vessel  and  covered  by  a 
layer  of  water  or  hydrobromic  acid,  when  the  following  reaction 
takes  place  : 

Br2  +  H2S  =  2HBr  +  S. 

The  liberated  sulphur  partially  combines  with  the  excess  of 
bromine,  forming  bromides  of  sulphur.  The  gas  evolved  is 
washed  by  passing  through  a  solution  of  potassium  bromide  in 
hydrobromic  acid  containing  a  little  amorphous  phosphorus  in 
suspension,  and  is  thus  obtained  free  from  bromine  and  sulphur- 
etted hydrogen  ;  with  this  method  the  evolution  of  the  gas  can 
be  regulated  very  exactly.1 

A  third  method  is  to  pass  a  mixture  of  hydrogen  and  bromine 
.  vapour  through  a  tube  in  which  a  platinum  spiral  is  heated  to 
bright  redness.  For  this  purpose  a  glass  tube  about  18  cm.  in 
length  and  15  mm.  in  width  is  fitted  at  each  end  with  a  cork 
carrying  a  small  tube  and  a  piece  of  stout  copper  wire  ;  the  ends 
of  the  stout  wires  are  joined  within  the  tube  by  a  platinum  spiral 
1  in.  in  length,  which  is  maintained  at  a  bright  red  heat  by 
means  of  an  electric  current ;  a  stream  of  hydrogen  is  then 
bubbled  through  bromine  heated  to  60°  and  passed  through  the 
wide  tube,  the  small  tube  being  plugged  with  glass  wool  to 
prevent  possible  explosions.  So  long  as  the  hydrogen  is  in 
slight  excess,  the  hydrogen  bromide  is  quite  free  from  bromine, 
and  the  presence  of  a  small  quantity  of  hydrogen  is  of  no 
importance  for  most  purposes,  especially  when  the  aqueous 
solution  of  the  acid  is  required.2 

1  Recoura,  Compt.  Rend.  HO,  784.       2  Newth,  Chcm.  News,  1891,  ii.  215. 


196 


THE  NON-METALLIC  ELEMENTS 


100  Properties. — Hydrobromic  acid  is  a  colourless  gas,  having  a 
strong  irritating  smell,  with  an  acid  taste  and  reaction.  It  fumes 
strongly  in  the  air,  and  on  exposure  to  a  temperature  of —73° 
(obtained  by  the  evaporation  of  a  mixture  of  ether  and  solid 
carbonic  acid),  it  condenses  to  a  colourless  liquid,  and  afterwards 
freezes  at  -  87°  to  a  colourless  ice-like  solid.1 

Like  hydrogen  chloride,  hydrogen  bromide  is  decomposed  by 
sunlight  in  presence  of  oxygen  and  moisture,  with  liberation  of 
bromine,  but  the  dry  mixture  of  the  gases  is  unaffected.2 


FIG. 


Pure  aqueous  hydrobromic  acid  is  colourless,  and  remains  so 
even  when  exposed  to  air ;  it  fumes  when  saturated  at  0°,  and 
then  possesses  a  specific  gravity  of  178.  The  weak  aqueous  acid 
becomes  stronger,  and  the  concentrated  acid  weaker  on  distilla- 
tion, until  an  acid  containing  from  47'38  to  47'86  per  cent,  of 
HBr,  distils  over  under  pressures  varying  from  0752  to  0762 

1  Faraday,  Phil.  Trans.  1845,  p.  155. 

2  Richardson,  Journ.  Chcm.  Soc.  1887,  i.  804. 


HYDROBROMIC  ACID  197 

metres.  When  the  pressure  under  which  the  distillation  occurs 
varies,  the  composition  of  the  constant  acid  changes  as  that  of 
hydrochloric  acid  does,  and  if  a  stream  of  dry  air  be  passed 
through  the  aqueous  acid  a  point  is  reached,  different  for  each 
temperature,  at  which  the  acid  no  longer  undergoes  change.  Thus 
the  acid  which  evaporates  unchanged  in  air  at  100°  contains 
49'35  per  cent.  HBr,  whilst  that  obtained  at  16°  contains 
51-65  per  cent  of  HBr. 

The  variation  of  the  specific  gravity  of  the  aqueous  acid  with 
the  percentage  of  hydrobromic  acid  dissolved  has  been  determined 
by  Topsoe,1  as  also  by  C.  R.  A.  Wright,'2  who  obtained  the 
following  numbers: — 

Per  Cent.  HBr.  Spec.  Grav.  at  15°. 

10-4 1-080 

23-5 1190 

30-0 .  1-248 

40-8 1-385 

48-5 1-475 

49-8 1-515 

The  composition  of  this  gas  is  analogous  to  that  of  hydro- 
chloric acid,  and  can  be  ascertained  in  a  similar  way  by  bringing 
a  given  volume  of  the  dry  gas  in  contact  with  sodium  amalgam, 
when  sodium  bromide  is  formed  and  hydrogen  liberated,  the 
volume  of  which  is  found  to  be  exactly  half  that  of  the  original 
hydrobromic  acid  gas. 

101  The  Bromides. — These  compounds  are  formed  in  a  similar 
manner  to  the  corresponding  chlorides.  They  possess  an  analogous 
composition  with  these,  and  exhibit  similar  properties.  Bromine 
unites  with  nearly  all  the  metals,  forming  bromides,  which  are 
also  produced  by  the  action  of  metals  on  hydrobromic  acid,  or 
by  the  action  of  vapour  of  bromine  on  the  metallic  oxides, 
oxygen  being  liberated. 

The  metallic  bromides  are  nearly  all  soluble  in  water,  the 
most  insoluble  being  silver  bromide,  AgBr,  mercurous  bromide, 
HgBr,  and  lead  bromide,  PbBr2,  which  latter  is  slightly  soluble. 
All  the  bromides  are  solid  at  the  ordinary  temperature,  but  when 
heated  they  fuse  and  volatilize,  some  undergoing  decomposition 
others  remaining  unchanged.  They  are,  however,  all  decomposed 
by  chlorine,  either  in  the  cold  or  upon  heating,  a  metallic  chloride 
being  formed  and  bromine  liberated ;  they  are  .also  decomposed 

1  Ber.  3,  404.  2  Chem.  News,  23,  242. 


198  THE  NON-METALLIC  ELEMENTS 

by  sulphuric   and  nitric  acids,  with  evolution  of  hydrobromic 
acid,  which  again  is  partly  oxidized,  bromine  being  set  free. 

102  Detection  and  Estimation  of  Bromine. — Bromine  when  in  the 
free  state  may  be  recognized  by  the  red  colour  of  its  vapour,  and 
by  its  exceedingly  disagreeable  odour,  and  by  imparting  to  starch 
paste  an  orange-yellow  colour.  When  present  in  small  quantities 
it  may  be  detected  by  shaking  up  with  chloroform  or  ether, 
which  dissolves  it,  and  acquires  thereby  a  red  or  brownish  colour. 
Bromine  in  the  state  of  a  soluble  bromide  may  be  detected  by 
giving  with  silver  nitrate  a  yellowish  white  precipitate  of  silver 
bromide,  which  is  insoluble  in  nitric  acid,  and  dissolves  only  with 
difficulty  in  ammonia,  but  readily  in  cyanide  of  potassium.  Also 
by  giving  with  nitrate  (but  not  chloride)  of  palladium  a  reddish- 
brown  precipitate  of  the  bromide ;  by  its  tinging  carbon  di- 
sulphide  yellow  in  presence  of  hydrochloric  acid  and  a  drop  of 
sodium  hypochlorite  ;  and  by  the  liberation  of  bromine  on  heat- 
ing with  sulphuric  acid,  with  sulphuric  acid  and  manganese 
dioxide,  or  with  sulphuric  acid  and  potassium  bichromate. 

When  the  bromine  is  present  as  a  soluble  bromide  it  is  usually 
estimated  by  precipitation  as  bromide  of  silver,  which  contains 
42*42  per  cent,  of  bromine.  In  presence  of  chlorine  the  two 
elements  are  precipitated  together  by  nitrate  of  silver,  the  pre- 
cipitate is  then  fused  and  weighed.  A  portion  of  it  is  next 
ignited  in  a  current  of  chlorine,  when  the  whole  of  the  bromine 
is  expelled,  the  residue  of  silver  chloride  weighed,  and  from  the 
weight  thus  obtained  and  that  of  the  mixed  silver  salts,  the 
quantities  of  chlorine  and  bromine  are  calculated.  For  every 
79*36  parts  of  bromine  expelled,  35*19  parts  of  chlorine  have  been 
substituted;  or,  if  a  difference  of  44*17  is  observed,  79*36  parts 
of  bromine  must  have  been  present.  Hence,  for  any  other 
difference  the  weight  of  bromine  is  found  by  multiplying  that 

difference  by  ^?|=  1'797. 
44*17 


CHLORINE  AND   BROMINE. 

103  Bromine  monochloride,  BrCl. — When  chlorine  gas  is  passed 
into  liquid  bromine  cooled  below  10°  it  is  largely  absorbed,  a  red- 
dish brown  volatile  mobile  compound  of  the  two  elements  being 
formed.1  It  dissolves  in  water  yielding  a  yellow  solution,  from 

1  Balard,  Ann.  CMm.  Phys.  32    337  :  Bornemann.  Annalcn,  189,  183. 


IODINE  199 


which,  on  cooling  below  0°,  a  crystalline  hydrate  BrCl  +  10H20 
separates  out,  which  melts  at  4-  7°.  This  is  possibly  simply  a 
mixture  of  chlorine  and  bromine  hydrates. 


IODINE.     1=125-91. 

104  Iodine  was  discovered  in  1812  by  Courtois,1  of  Paris,  in 
the  mother-liquors  of  the  soda  salts  which  are  prepared  from 
kelp  or  burnt  seaweed.     It  was  afterwards  examined  by  Davy,2 
and  much  more  completely  by  Gay-Lussac.3     Iodine  derives  its 
name  from  loeiSys,  violet-coloured,  owing  to  the  peculiar  colour 
of  its  vapour,  by  means  of  which  it  was  first  discovered. 

Like  chlorine  and  bromine,  iodine  does  not  occur  in  the  free 
state  in  nature,  but  is  found  combined  with  metals  to  form 
iodides,  which  occur  in  small  quantities,  but  widely  diffused, 
both  in  the  organic  and  inorganic  kingdoms,  having  been 
detected  in  sea-water,  in  sea-plants  and  animals,  and  in  many 
mineral  springs.  The  quantity  of  iodine  present  in  sea-water 
is  extremely  small,  but  certain  plants  and  even  animals  have 
the  power  of  absorbing  and  storing  up  the  iodine.  The  ash  of 
the  deep-sea-weed  (Fucus  palmatus  especially)  contains  more 
iodine  than  that  which  grows  in  shallow  water,  and  it  is 
from  the  weed  collected  on  exposed  and  rocky  coasts,  as  the 
north  and  west  coasts  of  Ireland,  Scotland,  and  France,  that  a 
large  portion  of  the  iodine  of  commerce  is  obtained.  Of  late 
years,  however,  the  quantity  manufactured  from  kelp  has 
considerably  decreased,  owing  to  the  discovery  of  iodine  in  the 
crude  Chili  saltpetre  or  Caliche  NaN03,  from  the  mother 
liquors  of  which  it  is  now  chiefly  obtained  ;  it  is  likewise  found 
in  combination  with  silver  in  a  Mexican  silver  ore,  in  some 
specimens  of  South  American  lead  ore,  in  certain  dolomites, 
and  in  small  quantities  in  almost  every  deposit  of  rock  salt. 
Iodine  has  also  been  found  in  coal,  and  it  has  been  detected  in 
some  few  land  and  fresh- water  plants,  and  in  many  sea  animals, 
as  in  sponges  and  oysters,  and  also  in  cod  liver  oil. 

105  Preparation  from  Seaweed. — The  stormy  months  of  the 
spring  are  those  in  which  the  deep-sea  tangle  is  thrown  up  on 

1  Courtois,  Clement,  and  Desormes,  Ann.  Chim.  88,  304. 

2  Phil.  Trans.  1814,  2,  74  and  487. 

3  Ann.  Chim.  88,  311,  319,  and  91,  5. 


200 


THE  NON-METALLIC  ELEMENTS 


the  north  coasts  of  France  and  Ireland  and  the  western  coasts 
of  Scotland.  The  inhabitants  collect  the  weed,  allow  it  to  dry 
during  the  summer,  and  then  burn  it  in  large  heaps.  The  ash 
thus  obtained  is  termed  kelp  in  Scotland  and  varec  in  Normandy ; 
it  contains  from  O'l  to  O3  per  cent,  of  iodine.  When  the 
seaweed  is  completely  burnt,  and  when  the  ash  is  fused,  a  con- 
siderable fraction  of  the  iodine  is  lost  from  volatilization  ;  hence 
it  is  preferable  to  carbonize  the  weed  in  closed  retorts,  the  whole 
of  the  iodine  remaining  in  the  ash,  whilst  the  tar  and  ammonia- 
cal  liquor  are  also  recovered ;  the  carbon  remaining  after  the 


FIG.  57 


lixiviation  of  the  ash  resembles  animal  charcoal,  and  is  used  as  a 
disinfectant. 

In  the  most  recent  process  for  obtaining  the  iodine  from  sea- 
weed the  latter  is  directly  lixiviated  without  previous  carboniza- 
tion, the  residual  apparently  unaltered  plants  being  converted 
into  algin,  a  substance  which  resembles  gelatin  and  is  employed 
as  a  substitute  for  bladder  skins,  &c.  On  lixiviating  systematic- 
ally either  the  kelp  or  the  carbonized  weed,  a  concentrated 
solution  of  the  alkaline  carbonates,  chlorides,  sulphates,  and  a 
small  quantity  of  sulphites  and  sulphides,  together  with  the 


MANUFACTURE  OF  IODINE  201 

iodides  and  bromides  of  the  alkali  metals,  is  obtained,  and  from 
this  solution  the  carbonates,  chlorides,  and  sulphates  are  allowed 
to  crystallize,  leaving  the  bromides  and  the  iodides  in  the 
mother-liquor.  This  liquor  is  then  treated  in  several  ways  in 
order  to  obtain  the  iodine. 

(1)  An  excess  of  sulphuric  acid  is  added  to  the  liquor,  when  the 
sulphides  and  sulphites  which  it  contains  are  decomposed  and  the 
iodine    and  bromine  liberated  as   hydriodic  and  hydrobromic 
acid.      In  this  process  the  liquor,  after  any  separated  crystals 
of  sodium  sulphate  which  may  have  formed  have  been  taken 
out,  is  placed  in  iron  boilers,  Fig.  57,  surrounded  with  brick- 
work, each  gently  heated  by  a  separate  fire  to  a  temperature  of 
60°,  and  fitted  with  leaden  hoods,  which  can  be  lifted  off  by 
means  of  a  chain  and  winch.     Each  cover  is  fitted  with  a  leaden 
pipe  (a),  and  this  is  connected  with  a  series  of  glass  or  earthen- 
ware condensers,  termed  udells,  fitting  one  into  the  other.   After 
the  introduction  of  the  liquor,  the  covers  are  luted  on  with  clay, 
the  pipes  (a)  fixed  in  their  receptacles  and  connected  with  the 
condensers.    Manganese  dioxide  is  then  thrown  little  by  little  into 
the  still  through  the  hole  (fr),  which  can  be  closed  by  a  stopper. 
The  iodine  thus  liberated  condenses  in  the  receivers,1  and  the 
accompanying  water  escapes  through  a  tubulus  at  the  bottom  of 
each  receiver  and  runs  away  along  the  channel  (c).     When  no 
more  iodine  distils  over,  the  leaden  pipes  are  dismounted,  and 
the  stills  are  connected   with   a   second   receiver    (D).      More 
manganese   dioxide   is   then   added,   and   the   bromine,   which 
hitherto  has  not  been  liberated,  is  now  disengaged  and  collects 
in  (D)  and  in  the  WoulfFs  bottles  (E). 

The  decomposition  occurring  during   the   formation   of  the 
iodine  is  represented  by  the  following  equation  : — 

2NaI  +  3H2SO4  +  MnO2  =  I2  +  2NaHSO4 + MnSO4 + 2H2O. 

The  iodine  thus  obtained  may  then  be  partially  purified  by 
resublimation,  but  even  then  invariably  contains  traces  of 
chloride,  bromide,  and  cyanide  of  iodine. 

(2)  The  iodine  contained  in  the  liquors,  after  separation  of 
the  crystallizable  alkaline  salts,  may  also  be  liberated  by  the 
addition  of  sulphuric  acid  containing  a  considerable  quantity  of 
nitric  acid.     The  acidified  liquor  is  then  agitated  with  the  most 
volatile   portion  of   petroleum    (petroleum-naphtha,  kcrosine), 
which  dissolves  the  iodine.     The  petroleum  solution  of  iodine 

1  One  ton  of  kelp  usually  yields  12  Ibs.  of  iodine. 


THE  NON-METALLIC  ELEMENTS 


is  next  drawn  off  from  the  aqueous  liquor  and  shaken  up  with 
an  aqueous  solution  of  caustic  soda,  whereby  the  iodine  is 
withdrawn  from  the  hydrocarbon  and  converted  into  iodide 
and  iodate  of  sodium.  The  iodine  is  then  liberated  from  these 
salts  by  the  addition  of  hydrochloric  acid,  thus  :  — 


(3)  In  France  the  mother  liquor  is  treated  with  a  slight 
excess  of  sulphuric  acid,  filtered  from  sulphur,  diluted  to  40° 
Tw.,  and  saturated  with  chlorine,  care  being  taken  to  avoid 
adding  an  excess  of  the  latter,  which  would  cause  loss  owing  to 
the  formation  of  chloride  of  iodine.  The  iodine  separates  out 
in  the  solid  form  and  is  filtered  off,  dried,  and  resublimed. 

Preparation  from  Caliche.  —  The  crude  Chili  saltpetre,  known 
as  Caliche,  now  forms  the  chief  source  of  iodine  ;  to  obtain  the 


FIG.  58. 

iodine  the  last  mother-liquor  obtained  in  the  preparation  of  the 
sodium  nitrate,  which  contains  about  20  per  cent,  of  sodium 
iodate,  is  treated  with  a  solution  of  sodium  bisulphite,  obtained 
by  burning  native  sulphur  and  passing  the  products  into  a  solu- 
tion of  sodium  carbonate.  The  iodine  separates  out  in  the  solid 
form,  and  is  filtered  off  and  purified  by  resublimation,  the 
vapours  being  condensed  in  a  series  of  udells  similar  to  those 
shown  in  Fig.  57. 

In  order  to  purify  the  commercial  iodine  it  is  washed  with  a 
small  quantity  of  water,  dried  on  porous  plates,  and  resublimed. 
According  to  Stas,1  the  only  mode  of  obtaining  chemically-pure 
iodine  (free  from  every  trace  of  chlorine  and  bromine)  is  to 
dissolve  the  commercial  resublimed  substance  in  iodide  of 
potassium  solution,  and  then  to  precipitate  the  iodine  by  water. 
The  precipitate  is  well  washed  with  water  and  then  distilled 

1  Rechcrches,  p.  136. 


PROPERTIES  OF  IODINE  203 

with  steam,  the  solid  iodine  in  the  distillate  collected  and 
dried  in  vacuo  over  solid  nitrate  of  calcium,  which  is  frequently 
changed,  and  distilled  afterwards  over  solid  caustic  baryta  to 
remove  the  last  traces  of  water  and  of  hydriodic  acid. 

1 06  Properties. — Iodine  is  a  bright,  shining,  crystalline,  opaque, 
blackish-grey  solid.  The  crystals  when  large  possess  almost  a 
metallic  lustre ;  it  crystallizes  by  sublimation  in  the  rhombic 
system,  in  the  form  of  prisms  or  pyramids.  Finer  crystals  are 
obtained  from  solution,  as  by  exposing  to  the  air  a  solution 
of  iodine  in  ether,  or  in  an  aqueous  solution  of  hydriodic  acid. 
The  crystals  thus  obtained  have  the  ratio  of  their  axes  represent- 
ed by  the  numbers  4  :  3  :  2.  The  crystal  represented  in  Fig. 
58  a  was  obtained  by  Marignac  from  solution  in  hydriodic 
acid. 

Iodine  is  a  heavy  substance,  having  a  specific  gravity  of  4'948 
at  17°,  melting  at  114°-2  and  solidifying  at  113°'6  (Stas).  It 
boils  at  184'35  under  760  mm.  pressure,1  giving  rise  to  a  vapour 
which,  seen  by  transmitted  white  light,  possesses,  when  chemi- 
cally pure,  a  splendid  deep  blue  colour,  but  when  mixed  with 
air  a  reddish-violet  colour  (Stas).  The  specific  gravity  of 
iodine  vapour  was  found  by  Deville  and  Troost  to  be  8*72 
(air  =  l),  which  corresponds  to  the  density,  125'9,  proving 
that  the  molecule,  or  two  volumes  of  iodine  gas,  weighs 
125'91  x  2  =  251-82  and  that  the  molecular  formula  is  I2. 
When  iodine  vapour  is  heated  above  700°  its  specific  gravity 
begins  to  diminish  until  at  1700°  it  becomes  constant,  and  is 
half  that  at  700°,  the  vapour  consisting  of  free  atoms.2  At  the 
ordinary  temperature  it  volatilizes  slowly,  shining  crystals 
being  deposited  on  the  sides  of  a  bottle  on  the  bottom  of  which 
a  little  iodine  has  been  placed.  It  is  a  bad  conductor  of 
electricity,  and  possesses  a  peculiar  smell  less  penetrating  than, 
though  similiar  to  that  of,  chlorine  and  bromine.  The  specific 
heat  of  solid  iodine  is,  according  to  the  experiments  of 
Regnault,  0'05412,  and  that  of  the  liquid  010882.  The  latent 
heat  of  fluidity  of  iodine  is  11 '7  thermal  units,  its  heat  of 
vaporization  23'95  thermal  units. 

When  an  electric  discharge  is  passed  through  a  heated 
Geissler's  vacuum-tube  containing  a  trace  of  iodine  vapour, 
a  spectrum  of  bright  lines  is  obtained,  characteristic  of  this 

1  Ramsay  and  Young,  Journ.  Chem.  Soc.  1886,  1,  453. 

2  V.  Meyer,  Bcr.  13,  394,  1010,  1103  ;  v.   Meyer  and  Biltz,  Ber.  22,  725  ; 
Meier  and  Crafts,  Compt.  Rend.  90,  690  ;  92,  39. 


204  THE  NON-METALLIC  ELEMENTS 

element.1  This  emission  spectrum  is,  however,  not  identical 
with  the  characteristic  absorption-spectrum  of  iodine,  so  care- 
fully mapped  by  Thalen,2  and  seen  when  white  light  is  passed 
through  iodine  vapour.  Salet  3  has  recently  shown  that  when 
an  electric  current  of  feeble  tension  is  passed  through  a  Geissler's 
tube  containing  iodine,  another  set  of  bright  bands  is  obtained, 
which  are  identical  in  position  with  the  dark  bands  of  Thalen's 
absorption-  spectrum,  each  bright  band  being  replaced  by  a  black 
band  when  the  vapour  is  illuminated  from  behind. 

107  In  its  chemical  properties  iodine  resembles  chlorine  and 
bromine  ;  the  two  latter  elements  have  the  power  of  displacing 
iodine  from  its  combination  with  metals  (or  electro-positive 
elements),  thus  :  — 

C1  =  2KC1      I. 


The  compounds  of  iodine  with  oxygen  (or  with  electro- 
negative elements)  are,  on  the  other  hand,  more  stable  than 
those  of  the  other  two  elements.  Thus  iodine  expels  chlorine 
from  the  chlorates  with  formation  of  iodate  and  free  chlorine  :  — 


These  differences  are  explained  when  we  examine  the  amount 
of  heat  evolved  by  the  several  decompositions  in  question.  This 
heat  may  be  taken  as  a  measure  of  the  relative  affinity  or  power 
of  combination  which  the  elements  exhibit  towards  one  another. 
Thus  the  heat  evolved  on  the  combination  of  chlorine,  bromine, 
and  iodine  with  hydrogen,  or  the  heat  modulus  of  the  reactions 
according  to  Julius  Thomsen's  experiments  4  is  :  — 

H  +  Cl  .     .     .     21836  heat  units 
H  +  Br  .     .     .       8337 
H  +  I     .    .    .  -  6060 

When  these  same  elements  unite  with  oxygen  and  hydrogen 
to  form  the  oxyacids  (HC1O8,  HBrO3,  HIO3)  the  heat  evolved 
is  as  follows  :  — 

Cl  +  3  O  +  H  .     .     23761  heat  units 
Br  +  3  O  +  H  .     .       5344 
I     +  3  0  +  H  .     .     43211 

1  Plucker  and  Hittorf,  Phil.  Trans.  1865,  28. 

2  Kon  Svenska  Acad.  Handb.  1869.  3  Phil  Mag.  [4],  44,  156. 
4  Journ.  Chem.  Soc.  1873,  1188. 


PROPERTIES  OF  IODINE  205 

Hence  we  see  that  as  regards  affinity    for  oxygen    chlorine 

stands     nearly    midway    between     bromine  and    iodine,    for 
43711  +  5344 


Iodine  dissolves  but  very  sparingly  in  water,  one  part  being 
soluble  in  5524  parts  of  water  at  10°  ;  but  it  dissolves  freely  in 
an  aqueous  solution  of  potassium  iodide,  and  in  alcohol,  yielding 
brown  solutions.  Tincture  of  iodine  of  the  pharmacopoeia 
contains  J  oz.  of  iodine,  J  oz.  of  iodide  of  potassium,  rectified 
spirit  1  pint.  It  is  also  soluble  in  chloroform,  carbon  disulphide, 
and  many  liquid  hydrocarbons,  imparting  to  these  liquids  a 
fine  violet  or  dark-red  colour  when  present  in  small  quantities, 
and  in  larger  quantities  forming  a  black  opaque  solution,  which 
in  the  case  of  carbon  disulphide  is  diathermous,  allowing  the 
invisible  heating  rays  of  low  refrangibility  to  pass,  though  it  is 
opaque  to  the  visible  rays. 

Although  iodine,  unlike  chlorine  and  bromine,  does  not  combine 
readily  with  hydrogen,  it  unites  with  many  of  the  metals  and 
non-metals  with  evolution  of  light  and  heat.  Thus  solid 
phosphorus,  when  brought  into  contact  with  iodine,  first  melts 
and  then  bursts  into  flame  owing  to  the  heat  evolved  in  the  act 
of  combination  :  and  powdered  antimony  takes  fire  when  thrown 
into  iodine  vapour,  antimony  iodide  being  produced,  whilst  if 
the  vapour  of  mercury  be  passed  over  heated  iodine,  immediate 
action  occurs,  the  iodides  of  mercury  being  formed.  When 
iodine  is  brought  into  contact  with  water  and  filings  of  iron  or 
zinc,  a  violent  reaction  occurs,  colourless  solutions  of  the 
respective  iodides  resulting.  The  action  of  iodine  upon  the 
alkali-metals  is  analogous  to  that  of  chlorine  and  bromine. 
Sodium  and  iodine  can  be  heated  together  without  any  alteration, 
whilst  if  potassium  be  employed  an  explosive  combination 
occurs. 

Potash  at  once  decolorises  a  solution  of  iodine,  iodide  and 
iodate  of  potassium  being  produced,  thus  :  — 

3I2  +  6KHO  =  5KI  +  KI03  +  3H2O. 

When  acted  upon  by  strong  nitric  acid,  iodine  is  completely 
oxidized  to  iodic  acid  HIO3. 

The  most  characteristic  property  of  free  iodine  is  its  power 
of  forming  a  splendid  blue  colour  with  starch-paste.  This  is 
formed  when  starch  granules  are  brought  into  contact  with  the 
vapour  of  iodine,  or,  better,  when  a  solution  of  iodine  is  added 


206  THE  NON-METALLIC  ELEMENTS 

to  starch  paste.  The  blue  colour  disappears  on  warming  the 
solution,  but  reappears  on  cooling,  and  its  formation  serves 
as  a  most  delicate  test  for  the  presence  of  iodine.1  In  order 
,to  exhibit  this  property,  a  few  grains  of  iodide  of  potassium 
may  be  dissolved  in  three  or  four  litres  of  water  placed 
in  a  large  glass  cylinder,  and  some  clear,  dilute,  well-boiled 
starch-paste  added.  As  the  iodine  is  here  combined  with  the 
metal,  no  coloration  will  be  seen,  but  if  a  few  drops  of  chlorine 
water  be  added,  or,  better,  if  a  little  of  the  air  (containing  free 
chlorine)  from  a  bottle  of  chlorine  water  be  poured  on  to  the 
surface  of  the  liquid,  a  blue  film  will  be  formed,  which  on 
stirring  will  impart  a  blue  tint  to  the  whole  mass.  Iodine 
both  free  and  in  combination  is  largely  used  in  medicine.  The 
atomic  weight  of  iodine  has  been  very  accurately  determined 
in  several  ways  by  Marignac  and  Stas,  the  mean  value  obtained 
from  a  large  number  of  closely-agreeing  experiments  being 
125-91  (O  =  15-88). 


IODINE  AND  HYDROGEN. 
HYDRIODIC  ACID.    HI  =  126-91. 

108  Iodine  and  hydrogen  undergo  partial  combination  when 
they  are  passed  over  finely-divided  platinum  heated  to  redness, 
or  over  charcoal  at  a  bright  red  heat,2  forming  a  strongly  acid 
gas,  having  properties  very  similar  to  hydrochloric  and  hydro- 
bromic  acid. 

Hydriodic  acid  can  also  be  obtained  by  heating  iodide  of 
potassium  with  phosphoric,  but  not  with  sulphuric,  acid  ;  for 
when  this  latter  acid  is  used,  sulphur  dioxide  SO2  and  free 
iodine  are  formed  at  the  same  time,  thus : — 

3H2  S04  +  SKI  =  2KHS04  +  I2  +  SO2  +  2H2O. 

On  the  other  hand,  hydriodic  acid  is  easily  prepared  by  allow- 
ing iodine  and  phosphorus  to  act  on  one  another  in  presence  of 
water,  thus  : — 

P  +  51  +  4H20  =  SHI  +  H3P04. 

Preparation. — For  this  purpose  1  part  by  weight  of  amorphous 
phosphorus  and  15  parts  of  water  are  brought  together  in  a 

1  Collin  and  Gaultier  de  Claubry,  Ann.  Chim.  90>  87. 

2  Merz  and  Holzmann,  Ber.  22,  867. 


HYDRIODIC  ACID 


207 


tubulated  retort  or  flask,  provided  with  a  caoutchouc  cork  and 
gas  delivery-tube,  and  to  these  20  parts  of  iodine  are  gradually 
added,  the  contents  of  the  flask  during  this  operation  being  kept 
cool  by  immersing  the  flask  in  cold  water.  When  all  the  iodine 
has  been  added,  and  as  soon  as  no  further  evolution  of  gas  can 
be  noticed,  the  flask  may  be  gently  warmed.  The  gas  thus 
obtained  may  either  be  received  in  dry  bottles  filled  with 


FIG.  59. 


mercury  in  the  mercurial  trough,  or  it  may  be  collected  by 
displacement,  as  it  is  more  than  four  times  as  heavy  as  air. 

If  we  possess  a  concentrated  solution  of  hydriodic  acid,  the 
gas  may  be  obtained  in  a  still  more  simple  manner.  Two  parts 
of  iodine  are  dissolved  in  aqueous  hydriodic  acid  of  specific 
gravity  1*7,  and  this  solution  is  allowed  to  fall,  drop  by  drop,  by 
means  of  a  stoppered  funnel-tube,  into  a  flask  containing  amor- 
phous phosphorus  covered  with  a  thin  layer  of  water.  The  evo- 
lution of  gas  occurs  at  first  without  any  application  of  heat  being 


208  THE  NON-METALLIC  ELEMENTS 

necessary,  but  after  a  time  the  flask  may  be  slightly  warmed. 
The  apparatus,  Fig.  59,  is  used  for  the  purpose  of  preparing, 
according  to  the  above  method,  a  saturated  aqueous  solution  of 
hydriodic  acid. 

When  hydriodic  acid  is  prepared  by  the  foregoing  methods 
it  frequently  contains  phosphuretted  hydrogen;  the  forma- 
tion of  this  impurity  may,  however,  be  avoided  if  the 
iodine  is  never  allowed  to  come  in  contact  with  an  excess  of 
phosphorus.  According  to  Lothar  Meyer,  this  may  be  readily 
carried  out  as  follows  : — 100  parts  of  iodine  moistened  with 
ten  parts  of  water  are  placed  in  a  tubulated  retort,  the  neck  of 
which  is  inclined  upwards ;  a  thin  paste  of  5  parts  of  amorphous 
phosphorus  and  10  parts  of  water  is  then  allowed  gradually  to 
drop  in  through  the  tubulus  of  the  retort.  For  this  purpose  a 
dropping  funnel  is  employed,  which,  in  place  of  a  stopcock,  is 
furnished  with  a  glass  rod,  ground  at  the  lower  end  to  fit  into 
the  funnel  tube  ;  by  raising  the  latter,  small  quantities  of  the 
paste  are  allowed  to  pass  into  the  retort.  The  first  few  drops 
musi:  be  added  cautiously,  waiting  each  time  for  the  reaction  to 
moderate,  as  otherwise  an  explosion  may  occur ;  after  a  short 
time,  however,  larger  quantities  may  be  added  at  once,  and  the 
mixture  may  be  completed  in  a  quarter  of  an  hour.  The  iodine 
carried  over  mechanically  settles  for  the  most  part  in  the  neck 
of  the  retort,  and  may  be  removed  completely  in  most  cases  by 
washing  with  a  little  water.1  The  collection  of  the  gas  or  the 
preparation  of  the  solution  may  be  carried  out  in  the  manner 
already  described. 

Properties. — Hydriodic  acid  exists  at  the  ordinary  temperature 
and  pressure  as  a  colourless  gas,  having  a  strongly  acid  reaction 
and  suffocating  odour,  and  fuming  strongly  in  the  air.  It  can 
be  condensed  to  a  colourless  liquid2  by  a  pressure  of  four 
atmospheres  at  0°,  or  by  exposure,  under  the  ordinary  atmos- 
pheric pressure,  to  the  low  temperature  of  a  bath  of  ether  and 
solid  carbonic  acid,  and  if  cooled  to  -  55°  it  freezes  to  a  colour- 
less ice-like  solid  mass. 

Its  specific  gravity  (air=l)  has  been  found  to  be  4*3737, 
or  62'94  (H  =  l),  thus  closely  corresponding  to  its  theoretic 
density,  63'45. 

Hydriodic  acid  gas  is  easily  decomposed  by  heat  into  iodine 
and  hydrogen,  as  is  seen  by  the  violet  colour  which  appears 
when  the  gas  is  passed  through  a  heated  glass  tube.  A  hot 

1  Ber.  20,  3381.  2  Faraday,  Phil.  Trans.  1845,  1,  170. 


HYDKIODIC  ACID  209 


metallic  wire  plunged   into   the  gas  also  causes  an  immediate 
decomposition,  violet  fumes  of  iodine  making  their  appearance. 

109  The  aqueous  acid  is  obtained  by  passing  the  gas  into  water, 
by  which  it  is  absorbed  quickly  and  in  large  quantities,  yield- 
ing, when  kept  cold  by  ice,  a  solution  which  is  twice  as  heavy  as 
water,  having  a  specific  gravity,  according  to  De  Luynes,of  T99. 
A  simple  mode  of  preparing  a  dilute  aqueous  solution  of  hydri- 
odic acid  consists  in  passing  a  current  of  sulphuretted  hydrogen 
gas  through  water  in  which  finely-divided  iodine  is  suspended, 
the  reaction  which  occurs  being  as  follows  — 


On  standing,  the  clear  liquid  may  be  poured  off  from  the 
precipitated  sulphur  and  boiled  to  expel  any  trace  of  sulphuretted 
hydrogen.  It  is  found  that  the  strongest  acid  which  can  in  this 
way  be  prepared  has  a  specific  gravity  of  I1  56. 

On  distillation,  aqueous  hydriodic  acid  behaves  like  aqueous 
hydrochloric  and  hydrobromic  acids,  both  the  strong  and  weak 
aqueous  acid  yielding  on  distillation  in  an  atmosphere  of  hy- 
drogen (to  prevent  oxidation  and  liberation  of  iodine)  an  acid 
of  constant  composition,  boiling  at  127°  (under  a  pressure  of 
774  mm.),  and  containing  57*0  per  cent,  of  hydriodic  acid. 
If  dry  hydrogen  be  led  through  aqueous  acids  of  varying 
strengths,  each  will  attain  the  same  constant  composition  at 
the  same  temperature  ;  thus  from  15°  to  19°  the  constant  acid 
contained  GO'S  to  6(V7  per  cent,  of  HI.  When  the  hydrogen 
was  passed  through  the  liquid  at  100°,  the  percentage  of  hy- 
driodic acid  in  the  constant  acid  was  58'S,1  and  hence  it  is 
seen  that  no  definite  hydrate  of  the  acid  is  obtained  by  boil- 
ing, as  was  formerly  supposed.  Aqueous  hydriodic  acid  also 
rapidly  undergoes  oxidation  with  liberation  of  iodine  when  ex- 
posed to  the  air,  the  colourless  solution  becoming  brown  owing 
to  the  solubility  of  iodine  in  the  acid.  Gaseous  hydrogen  iodide 
is  also  decomposed  by  dry  oxygen  when  the  mixed  gases  are 
exposed  to  bright  sunlight,  and  differs  in  this  respect  from 
hydrogen  chloride  and  bromide  which  are  only  decomposed  by 
oxygen  in  presence  of  moisture.2 

no  The  Iodides.  —  The  metallic  iodides  possess  great  analogy 
with  the  corresponding  chlorides  and  bromides  ;  they  are  all 

1  Roscoe,  Journ.  Chem.  Soc.  1861,  160. 

2  Richardson,  Journ.  Chem.  Soc.  1887,  i.  805. 
15 


210  THE  NON-METALLIC  ELEMENTS 

solid  bodies,  less  fusible  and  volatile  than  the  corresponding 
chlorides  and  bromides.  Silver  iodide,  Agl,  mercurous  iodide, 
Hgl,  and  mercuric  iodide,  HgI2,  are  insoluble  in  water,  and  lead 
iodide,  PbI2,  sparingly  soluble,  whilst  the  other  metallic  iodides 
dissolve  readily  in  water.  Most  of  the  iodides  are  decomposed 
on  heating,  either  the  metal  or  an  oxide  being  formed  and 
iodine  set  free. 

All  the  iodides,  whether  soluble  or  insoluble  in  water,  are 
decomposed  by  chlorine  and  nitrous  acid,  the  iodine  being  liber- 
ated. Some  of  the  insoluble  iodides  possess  a  brilliant  colour. 
Thus,  on  adding  a  solution  of  corrosive  sublimate  (mercuric 
chloride)  to  a  soluble  iodide,  a  salmon-coloured  precipitate  is 
thrown  down,  which  rapidly  changes  to  a  brilliant  scarlet  one  of 
mercuric  iodide,  HgI2,  soluble  in  excess  of  either  reagent ;  a 
soluble  lead  salt,  such  as  the  nitrate  or  acetate,  produces  a  bright 
yellow  precipitate  of  lead  iodide,  PbI2 ;  silver  nitrate  gives  a 
light  yellow  precipitate  of  silver  iodide,  Agl,  insoluble  in  nitric 
acid  and  in  ammonia.  If  a  mixture  of  ferrous  sulphate,  FeSO4, 
and  copper  sulphate,  CuS04,  be  added  to  that  of  a  soluble 
iodide,  a  greenish  white  precipitate  of  cuprous  iodide,  Cul,  is 
formed.  This  reaction  depends  upon  the  fact  that  ferrous 
sulphate  is  oxidized  to  ferric  sulphate,  Fe2(SO4)3,  whilst  cuprous 
iodide  is  precipitated,  thus  : — 

2CuSO4  +  2FeSO4  +  2KI  =  2CuI  +  K2SO4  +  Fe2(S04)3 

This  reaction  serves  as  a  means  of  roughly  separating  iodine 
from  a  mixture  containing  chlorides  and  bromides. 

The  metallic  iodides  can  be  prepared  by  similar  processes  to 
those  which  yield  the  chlorides  arid  bromides. 

(1)  By  the  direct  action  of  iodine  on  the  metal,  as  in  the  cases 
of  the  iodides  of  iron  and  mercury. 

(2)  By  the  action  of  iodine  on  certain  of  the  metallic  oxides, 
hydroxides,  or  carbonates,  as  those  of  potassium,  sodium,  barium,, 
calcium,  and  silver. 

(3)  By  the  action  of  hydriodic  acid  on  certain  metals,  such 
as  zinc,  hydrogen  being  liberated. 

(4)  By  the  action  of  hydriodic  acid  on  the  metallic  oxides, 
hydroxides,  or  carbonates. 

(5)  By  adding  a  soluble  iodide,  such  as  potassium  iodide,  to 
a  solution  of  the  salt  of  the  metal,  when  the  metallic  iodide  is 
thrown  down  in  the  form  of  a  precipitate  ;  this  method,  however, 
can  only  be  used  when  the  iodide  required  is  insoluble. 


ESTIMATION  OF  IODINE  211 

in  Detection  and  Estimation  of  Iodine. — For  the  detection  of 
iodine,  the  starch  reaction,  the  violet-coloured  vapours,  and  the 
above-mentioned  coloured  precipitates  are  sufficient. 

To  estimate  iodine  in  the  free  state,  a  standard  solution  of 
sulphurous  acid  is  employed,  and  the  point  ascertained  at  which 
sufficient  of  this  solution  has  been  added  to  reduce  all  the  iodine 
to  hydriodic  acid,  thus  : — 

I2  +  2H20  +  S02  =  2HI  +  H2S04. 

The  solution  of  sulphurous  acid  may  be  replaced  by  one  of 
sodium  thiosulphate,  a  substance  which  reacts  with  free  iodine 
in  the  following  manner : — 

2Na2S203  +  I2  =  2NaI  +  Na2S406. 

For  the  quantitative  determination  of  iodine  in  a  soluble  iodide 
and  for  the  exact  separation  from  chlorine  or  bromine,  use  may 
be  made  of  the  fact  that  the  palladium  nitrate,  Pd  (NO3)2,  pro- 
duces with  solutions  of  an  iodide,  an  insoluble  precipitate  of 
PdI2,  which  on  ignition  yields  metallic  palladium.  Iodine  when 
in  the  form  of  an  alkaline  iodide  can  be  weighed  also  as  iodide 
of  silver,  when  neither  chlorine  nor  bromine  is  present;  100 
parts  of  silver  iodide  contain  54*128  parts  of  iodine.  In  the  case 
of  the  insoluble  iodides,  it  is  best  either  to  transform  them  into 
soluble  iodide  of  sodium  by  fusing  them  with  carbonate  of  soda, 
or  to  digest  them  with  zinc  and  dilute  sulphuric  acid,  when 
hydriodic  acid  is  liberated,  thus  : — 

2AgI  +  Zn  +  H2S04  =  2HI  +  2Ag  +  ZnSO4. 

If  it  is  required  to  determine  chlorine,  bromine,  and  iodine 
when  mixed  in  solution  together  the  following  method  may  be 
employed  : — 

Field  has  shown1  that  chloride  of  silver  is  completely  de- 
composed by  digestion  with  bromide  of  potassium,  the  chlorine 
and  bromine  changing  places  ;  and  that  both  bromide  and 
chloride  of  silver  are  decomposed  in  like  manner  by  iodide  of 
potassium.  Hence,  if  a  solution  containing  chlorine,  bromine, 
and  iodine,  be  divided  into  three  equal  parts,  each  portion  precipi- 
tated by  nitrate  of  silver,  the  first  precipitate  dried  and  weighed 

1  Ohem.  Soc.  Quart.  Journ.  1858,  234. 


212  THE  NON-METALLIC  ELEMENTS 


the  second  digested  with  bromide  of  potassium,  then  dried  and 
weighed,  and  the  third  digested  with  iodide  of  potassium,  then 
dried  and  weighed,  the  relative  quantities  of  the  three  elements 
may  be  determined  from  the  following  equations  :— 

x+y+z=w 
186-49 


233-04      233-04 


where  w,  w'  w",  are  the  weights  of  the  three  precipitates,  and 
x,  y,  and  z,  the  unknown  quantities  of  chloride,  bromide,  and 
iodide  of  silver  respectively. 

The  mixture  of  the  three  salts  of  silver  may  also  be  treated 
with  a  solution  of  potassium  bichromate  in  sulphuric  acid,  which 
converts  the  chloride  and  bromide  into  the  soluble  sulphate, 
whilst  the  iodide  is  converted  into  the  insoluble  iodate.  After 
dilution  and  filtration  the  iodate  may  be  reduced  and  the  silver 
in  it  determined,  whilst  the  silver  originally  present  as  chloride 
and  bromide  may  also  be  determined  in  the  filtrate.1 

In  this  case  the  equations  became  :— 

x  +  y  4  z  ^  w 


Ag 

Ag  Ag 


where  w  is  the  weight  of  the  mixed  salts,  wp  the  weight  of 
silver  from  silver  iodate,  and  w9  the  weight  of  silver  from  the 
chloride  and  bromide. 


IODINE     COMPOUNDS     WITH     OTHER 
HALOGENS 

112  Iodine  burns  with  a  pale  flame  in  fluorine,  yielding  a 
colourless  fuming  oil,  which  rapidly  attacks  glass,  and  is  probably 
an  iodine  fluoride.  It  is  most  likely  identical  with  the  compound 
obtained  by  Gore  by  the  action  of  iodine  on  silver  fluoride. 
(Moison.) 

1  Macnair,  Proc.  Chem.  Soc.  1893,  181. 


IODINE  TRICHLORIDE  213 

Two  compounds  of  iodine  and  chlorine  are  known : — (1)  Iodine 
monochloride,  IC1.  (2)  Iodine  trichloride,  IC13.  They  are  both 
obtained  by  the  direct  union  of  chlorine  and  iodine,  the  higher 
chloride  being  formed  when  the  former  element  is  in  excess. 

Iodine  Monochloride,  IC1,  is  prepared  (1)  by  passing  dry 
chlorine  gas  over  dry  iodine  until  the  latter  is  completely 
liquefied  ;  (2)  according  to  Berzelius,  by  distilling  1  part  of 
iodine  with  4  parts  of  potassium  chlorate  the  chlorine  evolved 
by  the  reaction  combining  with  a  portion  of  the  iodine ; x  (8) 
by  boiling  iodine  with  strong  aqua  regia ;  after  dilution  with 
water  the  liquid  is  shaken  up  with  ether  in  which  the  chloride 
of  iodine  dissolves  and  remains  behind  when  the  ether  is 
evaporated.2 

The  product  thus  obtained  is  a  reddish  brown  oil,  which,  on 
standing,  solidifies,  forming  long  well-defined  crystals  melting  at 
240>7 ;  it  boils  at  101°'3  and  has  a  sp.  gr.  at  that  temperature  of 
2'88196  (Thorpe).  It  smells  like  a  mixture  of  chlorine  and 
iodine,  bleaches  indigo  solution,  but  does  not  colour  starch-paste 
blue.  The  following  analyses  show  the  composition  of  this 

substance  : — 

Found 

Liquid.  Solid. 

Calculated.  (Burisen.)      (Schiitzenberger.) 

Iodine     .      12591         78*85  77'85  7S"52 

Chlorine         3519         2M5  22'15  21'48 

161-10       10000  100-00  100-00 


A  second  modification,  known  as  ft  iodine  monochloride,  is 
obtained  by  heating  the  crude  monochloride  till  it  is  free  from 
trichloride,  and  then  cooling  to  -  10°.  It  melts  at  13-90.3 

Iodine  Trichloride,  IC13,  is  obtained  (1)  by  acting  on  iodine, 
gently  heated,  with  a  large  excess  of  chlorine  ;  (2)  by  treating 
iodic  acid,  HIO3,  with  hydrochloric  acid  ;  (3)  by  heating  iodine 
pentoxide,  I205  with  pentachloride  of  phosphorus,  PC15. 

This  compound  forms  long  lemon-coloured  crystals,  and  very 
readily  undergoes  dissociation.  When  heated  in  the  air  to  25° 
it  decomposes,  giving  off  chlorine  gas,  forming  the  monochloride  ; 
but  when  heated  in  an  atmosphere  of  chlorine  it  only  de- 


1  Thorpe  and  Perry,  Journ.  Chem.  Soc.  1892,  i.  925. 

2  Bunsen,  Annalen,  84,  1. 

3  Tanatar,  Journ.  Russ.  Chem,  Soc.,  25,  97. 


214  THE  NON-METALLIC  ELEMENTS 

composes  at  a  much  higher  temperature,  which  rises  as  the 
pressure  of  the  chlorine  is  increased.  Thus,  under  a  pressure 
of  one  atmosphere  it  decomposes  at  67°  into  the  monochloride 
and  free  chlorine,  and  these  again  unite  on  cooling  to  form  a 
yellow  sublimate  of  the  trichloride.1  The  composition  of  the 
compound  is  seen  from  the  following  analysis  : — 

Calculated.  Found. 

Iodine  .     .     .     125*91         54'39  54'34 

Chlorine    .    .     105*57         45*61  45'66 

^31-48       100-00          100-00 

Both  the  chloride  and  trichloride  dissolve  in  water,  ether,  and 
alcohol  apparently  without  decomposition.  When  either  of  them 
is  acted  upon  by  a  small  quantity  of  an  alkali,  an  iodate  and 
chloride  are  formed  and  iodine  is  liberated.2 

6KHO+5C1I=5KC1+KI03+2I2+3H20. 

They  are  both  very  hygroscopic  and  give  off  irritating 
vapours. 

The  formation  of  both  of  these  compounds  can  be  demon- 
strated by  inverting  a  small  cylinder  containing  chlorine  and 
bringing  its  mouth  in  contact  with  that  of  another  of  the  same 
size  filled  with  hydriodic  acid  gas.  Iodine  is  first  liberated, 
which  combines  with  the  excess  of  chlorine,  the  yellow  tri- 
chloride being  deposited  on  the  sides  of  the  upper  jar,  in  which 
chlorine  is  in  large  excess,  whilst  the  brown  monochloride, 
mixed  with  iodine,  is  formed  in  the  lower  jar. 

113  Iodine  unites  with  bromine  to  form  a  solid,  volatile, 
crystalline  compound  which  is  probably  the  monobromide,  and 
also  a  dark  liquid,  possibly  the  tribromide.  These  bodies 
possess  properties  similar  to  those  of  the  chlorides  of  iodine. 


OXYGEN.     0  =  15-88. 

114  Of  the  elements  which  occur  on  our  planet,  oxygen  is 
the  most  widely  diffused,  and  is  found  in  the  largest  quantity. 
The  old  crystalline  rocks,  which  constitute  the  chief  mass  of 
the  earth's  crust,  consist  of  silicates,  or  compounds  of  silicon 
and  various  metals  with  oxygen.  These  rocks  contain  from 
44  to  48  per  cent,  of  oxygen.  Water  likewise  is  a  compound 

1  Brenken,  Ber.  8,  487.  2  Philip,  Ber.  3,  4. 


PREPARATION  OF  OXYGEN  215 

of  oxygen  and  hydrogen,  containing  88*81  per  cent,  of  the 
former  element.  .Oxygen  also  exists  in  the  free  state  in  the 
atmosphere,  which  contains  about  21  per  cent,  of  its  volume  of 
this  gas.  Although  the  absolute  amount  of  free  oxygen  con- 
tained in  the  air  is  very  great,  yet  the  proportion  which  the  free 
oxygen  bears  to  that  in  a  state  of  combination  is  but  very  small. 

It  has  already  been  mentioned,  in  the  Historical  Intro- 
duction, that  the  air  was  believed  to  be  a  simple  or  elementary 
substance  until  the  investigations  of  Priestley,  Rutherford,  and 
Scheele  x  showed  distinctly  that  it  is  a  mixture  of  two  different 
gases,  only  one  of  which  is  capable  of  supporting  combustion 
and  respiration.  This  constituent  of  the  atmosphere  is  oxygen, 
discovered  on  the  1st  of  August,  1774,  by  Priestley,  who,  by 
heating  "red  precipitate"  (mercuric  oxide)  by  means  of  the 
sun's  rays,  decomposed  it  into  oxygen  and  metallic  mercury. 
The  discovery  of  oxygen  enabled  Lavoisier  to  put  forward  the 
true  theory  of  combustion,  and  to  the  body  capable  of  support- 
ing this  combustion  was  given  the  name  "oxygene"  (ofvs 
sour,  and  yevvdco  I  produce),  from  the  fact  that  the  products  of 
combustion  are  frequently  of  an  acid  nature. 

Preparation. — (1)  The  simplest  method  of  preparing 
oxygen  is  to  heat  mercuric  oxide,  HgO,  in  a  small  retort  of  hard 
glass.  The  oxide  decomposes  at  a  red-heat  into  metallic  mercury 
and  oxygen ;  100  parts  by  weight  yield  7 '4  parts  by  weight  of 
oxygen. 

2  HgO=2Hg  +  02. 

The  apparatus  in  which  this  decomposition  can  be  shown  is  seen 
in  Fig.  60.  Owing  to  the  comparatively  high  price  of  oxide 
of  mercury,  this  process  is  only  used  as  a  means  of  illustrating 
the  decomposition. 

(2)  The  best  and  most  usual  mode  of  preparing  oxygen  con- 
sists in  heating  potassium  chlorate,  commonly  called  chlorate  of 
potash,  KC1O3.  This  salt  loses  the  whole  (39' 14  per  cent, 
of  its  weight)  of  its  oxygen,  leaving  potassium  chloride,  thus  : — 

2KC103  =  2KC1  +  3O2. 

The  preparation  and  collection  of  the  gas,  according  to  this 
method,  may  be  carried  on  in  the  apparatus  shown  in  Fig.  61. 

1  It  appears  that  Scheele  had  prepared  oxygen  prior  to  the  date  of  Priestley's 
discovery,  but  that  these  results  were  not  published  until  after  Priestley's 
experiments  had  been  made  known.  See  Carl  Wilhelm  Scheele :  Nachgelassene 
Brief e  und  Aufzeichnungen,  edited  by  A.  E.  Nordenskiold  (Stockholm,  1892). 


216 


THE  NON-METALLIC  ELEMENTS 


The  temperature  has  to  be  raised  much  above  the  melting  point 
of  the  salt  (372°),  before  the  evolution  of  the  gas  begins ;  and 
after  a  certain  time  has  elapsed,  the  fused  mass  becomes  thick, 


FIG.  60. 


owing  to  the  formation  of  potassium  perchlorate,  KC1O4,  a  part 
of  the  evolved  oxygen  having  united  with  the  chlorate,  whilst 


FIG.  61. 

potassium   chloride,    KC1,   and   oxygen    are   at  the   same  time 
formed,1  thus : — 

10KC103  =  6KC104  +  4KC1  +  302. 

1  Frankland  and  Dingwall,  Journ.  Chem.  Soc.  1887,  274  ;  Teed,  Journ  Chem 
Soc.  1887,  283. 


PEEPARATION  OF  OXYGEN  217 

When  more  strongly  heated,  the  perchlorate  also  decomposes 
into  potassium  chloride  and  oxygen. 

The  gas  can  be  obtained  at  a  lower  temperature  by  employ- 
ing a  mixture  of  potassium  and  sodium  chlorates  (Shenstone). 

(3)  In  order  to  obtain  the  evolution  of  oxygen  at  a  lower 
temperature,  a  small  quantity  of  manganese  dioxide  is  generally 
mixed  with  the  powdered  chlorate  ;  the  oxygen  then  comes  off  at 
about  350°,  before  the  salt  fuses,  and  thus  the  preparation  of  the 
gas  is  greatly  facilitated.  The  manganese  dioxide  is  found, 
mixed  with  potassium  chloride,  in  the  residue  unaltered  in 
composition. 

115  In  order  to  prepare  oxygen  on  a  larger  scale  this  mixture 


FIG,  62, 


, 


of  potassium  chlorate  and  manganese  dioxide  is  heated  in  a 
thick  copper  vessel  a,  Fig.  62,  provided  with  a  wide  tube  con- 
nected with  the  wash-bottle  b,  containing  caustic  soda,  for  the 
purpose  of  absorbing  chlorine  gas,  which  is  always  evolved 
along  with  the  oxygen.  It  is  difficult  to  give  a  satisfactory 
explanation  of  this  peculiar  action  of  the  manganese  dioxide. 
It  may  possibly  be  due  to  the  fact  that  certain  oxides,  such  as 
this  one,  are  capable  of  undergoing  a  higher  degree  of  oxidation, 
but  that  these  higher  oxides  part  very  readily  with  a  portion  of 
their  oxygen,  forming  again  the  lower  oxide.  In  this  way  such 
oxides  would  perform  the  part  of  carriers  of  oxygen,  first  taking 
it  up  and  then  setting  it  free.  Indeed  it  has  been  rendered 


218  THE  NON-METALLIC  ELEMENTS 


probable   that   when   manganese   dioxide    is    used,   potassium 
permanganate  is  first  formed  according  to  the  equation : 


2KC103  4-  2MnO2  =  2KMnO4  +  C12  +  0 


This  is  borne  out  by  the  facts  that  traces  of  chlorine  are  in- 
variably evolved  and  that  when  only  a  small  quantity  of 
manganese  dioxide  is  added,  the  pink  colour  of  the  permanga- 
nate can  be  distinctly  seen.  The  latter  is  then  broken  up  by 
the  action  of  heat  and  the  chlorine  evolved  by  the  further 
decomposition  of  the  chlorate,  into  potassium  chloride,  manga- 
nese dioxide  and  oxygen,  after  which  the  reaction  is  repeated.1 

According  to  Brunck  the  gas  prepared  in  this  manner 
invariably  contains  ozone.2 

The  decomposition  of  the  chlorate  is  also  aided  by  many 
other  oxides,  the  action  of  which  is  probably  to  be  explained  in 
a  similar  manner  to  that  of  manganese  dioxide.  Finely  divided 
platinum  (platinum  black),  kaolin,  and  other  powdered  sub- 
stances also  produce  the  same  effect,  but  the  above  explanation 
does  not  hold  good  in  these  cases.3 

It  not  unfrequently  happens  that  the  commercial  black  oxide 
of  manganese  may  be  accidentally  mixed  or  adulterated  with 
carbon  (pounded  coal),  and  this  impure  material,  when  mixed 
with  chlorate  of  potash  and  heated,  ignites,  giving  rise  to  even 
fatal  explosions.  Hence  care  should  be  taken  to  try  any  new  or 
doubtful  sample  on  a  small  scale  beforehand  by  heating  it  with 
chlorate  of  potash  in  a  test-tube. 

(4)  Many  other  salts  behave  like  potassium  chlorate  in  yield- 
ing oxygen   on  heating:   among  these   are  the  hypochlorites, 
chlorites,  perchlorates,  bromates  and  perbromates,  as  well  as 
the  iodates,    periodates,  nitrates,  nitrites  and   permanganates; 
but  these  compounds  are  not  usually  employed  for  this  purpose. 

(5)  Several  oxides,  such  as  manganese  dioxide,  MnO2,  lead 
dioxide,  Pb02,  barium  dioxide,  Ba02,  chromium  trioxide,  CrO3, 
lose  a  portion  of  their  oxygen  when  strongly  heated,  and  all 
these  may,  therefore,  be  used  for  preparing  oxygen.     In  order 
to  obtain  oxygen  by  heating  the  first-named  oxide,  the  sub- 
stance is  placed  in  a  strong  iron  bottle  which  can  be  heated  in 
a  furnace  to  bright  redness  (Fig.  63).     The   pure   manganese 

1  McLeod,    Journ.  Chem.  Soc.  1889,  184.  2  Ber.  26,  1790. 

3  Veley,  Phil.  Trans.  1888,  i.,  271  ;   Fowler  and  Grant,  Journ.   Chem.  Soc. 
1890,  272. 


PREPARATION  OF  OXYGEN  219 

dioxide  loses  one-third  (12'4  per  cent.)  of  its  oxygen,  being  con- 
verted into  the  brown  oxide,  Mn3O4,  thus  : — 

3MnO2  =  Mn8O4  +  O2. 

(6)  By  heating  manganese  dioxide  in  a  glass  flask  with  sul- 
phuric acid,  one-half  of  its  oxygen  is  given  off,  and  manganous 
sulphate,  MnS04,  is  formed. 

2MnO2  +  2H2  SO4  =  2MnSO4  +  2H2O  +  O2. 

(7)  Chromium  trioxide  can  also  be  employed  for  the  prepara- 
tion of  oxygen,  but  it  is  not  necessary  to  obtain  this  substance 


FIG.  63. 

in  the  pure  state,  for  if  bichromate  of  potash,  K2Cr2O7,  be 
heated  with  sulphuric  acid,  chromium  trioxide  is  formed,  thus  : — 

K2Cr2O7  +  2H2SO4  =  2KHSO4  +  2CrO3  +  H2O. 

The  chromium  trioxide  is  then  further  decomposed  by  the 
action  of  sulphuric  acid  with  the  formation  of  chromium  sul- 
phate, a  decomposition  which  is  rendered  visible  by  the  change 
of  colour  from  the  original  red  to  a  deep  green,  thus : — 

4Cr03  +  6H2SO4  =  2Cr2(SO4)3  +  6H20  4  3O2. 

(8)  Oxygen  can  be  obtained  by  the  decomposition  of  bleach- 
ing powder  (Mitscherlich,  1843 ;  Fleitmann,  1865).  For  this 
preparation  a  clear  concentrated  solution  of  bleaching  powder 
—which  contains  calcium  hypochlorite,  CaCl2O2 — is  placed  in 
a  flask,  and  a  few  drops  of  cobalt  chloride  solution  added.  An 


220  THE  NON-METALLIC  ELEMENTS 


oxide  of  cobalt,  which  probably  has  the  formula1  CoO2,  is  pre- 
cipitated, and  on  heating  the  mixture  to  about  80°  a  rapid 
effervescence  of  oxygen  occurs.  The  cobalt  oxide,  which  is 
formed,  is  left  unchanged  after  the  operation,  and  may  be 
employed  again ;  it  probably  acts,  like  the  manganese  dioxide, 
by  the  formation  of  a  higher  oxide,  which  is  again  quickly 
reduced,  the  oxygen  being  liberated  as  a  gas.  Instead  of  a 
clear  solution,  a  thick  paste  of  bleaching  powder  may  be  used, 
with  the  addition  of  a  little  cobalt  salt  and  a  small  quantity  of 
paraffin  oil,  to  prevent  the  frothing  which  usually  occurs.  The 
best  temperature  for  the  evolution  of  gas  is  from  70—80°. 

CaCl202  =  CaCl2  +  O2. 

The  same  decomposition  and  the  replacement  of  chlorine  for 
oxygen  may  be  shown  in  a  striking  manner  by  passing  chlorine 
gas,  generated  in  a  flask  from  manganese  dioxide  and  hydro- 
chloric acid,  into  a  second  flask,  which  contains  boiling  milk  of 
lime,  to  which  a  little  cobalt  nitrate  solution  has  been  added. 
Oxygen  gas  is  then  liberated  in  the  second  flask,  and  may  be 
collected  as  usual.  The  following  equation  explains  the 
replacement,  and  we  see  that  two  volumes  of  chlorine  yield 
their  equivalent,  or  one  volume  of  oxygen  : — 

2C12  +  2Ca  (OH)2  =  2CaCl2  +  2H20  +  O2. 

(9)  Oxygen  can  also  be  prepared  by  the  decomposition  of 
sulphuric  acid.  For  this  purpose  a  thin  stream  of  sulphuric  acid 
flows  into  a  platinum  retort  heated  to  redness ;  the  acid  splits 
up  into  sulphur  dioxide,  water,  and  oxygen,  yielding  15*68  per 
cent,  of  its  weight  of  the  gas,  or  in  practice  55  grams,  of  acid 
yield  6  litres  of  gas,  thus : — 

2H2S04  =  2S02  +  2H2O  +  O2. 

The  resulting  sulphur  dioxide  and  water  can  be  absorbed,  whilst 
the  oxygen  can  be  collected  in  a  gas-holder.  An  apparatus  for 
illustrating  this  decomposition  is  described  under  Sulphuric 
Acid. 

In  order  to  prepare  oxygen  cheaply  on  the  large  scale  several 

1  Vortmann,  quoted  by  McLeod,  British  Ass.  1892,  669. 


PROPERTIES  OF  OXYGEN  221 

other  processes    have    been    suggested.       Amongst   them   the 
following  is  now  widely  used. 

(10)  When  baryta,  BaO,  is  gently  heated  to  dull  redness  in 
the  air,  it  takes  up  an  additional  atom  of  oxygen,  forming  the 
dioxide,   BaO2,  but    at  a  bright-red  heat    this  parts  with  the 
additional  atom  of  oxygen  with    the    reproduction    of  baryta. 
By  thus  alternately   varying  the  temperature,  first  leading  air 
over  the  baryta  contained  in  a  porcelain  tube,  and  then  placing 
the   tube    in   connection    with    a    gas-holder    and    raising   the 
temperature,  and  again  repeating  the  process,   a  regular  pro- 
duction  of   gas    can    be    obtained    from   a   small   quantity    of 
baryta.1     This    simple  method  is  now  carried    out  on  a    large 
scale  according   to   a  process   patented   by  the    Brin    Oxygen 
Company  2 

Oxide  of  barium,  prepared  from  the  nitrate,  is  introduced  in 
pieces  about  the  size  of  walnuts  into  steel  or  cast-iron  retorts. 
These  retorts,  placed  in  a  vertical  position  in  a  gas  furnace, 
are  heated  to  about  700°,  and  air,  carefully  purified  from 
moisture  and  carbonic  acid  then  pumped  through  them  under  a 
pressure  of  about  15  Ibs.  on  the  square  inch.  The  baryta  absorbs 
the  oxygen,  becoming  converted  into  peroxide,  and  as  soon  as  this 
peroxidation  has  been  carried  as  far  as  is  economical,  the  pump 
is  reversed  and  the  pressure  in  the  retorts  thus  reduced.  When 
the  reduction  of  pressure  has  reached  about  26 — 28  inches  of 
mercury  below  the  normal,  the  peroxide  begins  to  give  up  its 
oxygen,  which  passes  through  the  pump  and  is  delivered  to  a 
gas-holder.  A  complete  operation  lasts  about  ten  minutes,  and 
•nearly  140  operations  can  be  conducted  per  diem.  The  oxygen 
is  then  compressed  at  120  atmospheres  in  wrought  iron 
cylinders.3 

(11)  Potassium  manganate,  K2Mn04,  loses  oxygen  when  heated 
in  a  current  of  steam,  forming  caustic  potash  and   lower  oxides 
of  manganese,  which  when  again  heated  absorb  oxygen,  the 
manganate  being  reproduced,  so  that  the  same  portion  may  be 
used  over  and  over  again  (Tessie  du  Motay).     A  modified  form 
of  this    process    in    which    sodium    manganate    is    used,    has 
been  applied   by  Fontana4    for  the    industrial  preparation    of 
oxygen. 

It   has   also   been    proposed    to    utilise   calcium   plumbate 

1  Boussingault,  Ann.  Chim.  Phys.  [3]  35. 

2  Pat.  157,  Oct.  5,  1885. 

3  Journ.  Soc.  Chem.  2nd.  1890,  246.     4  Journ.  Soc.  Chem.  Ind.  1892,  312. 


THE  NON-METALLIC  ELEMENTS 


Ca2Pb04,  for  this  purpose.1  The  material  is  heated  at  a  low 
temperature  in  the  presence  of  carbonic  acid,  which  causes 
its  decomposition  into  calcium  carbonate,  CaCO3,  and  lead 
peroxide,  Pb02.  When  the  mixture  of  these  is  more  strongly 
heated  oxygen  is  first  given  off  and  a  lower  oxide  of  lead 
formed,  after  which  the  calcium  carbonate  loses  its  carbon 
dioxide.  The  residual  mass,  consisting  of  a  mixture  of  lime 
and  the  lower  oxides  of  lead  is  reconverted  into  calcium 
plumbate  by  heating  in  a  current  of  air. 

116  Properties — Oxygen  is  a  colourless,  invisible,  tasteless, 
inodorous  gas  which  is  slightly  heavier  than  atmospheric  air. 
According  to  Regnault's  experiments  2  the  density  of  the  gas  is 
1-10562  (air=l),  one  litre  at  0°  and  760  mm.  weighing  T43011 
grams,  whilst  according  to  Leduc3  the  density  is  1*10503,  and  to 
Rayleigh,4  T10535,  one  litre  weighing  1*42961  grams. 

The  density  of  the  gas  compared  with  hydrogen  has  been 
found  by  Rayleigh  to  be  15 '882  (see  p.  260)  ;  its  molecular 
weight  is  therefore  about  31 '76  and  its  formula  O2. 

Oxygen  was  first  liquefied  by  Cailletet  and  Pictet  in  December, 
1877.    It  forms  a  light  blue  liquid,  boiling  at  - 181'4°,  at  which 
temperature  the  density  of  the  liquid  is  1124.     The  critical 
temperature  of  the  gas  is  - 118°,  the  corresponding  pressu^^ 
being  50  atmospheres. 

As  long  ago  as  1847  Faraday  discovered  that  oxygen  is  less 
diamagnetic  than  air,5  and  it  was  subsequently  shown  to  be 
actually  paramagnetic.  Liquid  oxygen  is  strongly  magnetic, 
and  when  placed  in  a  cup-shaped  piece  of  rock  salt  between 
the  poles  of  a  powerful  electro-magnet  suddenly  leaps  up  to 
the  poles  and  remains  there  permanently  attached  until  it 
evaporates.6 

Liquid  oxygen  presents  the  same  absorption  spectrum  as  the 
gas,  characteristic  bands  being  present  in  the  orange,  yellow, 
green,  and  blue,  but  the  bands  are  much  more  intense  and  well 
marked  than  those  of  the  gas.  It  also  possesses  a  measurable 
thermal  absorption  and  presents  a  very  high  resistance  to  an 
electric  current.  When  cooled  to  —210°  by  its  own  rapid 
evaporation,  it  is  no  longer  capable  of  supporting  combustion 
or  of  combining  with  substances  like  phosphorus  and  sodium. 

1  Kassner,  Chem.  2nd.  1890,  104,  120  ;  Le  Chatelier,  Compt.  Rend.  H7,  109. 

2  Regnault,  Mem.  Acad.  Science,  21,  144  ;  Crafts,  Compt.  Rend.  106,  1662. 

3  Leduc,  Compt.  Rend.  113,  186.  4  Rayleigh,  Proc.  Roy.  Soc.  53,  134. 
5  Faraday,  Phil  Mag.  (3),  31,  401.  6  Dewar,  Proc.  Roy.  Soc.  50,  247. 


PROPERTIES  OF  OXYGEN  223 

Pictet 1  has  indeed  found  that  chemical  action  in  general  entirely 
ceases  at  temperatures  approaching  — 150°.  Thus  sulphuric 
acid  and  caustic  potash  when  compressed  together  at  this 
temperature  do  not  react,  although  the  normal  action  takes 
place  at  —90°.  In  the  same  manner  sodium  preserves  its 
metallic  lustre  in  liquid  alcohol  of  84  per  cent,  at  —78°  and 
does  not  commence  to  act  upon  it  until  the  temperature  reaches 
—  48°;  and  alcoholic  litmus  solution  remains  blue  in  contact 
with  solid  sulphuric  acid  at  all  temperatures  below  —105°,  at 
which  reddening  suddenly  takes  place. 

Oxygen  dissolves  appreciably  in  water ;  at  0°  one  volume  of 
water  absorbs  O04890  volume  of  oxygen,  measured  under 
the  normal  temperature  and  pressure.  When  the  tempera- 
ture rises,  the  quantity  of  oxygen  absorbed  becomes  less, 
according  to  a  complicated  law,  which  is  expressed  by  the 
empirical  formula.2 

G  =  0-04890  -  0'0013413t  +  0'0000283t2  -  0'00000029534t3. 

Certain  metals  also  absorb  oxygen  when  in  the  molten  state, 
and  give  it  off  again  on  solidifying ;  thus,  melted  silver  absorbs 
about  ten  times  its  bulk  of  oxygen,  and  this  is  nearly  all 
emitted  when  the  metal  cools,  giving  rise  to  the  peculiar 
phenomenon  of  the  "  spitting  "  of  silver. 

As  oxygen  is  the  constituent  of  the  air  which  supports  com- 
bustion, it  naturally  follows  that  bodies  burn  in  oxygen  with 
much  greater  brilliancy  than  they  do  in  common  air.  A 
glowing  chip  of  wood,  or  the  red-hot  wick  of  a  taper,  ignites 
with  a  slight  detonation  when  plunged  into  oxygen  gas,  and 
even  metals  such  as  iron,  which  oxidize  only  slowly  in  the  air, 
burn  brilliantly  in  oxygen.  The  following  experiments  serve  to 
illustrate  this  property  of  oxygen : — 

A  bundle  of  thin  iron  wire,  with  the  ends  tipped  with 
lighted  sulphur  or  a  burning  piece  of  twine,  burns  when 
plunged  into  a  jar  of  oxygen,  forming  the  black  oxide,  Fe3O4> 
which  falls  down  in  glowing  drops.  A  piece  of  watch  spring 
also  burns  easily  with  splendid  scintillations  if  held  in  a  flame 
obtained  by  blowing  a  jet  of  oxygen  into  the  flame  of  a  spirit 
lamp.  An  even  more  striking  mode  of  showing  the  combustion 
of  iron  is  to  place  a  heap  of  cast  iron  nails  on  a  brick  and 
burn  them  by  means  of  a  blow-pipe  fed  with  oxygen  and  coal- 

*  Compt.  Rend.  115,  814. 

2  L.  W.  Winkler,  Ser.  22,  1764  ;  24,  3607. 


224  THE  NON-METALLIC  ELEMENTS 


gas  contained  in  separate  gas-holders.  Substances  like  sulphur 
and  phosphorus,  which  take  fire  readily  in  the  air,  burn  with 
much  greater  brilliancy  in  oxygen  ;  combination  takes  place 
much  more  rapidly,  and,  therefore,  the  temperature  reached  is 
much  higher  in  oxygen  than  in  the  air,  in  which,  moreover, 
the  inert  nitrogen  takes  up  a  share  of  the  heat.  The  best 
method  of  exhibiting  combustion  in  oxygen  is  to  place  the 
substance  to  be  burnt  in  a  metal  cup,  riveted  on  to  an 
upright  stem,  carrying  a  round  saucer  containing  water.  As 
soon  as  the  body  has  been  ignited,  a  large  glass  globe  filled 
with  oxygen  gas  is  placed  over  it,  so  that  the  cup  occupies  a 


FIG.  64. 

central  position  in  the  lower  half  of  the  globe,  and  then  the 
combustion  can  proceed  with  great  rapidity  without  fear  of 
the  globe  being  cracked  by  the  heat  evolved  (Fig.  64).  In 
this  way  sulphur  burns  with  a  bright  violet  flame,  with  form- 
ation of  colourless  sulphur  dioxide  gas,  SO., :  whilst  phosphorus, 
thus  burnt,  emits  a  brilliant  white  light,  which  vies  with 
sunlight  in  intensity.  In  this  case  the  white  solid  phosphorus 
pentoxide,  P2O5,  is  the  product  of  the  combustion. 

117  An  act  of  chemical  union  accompanied  by  the  evolution 
of  light  and  heat,  is  termed  a  combustion,  and  hence  oxygen  is 
commonly  termed  a  supporter  of  combustion,  whilst  those  bodies 
which  thus  unite  with  oxygen  are  called  combustible  substances. 


COMBUSTIONS  IN  OXYGEN  AND  IN  HYDROGEN        225 


A  little  'consideration,  however,  shows  that  these  terms  are  only 
relative,  and  an  experiment  makes  this  plain.  One  of  the  stop- 
pered bell-jars  (Figs.  65  and  66)  is  filled  with  oxygen  gas,  the 
other  with  hydrogen  ;  two  gas-holders,  one  containing  hydrogen, 
the  other  oxygen,  are  provided  with  flexible  gas  delivery-tubes  at 
the  end  of  which  is  fixed  a  perforated  caoutchouc  stopper 
•carrying  a  metal  tube  with  a  nozzle.  The  hydrogen  gas  is 
.allowed  to  escape  through  the  nozzle,  then  ignited,  and  the 
flame  of  hydrogen  plunged  into  the  bell-jar  filled  with  oxygen, 
the  caoutchouc  stopper  fitting  tightly  into  the  tubulus.  A  flame 


FIG.  65. 

of  hydrogen  burning  in  oxygen  is  then  seen,  the  hydrogen  being 
the  burning  body  and  the  oxygen  the  supporter  of  combustion. 
A  stream  of  oxygen  gas  is  next  allowed  to  issue  from  the  nozzle 
of  the  second  gas-holder,  the  stopper  of  the  bell-jar  containing 
hydrogen,  is  then  removed,  and  the  jet  of  oxygen  is  plunged 
into  the  bell-jar,  whilst  the  flame  of  a  candle  is  brought  at 
the  same  instant  to  the  tubulus.  On  pressing  the  caoutchouc 
stopper  into  its  place,  a  flame,  not  to  be  distinguished  from 
that  burning  in  the  other  bell-jar,  is  seen,  in  which  oxygen 
is  the  burning  body,  and  hydrogen  is  the  supporter  of  com- 
bustion. 

16 


THE  NON-METALLIC  ELEMENTS 


Most  bodies  do  not  combine  with  oxygen  rapidly  enough  at 
the  ordinary  atmospheric  temperatures  to  give  rise  to  the 
phenomena  of  combustion,  but  require  to  be  heated  before  this 
begins.  Oxidation  is,  however,  often  slowly  going  on,  as  in 
the  case  of  the  rusting  of  metals,  the  decay  of  wood  and 
organic  bodies.  Thus,  we  come  to  distinguish  between  quick 
and  slow  oxidations  in  which  the  intensity  of  the  heat  and 
light  evolved  is  very  different. 

This  slow  oxidation  frequently  occurs  in  presence  of  certain 
finely-divided  metallic  particles,  probably  owing  to  the  con- 


FIG. 


densation  of  the  gases  on  the  surface  or  in  the  pores  of  the 
metal.  Thus  a  small  quantity  of  spongy  platinum  (obtained 
by  heating  the  double  chloride  of  platinum  and  ammonium) 
when  held  over  a  jet  of  coal  gas  or  hydrogen,  first  becomes 
red  hot,  owing  to  the  combustion  of  the  gas  occurring  on  its 
surface,  and  afterwards  the  temperature  of  the  metal  ma|[  rise 
so  high  that  the  jet  of  gas  is  ignited. 

In  some  cases  the  products  of  the  slow  oxidation  of  the  sub- 
stance are  different  from  those  formed  by  its  rapid  oxidation  or 


SLOW  OXIDATION  227 


combustion.  When  a  coil  of  fine  platinum  wire  rs  first  heated  in 
a  flame  and  then  hung  whilst  warm  over  the  surface  of  some 
alcohol  contained  in  a  small  beaker-glass,  the  coil  soon  begins 
to  glow,  and  remains  red-hot  until  all  the  alcohol  is  consumed, 
but  no  flame  is  seen.  Alcohol  has  the  formula  C2H6O,  and 
when  it  burns  with  a  flame  its  constituents  unite  with  oxygen 
to  form  water,  H2O,  and  carbon  dioxide,  CO2.  When  oxidised 
at  the  lower  temperature,  a  peculiar  smelling  body  termed 
aldehyde  is  formed,  having  the  formula  C2H40  ;  hence  the 
oxidation  of  the  alcohol  is  partial  or  incomplete.  Only  two  of 
the  hydrogen  atoms  of  alcohol  are  then  withdrawn,  water  being 
formed,  whilst  the  volatile  aldehyde  escapes,  giving  rise  to  a 
peculiar  choking  smell. 

The  effect  of  mechanical  division  on  the  combustibility  of 
substances,  especially  of  metals,  is  well  known,  and  advantage 
is  taken  of  this  in  the  preparation  of  the  various  pyrophori.  If 
tartrate  of  lead  be  gently  heated  in  a  glass  tube,  the  lead  is  left 
in  a  state  of  very  fine  mechanical  division,  and  mixed  with 
carbon.  After  heating,  the  tube  is  hermetically  sealed,  and  on 
cooling  it  may  be  opened  and  the  contents  shaken  out  into  the 
air,  when  the  finely  divided  particles  will  at  once  take  fire.  In 
the  same  way,  if  the  oxides  of  iron,  cobalt,  or  nickel  be  reduced 
by  hydrogen  at  a  moderate  temperature,  the  metal  is  formed 
in  a  pulverulent  state,  in  which  it  takes  fire  spontaneously 
on  exposure  to  the  air.  The  explanation  of  this  is,  that  by 
fine  division,  the  ratio  of  the  surface  exposed,  to  the  mass 
to  be  heated  becomes  so  great  that  the  heat  generated  by 
the  oxidation  of  the  surface  is  sufficient  to  bring  the  mass  to 
incandescence. 

The  spontaneous  ignition  of  a  mass  of  inflammable  materials 
like  cotton  or  woollen  rags,  when  mixed  with  a  substance,  such 
as  oil,  capable  of  rapidly  absorbing  oxygen,  and  thereby 
generating  heat,  is  one  of  the  most  common  sources  of  fire, 
both  in  manufactories  and  on  board  ship.  Similar  cases  of 
spontaneous  combustion  occur  in  hay-ricks,  in  which  the  hay 
has  been  put  up  damp,  for  moisture  greatly  assists  the  process 
of  slow  oxidation.  Other  examples  of  the  same  thing  are  seen 
in  the  fires  which  break  out  in  ships  carrying  coal,  or  in  heaps 
of  coal  or  shale ;  these  seem  to  be  due  to  the  oxidation  of  the 
bituminous  constituents  of  the  coal,  into  carbon  dioxide  and 
water  by  the  oxygen  of  the  air,  which  is  absorbed  by  the 
coal,  especially  when  much  broken  up,  and  thus  evolve  heat 


228  THE  NON-METALLIC  ELEMENTS 

enough  to  set  the  mass  on  fire.  All  the  supposed  cases  of 
spontaneous  combustion  occurring  in  the  human  body  have 
been  clearly  proved  to  be  mistakes  or  deceptions,  as  may 
be  seen  by  reading  Chapter  xxv.  of  Liebig's  admirable  Letters 
on  Chemistry,  in  which  this  matter  is  fully  discussed. 

118  Temperature  of  Ignition. — In  order  that  a  body  may  take 
fire  in  air  or  in  oxygen,  a  certain  temperature  must  be  reached : 
this  point  is  termed  the  temperature  of  ignition.  The  tempera- 
ture at  which  inflammation  occurs  varies  widely  with  different 
substances;  thus  while  the  vapour  of  carbon  bisulphide  is  ignited 
by  bringing  in  contact  with  it  a  glass  rod  heated  only  to  149°,  a 
jet  of  coal  gas  cannot  be  lighted  with  a  piece  of  iron  at  a  dull 
red-heat :  and,  again,  certain  substances,  such  as  the  liquid 
phosphuretted  hydrogen,  or  zinc  ethyl,  only  require  to  be 
exposed  to  the  air  at  the  ordinary  temperature  in  order  to 
ignite,  whilst  nitrogen  can  only  be  made  to  unite  with  oxygen 
by  heating  the  mixture  to  the  temperature  of  the  electric 
spark.  The  temperature  at  which  slow  oxidation  commences 
is  of  course  lower  than  that  of  ignition ;  thus  phosphorus  begins 
to  enter  into  slow  combustion  in  the  air  (exhibiting  phospho- 
rescence) below  10°  C. ;  but  we  must  heat  it  up  to  60°  C., 
before  it  begins  to  burn  brightly,  or  to  enter  into  quick  com- 
bustion. 

The  Davy  Lamp. — A  most  striking  example  of  the  fact  that  a 
certain  temperature  must  be  reached  before  a  mixture  of  in- 
flammable gas  with  air  can  take  fire,  is  seen  in  the  safety  lamp 
for  coal  mines,  invented  by  Sir  Humphry  Davy.1  The  principle 
upon  which  this  depends  is  well  illustrated  by  holding  a  piece 
of  wire  gauze,  containing  about  700  meshes  to  the  square  inch, 
over  a  jet  of  gas  (Fig.  67).  If  the  gas  is  lit,  it  is  possible  to 
remove  the  gauze  several  inches  above  the  jet,  and  yet  the  in- 
flammable gas  below  does  not  take  fire/  the  flame  burning  only 
above  the  gauze.  The  metallic  wires  in  this  case  conduct  away 
the  heat  so  quickly  that  the  temperature  of  the  gas  at  the 
lower  side  of  the  gauze  cannot  rise  to  the  point  of  ignition.  In 
a  similar  way  we  may  cool  down  a  flame  so  much  that  it 
goes  out,  by  placing  over  it  a  small  coil  of  cold  copper  wire 
whereas  it  is  impossible  to  extinguish  the  flame  if  the  coil  of 
wire  be  previously  heated.  The  "  Davy  lamp  "  consists  of  an 
oil  lamp  (Figs.  68  and  69)  the  top  of  which  is  inclosed  in  a 
covering  of  wire  gauze,  so  that  the  products  of  combustion  of 
1  Phil.  Trans.  1817,  pp.  45—77. 


TEMPERATURE  OF  IGNITION 


229 


the  oil  can  escape,  while  no  flame  can  pass  to  the  outside  of  the 
gauze.  Hence  no  ignition  is  possible,  even  if  the  lamp  is  placed 
in  the  most  inflammable  mixture  of  fire-damp  and  air,  although 
the  combustible  gases  may  take  fire  and  burn  inside  the  gauze. 
It  is,  however,  necessary,  to  be  careful  that  the  flame  thus 


FIG.  67. 

kindled  inside  the  gauze  does  not  heat  it  up  to  the  point  of 
ignition  of  the  inflammable  gas,  and  especially  to  avoid  placing 
the  lamp  in  draughts,  which  might  blow  the  flame  against  a 
point  of  the  gauze  and  thus  heat  it  above  the  point  of  safety. 


FIG.  68* 


FIG.  69. 


Indeed,  it  was  pointed  out  by  Davy  himself  that  the  lamp  is 
no  longer  safe  if  exposed  to  a  draught  of  air,  and  several  serious 
accidents  have  occurred  from  the  neglect  of  these  precautions. 
It  has  also  been  shown  that  the  flame  burning  inside  a  wire 
gauze  may  be  mechanically  blown  through  the  gauze  by  a  cur- 


230  THE  NON-METALLIC  ELEMENTS 

rent  or  blast  of  air  passing  at  the  rate  of  eight  feet  per  second,1 
and  this  has  doubtless  given  rise  to  many  serious  accidents. 
Several  modifications  of  the  original  Davy  lamp  have  been  intro- 
duced to  lessen  this  danger,  but  the  firing  of  shots  in  fiery  pits 
is,  in  any  case,  much  to  be  condemned.  It  is  almost  unneces- 
sary to  say  that  the  lamp  ought  not  be  opened  whilst  in  use 
in  the  pit. 

119  Heat  of  Combination. — In  every  chemical  reaction  the 
formation  of  new  compounds  is  accompanied  by  a  change  in  the 
energy  of  the  system.  Thus  when  two  grams  of  hydrogen  at  0° 
unite  >.  with  15*88  grams  of  oxygen,  also  at  0°,  to  form  17*88 
grams  of  water  at  the  same  temperature,  we  not  only  have  the 
material  conversion  of  the  33*3  litres  of  mixed  gases  into  17'88  cc. 
of  liquid  water,  but  we  find  that  sufficient  heat  is  evolved  to 
raise  the  temperature  of  67,940  grams  of  water  (at  0°  C)  one 
degree.  The  amount  of  heat  necessary  to  raise  the  tempera- 
ture of  one  gram  of  water  (at  0°  C.)  through  one  degree  is  called 
a  calorie,  and  the  heat  evolved  in  chemical  reactions  is  measured 
in  terms  of  this  unit. 

In  order  to  measure  the  heat-change  which  accompanies  the 
chemical  transformation  of  a  mixture  of  hydrogen  and  oxygen 
into  water,  oxygen  is  burnt  in  hydrogen  contained  in  a  platinum 
globe  immersed  in  water  contained  in  a  calorimeter.2  The  latter 
consists  of  a  gilded  brass  cylindrical  vessel,  surrounded  by  two 
concentric  brass  cylinders  to  prevent  loss  or  gain  of  heat  by 
radiation.  The  air  in  the  platinum  vessel  is  displaced  by  a 
current  of  hydrogen,  and  dry  oxygen  then  introduced  by  means 
of  a  tube  and  ignited  by  an  electric  spark,  so  that  the  jet  of 
oxygen  continues  to  burn  in  the  atmosphere  of  hydrogen 
which  is  supplied  through  another  tube.  The  excess  of 
hydrogen  passes  out  of  the  globe  by  a  third  tube  through 
a  weighed  calcium  chloride  tube,  which  retains  the  water 
vapour.  The  water  of  the  calorimeter  is  stirred  throughout 
the  experiment. 

The  temperature  of  the  water  is  observed  just  before  the 
commencement  of  the  combustion  and  a  series  of  observations  is 
taken  at  the  close  of  the  experiment,  the  final  temperature  of  the 
water  being  calculated  from  these  after  allowing  for  loss  of  heat 
by  cooling.  The  amount  of  water  formed  is  found  by  displacing 
the  hydrogen  by  air  and  weighing  the  platinum  globe  after  the 

1  Galloway,  Proc.  Roy.  Soc.  22,  441. 

2  Thomsen,  Thermochemischc  Untersuchungen  2.  45. 


HEAT  OF  COMBINATION  231 

experiment,  the  moisture  carried  off  by  the  escaping  gases  being 
retained  by  calcium  chloride  and  weighed.  An  actual  experi- 
ment gave  the  following  results : — 

Initial  temperature  16*075°. 

Final  temperature  (corr.)  19'357°. 

Rise  of  temperature  3'282°. 

Water  value  of  calorimeter  2,460  grams. 

Total  weight  of  water  formed  2'129  grams. 

The  water  value  of  the  calorimeter  represents  the  amount  of 
water  contained  in  it  together  with  the  amount  which  would 
require  as  much  heat  to  raise  its  temperature  one  degree  as  the 
metal  work  of  the  calorimeter. 

The  heat  developed,  therefore,  amounts  to  2,460  x  3'282  = 
8,074  cal.  To  this  must,  however,  be  added  the  heat  required 
to  raise  the  water  produced  by  the  combination  to  this  final 
temperature,  and  the  latent  heat  which  was  absorbed  by  the 
water  passing  off  with  the  hydrogen  as  vapour,  amounting  in 
all  to  15  cal.  The  total  heat  for  2'129  grams  of  water  is 
therefore  8,089  units,  and  hence  the  heat  of  formation  of 
water  is 

8,089  x  17-88 

2.129      -  -  67,934  cal. 

The  mean  of  a  number  of  experiments  gives  67,940  cal.  for 
the  production  of  17 '88  grams  of  water.  This  may  be  ex- 
pressed in  an  equation  in  the  following  manner,  the  chemical 
formulae  being  understood  to  represent  simply  the  quantities  of 
the  reacting  substances  and  not  the  molecular  weights : — 

2H  +  0  =  H20  +  67,940. 

The  product  of  this  reaction  possesses  less  energy  than  its 
constituents,  and  the  mixed  gases  are,  therefore,  said  to  possess 
potential  energy,  which  is  converted  on  their  combination  into  the 
kinetic  energy  or  energy  of  motion  of  the  molecules  of  the  heated 
products.  In  order  to  reconvert  the  liquid  water  into  the  same 
weight  of  the  mixed  gases  this  exact  amount  of  energy  must  be 
restored  to  it,  and  generally,  the  heat  produced  (or  absorbed)  in 
the  formation  of  a  compound  is  exactly  equal  to  that  absorbed  (or 
evolved)  by  the  decomposition  of  the  substance  into  its  original 
constituents.  Most  substances  are  formed  with  evolution  of 
heat,  but  in  some  cases  heat  is  absorbed  in  the  formation  of 


232  THE  NON-METALLIC  ELEMENTS 

the  compound  so  that  it  possesses  more  energy  than  its 
constituents.  This  is  the  case,  for  example,  with  carbon  bisul- 
phide, the  oxides  of  chlorine  and  nitrogen,  ozone  and  many 
other  bodies.  Such  substances  give  out  heat  on  decomposition, 
and  consequently  can  often  be  made  to  undergo  explosive 
decomposition.  In  these  cases  the  heat  evolved  by  the  resolu- 
tion of  a  small  portion  of  the  substance  into  its  constituents 
is  sufficient  to  bring  surrounding  parts  to  the  decomposition 
point,  and  thus  the  whole  mass  rapidly  breaks  up.  Chlorine 
monoxide,  for  example,  gives  out  17,670  cal.  on  decomposition: 

C12O  =  2  Cl  +  O  +  17,670  cal. 

and  is  an  exceedingly  unstable  compound,  exploding  violently 
when  heated  or  even  shaken. 

1 20  It  is  frequently  impossible  to  ascertain  the  heat  of  form- 
ation of  a  compound  directly,  but  an  indirect  determination  is 
rendered  possible  by  the  fact  that  the  heat  evolved  in  a  chemical 
reaction  depends  only  on  the  initial  and  final  states  of  the  system 
and  is  independent  of  the  intermediate  stages.1 

Thus  if  we  wish  to  find  the  heat  evolved  in  the  production  of 
a  dilute  solution  of  ammonium  chloride  from  the  two  gases, 
ammonia  and  hydrogen  chloride,  we  may  either  (1),  allow 
these  two  gases  to  combine  directly  and  dissolve  the  product  in 
water ;  or  (2),  dissolve  the  two  gases  separately  in  water  and 
mix  the  solutions.  In  both  cases  we  start  with  ammonia, 
hydrochloric  acid  gas,  and  water,  and  end  with  a  dilute  solution 
of  ammonium  chloride,  and  the  amount  of  the  heat-change  is 
found  to  be  the  same  in  both  cases. 

The  determination  of  the  heat  of  formation  of  hydriodic  acid 
gas  from  its  elements  may  serve  as  an  instance  of  the  applica- 
tion of  this  indirect  method. 

When  hydriodic  acid,  dissolved  in  water,  is  decomposed  by 
chlorine  we  have  the  reaction  : — 

HI  Aq  +  Cl  =  HC1  Aq  +  I  +  26,000  cal. 

In  this  reaction  the  hydriodic  acid  has  been  decomposed  and 
the  hydrogen  and  chlorine  have  combined  to  form  hydrochloric 
acid  which  has  dissolved  in  the  water.  The  second  part  of  this 
change  gives  rise  to  39,000  cal. 

H  +  Cl  +  Aq  =  HC1  Aq  +  39,000  cal. 

1  Hess.  Pogg.  Ann.  50,  385. 


HEAT  OF  COMBINATION 


233 


From  this  it  follows  that  the  heat  absorbed  in  the  decomposi- 
tion of  the  aqueous  hydriodic  acid  into  hydrogen  and  iodine  is 
39,000  —  26,000  =  13,000  cal.  and  therefore  that  the  heat  of 
formation  of  the  dilute  acid  is  accompanied  by  an  evolution  of 

13,000  cal. 

H  +  I  +  Aq  =  HI  Aq  +13,000  cal. 

It  has  further  been  found  that  the  solution  of  hydriodic  acid 
gas  in  water  gives  rise  to  an  evolution  of  19,060  cal.,  so  that  the 
actual  formation  of  the  gas  from  its  elements  is  accompanied 
by  a  heat-change  of  13,000  -  19,060  =  -  6,060  cal.  Gaseous 
hydriodic  acid  is,  therefore,  produced  from  its  elements  with 
absorption  of  this  amount  of  heat. 

This  branch  of  chemical  science,  which  is  known  as  Thermo- 
chemistry, has  been  studied  by  many  chemists,  among  whom 
may  be  mentioned  Andrews,  Favre  and  Silbermann,  Julius 
Thomsen  and  Berthelot,  from  whose  researches  the  numbers 
given  in  the  following  table  are  taken.1 


MOLECULAR  HEAT  OF  FORMATION  FROM  THE  ELEMENTS. 


HC1 21835 

HBr 8337 

HI -  6060 

O3 -  29378 

H20 67940 

H202 -  22927 

C12O -  17670 

I205 44961 

SO2 70567 

SO3 102526 

NH3 11910 

N20 -17965 

NO  .  -  21438 


NO, 


2035 


N2O5 13000 


PH, 


4267 


POL 


104213 


POC13 144905 

.  308626 


As40G    .    .    . 

As2O5 217751 


AsR 


-  43770 


B203 314821 

CH4 21637 

C2H4 -  2679 

C02 

(a)  charcoal  .    .    .    96253 

(&)  gas  carbon  .    .    95806 

(c)  diamond      .    .    93190 

(d)  graphite      .    .    92660 
CO    .  67490 


CS, 


-2581 


COC12 55183 

CC14 20843 


P2O5 177062 

PC13 74933 

1  These  numbers  have  been  recalculated  from  the  original  results  on  the  basis 
of  0  =  15-88. 


234  THE  NON-METALLIC  ELEMENTS 


THE  OXIDES. 

121  All  the  elements  with  the  single  exception  of  fluorine  are 
found  to  unite  with  oxygen  to  form  an  important  class  of  com- 
pounds termed  oxides,  possessing  very  various  properties  acccord- 
ing  to  the  nature  of  the  combining  element  and  the  quantity  of 
oxygen  with  which  it  unites.  In  many  instances  one  element 
is  found  to  combine  with  oxygen  in  several  proportions,  giving 
rise  to  distinct  oxides.  Oxides  may  be  divided  into  three 
classes,  distinguished  as  Basic  oxides,  Peroxides,  and  Acid- 
forming  oxides. 

(1.)  The  basic  oxides,  such  as  K2O,  potassium  oxide  ;  BaO, 
barium  oxide ;  Fe.2O3,  ferric  oxide,  form  in  combination  with 
water  a  class  of  compounds  termed  hydroxides  or  Jiydrated  oxides, 
such  as  caustic  potash,  KOH ;  barium  hydroxide  or  caustic 
baryta,  Ba(OH)2 ;  ferric  hydroxide,  Fe(OH)3  ;  thus  : 

K20  +  H20  =  2KOH. 
BaO  +  H9O  =  Ba(OH)2. 
Fe203  +  3H20  =  2Fe(OH)3. 

The  characteristic  property  of  these  oxides  as  well  as  of  the 
corresponding  hydroxides  is  their  power  of  neutralizing  acids 
and  forming  compounds  which  are  termed  salts. 

(2.)  The  peroxides  contain  more  oxygen  than  the  basic  oxides. 
A  portion  of  it  is  loosely  combined  and  is  given  off  on  heating  ; 
thus  3Mn02=  Mn3O4  +  O2;  and  although  they  can  form  hydrox- 
ides, these  peroxides  have  not  generally  the  power  of  neutralizing 
acids  and  forming  stable  salts.  The  following  is  a  list  of  some 
of  the  more  important  peroxides.  Barium  dioxide  BaO2 ; 
potassium  tetroxide  K2O4 ;  sodium  peroxide  Na2O2 ;  manganese 
dioxide  Mn02 ;  lead  dioxide  PbO2.  The  term  peroxide  is  a 
somewhat  vague  one  and  is  applied  to  oxides  possessing  very 
different  properties. 

(3.)  The  acid-forming  oxides  combine  with  water  to  form 
hydrates,  which  are  termed  acids  ;  thus — 

Sulphur  trioxide  S03  yields  Sulphuric  acid  H2S04. 
Nitrogen  pentoxide  N205  yields  Nitric  acid  HN03. 
Phosphorus  pentoxide  P2O5  yields  Phosphoric  acid  H3P04. 

S03  +  H20  =  H0S04. 
N205  +  H20  -  2HN03. 
P205  +  3H20  -  2H3P04. 


THE  OXIDES  235 


Acids  possess  a  sour  taste,  turn  blue  litmus  red,  and  neutralize 
the  basic  oxides,  which  when  they  are  soluble  have  the  opposite 
property,  and  turn  red  litmus  blue. 

Salts  may  be  considered  to  be  acids  in  which  the  hydrogen 
is  replaced  by  a  metal.  They  are  obtained  by  a  variety  of 
reactions,  of  which  the  following  are  the  most  important. 

(1)  When  certain  metals  are  brought  in  contact  with  an  acid; 
thus  : — 

Zn  +  H2S04  =  ZnS04  +  H2. 

(2)  When  a  basic  oxide,  or  a  hydroxide  acts  upon  an  acid  or 
an  acid-forming  oxide,  thus  : — 

PbO  +  H2S04  -  PbSO4  +  H2O. 
Ba(OH)2  +  H2S04  =  BaS04  +  2H2O. 

BaO  +  SO3  =  BaSO4 
Ba(OH)2  +  SO3  =  BaS04  +  H2O. 

(3)  When  a  carbonate  of  a  metal  is  acted  upon  by  an  acid : 

CaC03  +  2HN03  =  Ca(N03)2  +  H2O  +  C02. 

The  division  into  these  three  classes  of  oxides  cannot,  however, 
be  strictly  carried  out.  Thus,  whilst  the  position  of  the  extreme 
members  of  each  series,  such  as  the  strong  bases  or  alkalis, 
on  the  one  hand,  and  the  acids  on  the  other,  can  be  sharply 
defined,  it  is  often  difficult  to  classify  the  middle  terms  such 
as  alumina,  A12O3,  manganese  dioxide,  MnO2,  and  tin  oxide 
Sn02,  which  act  sometimes  as  weak  bases  and  at  other  times  as 
weak  acids. 

OZONE.    O3  =  47-64. 

122  So  long  ago  as  1785  Van  Marum  observed  that  oxygen 
gas  through  which  an  electric  spark  had  been  passed  possessed  a 
peculiar  smell,  and  at  once  tarnished  a  bright  surface  of  mer- 
cury ;  but  it  was  not  until  the  year  1840  that  the  attention  of 
chemists  was  recalled  to  this  fact  by  Schonbein.1  This  chemist 
showed  that  the  peculiar  strongly-smelling  substance,  to  which 
he  gave  the  name  of  ozone,  from  ojo>,  I  smell,  is  capable  of 
liberating  iodine  from  potassium  iodide,  and  of  effecting  many 
other  oxidising  actions.  Schonbein,  moreover,  showed  that 
ozone  is  produced  in  other  ways. 

1  Pogg.  Ann.  4,  616. 


236  THE  NON-METALLIC  ELEMENTS 

(1.)  It  is  evolved  at  the  positive  pole  in  the  electrolysis  of 
acidulated  water. 

(2.)  It  is  obtained  by  the  slow  oxidation  of  phosphorus  in  the 
air. 

(3.)  It  is  formed  by  the  discharge  from  an  electrical  machine 
through  air  or  through  oxygen  gas. 

For  many  years  much  doubt  existed  respecting  the  exact 
chemical  nature  of  this  oxidising  principle.  Williamson  and 
Baumert  came  independently  to  the  conclusion  that  ozone  is 
an  oxide  of  hydrogen  having  the  formula  H2  O3 ;  while  Marignac 
and  De  la  Rive,  as  well  as  Fremy  and  Becquerel,  found  that  ozone 
is  formed  when  electric  sparks  are  passed  through  perfectly 


FIG.  70. 

dry  oxygen  gas.  The  explanation  of  these  contradictory  results 
lies  in  the  fact  that  it  was  found  impossible  to  obtain  ozone 
except  in  very  small  quantities,  and  that  an  exact  investi- 
gation of  its  composition  is  rendered  still  more  difficult  by  its 
extremely  energetic  properties.  Further  researches,  conducted 
with  the  greatest  care,  have,  however,  shown  that  ozone  is 
nothing  more  than  condensed  oxygen,  and  the  steps  by 
which  this  conclusion  has  been  arrived  at  constitute  an  admir- 
able example  of  the  successful  resolution,  by  the  convergence 
of  many  independent  investigations,  of  an  apparently  insoluble 
problem. 

To  Andrews  l  belongs  the  credit  of  having  first  proved  that 

1  Phil.  Trans.  1856,  p.  13. 


OZONE  237 


ozone,  from  whatever  source  derived,  is  one  and  the  same  body, 
having  identical  properties,  and  the  same  constitution,  and  also 
that  it  is  not  a  compound  of  two  or  more  elements,  but  oxygen 
in  an  altered  and  allotropic  condition. 

If  a  series  of  electric  discharges  be  sent  through  a  tube  con- 
taining pure  and  dry  oxygen,  only  a  small  portion  of  the  gas  is 
converted  into  ozone  ;  but  if  the  ozone  is  absorbed  as  soon  as  it 
is  formed,  by  a  solution  of  iodide  of  potassium,  for  example,  the 
whole  of  the  oxygen  can  be  gradually  converted  into  ozone.  In 
order  to  obtain  the  maximum  production  of  ozone,  pure  oxygen 
gas  is  allowed  to  pass  through  an  apparatus  (Fig.  70),  which  con- 
sists essentially  of  an  iron  tube  (BB)  turned  very  truly  on  its 
outside,  through  which  a  current  of  cold  water  can  be  passed  by 
means  of  the  tubes  (cc).  Outside  this  metal  cylinder  is  one  of 
glass  (AA)  very  slightly  larger  than  the  iron  one.  By  means  of 
the  tubes  (DD)  air  or  oxygen  can  be  passed  through  the  annular 
space  between  the  two  cylinders.  Part  of  the  outer  cylinder  at 
G  is  covered  with  tinfoil.  The  outer  tinfoil  coating  and  the  inner 
metal  cylinder  are  connected  with  the  poles  of  an  induction  coil 
at  E  and  F.  By  this  means  the  oxygen  is  subjected  to  a  series  of 
silent  discharges,  by  which  it  is  converted  partially  into  ozone. 
The  action  of  this  stream  of  ozonised  oxygen  upon  a  sheet  of 
paper  covered  with  a  solution  of  iodide  of  potassium  and  starch 
is  strikingly  shown  when  the  paper  is  held  in  front  of  the 
current  of  issuing  gas.  The  white  surface  assumes  instantly 
a  deep  blue  colour. 

123  That  this  ozonisation  is  accompanied  by  a  change  of  bulk 
was  shown  by  Andrews  and  Tait.1  These  chemists  filled  a 
glass  tube  (Fig.  71)  with  dry  oxygen ;  one  end  was  then  sealed 
off,  whilst  the  other  ended  in  a  capillary  tube,  bent  in  form 
of  a  syphon,  and  containing  a  liquid,  such  as  strong  sulphuric 
acid,  upon  which  ozone  does  not  act.  On  passing  through  the 
gas  a  silent  discharge,  obtained  by  attaching  one  platinum  wire 
to  one  pole  of  a  Ruhmkorff's  coil,  or  to  the  conductor  of  a 
frictional  electrical  machine,  a  gradual  diminution  of  volume 
occurred,  but  this  never  reached  more  than  TV*h  of  the  whole. 
After  the  ozonized  gas  was  heated  to  about  300°  C.  it  was  found 
to  have  returned  to  its  original  bulk,  and  had  lost  all  its  active 
properties. 

This  decomposition  of  ozone  into  oxygen  can  be  readily  shown 
by  allowing  the  stream  of  ozonized  oxygen  to  pass  through  a 

1  Phil.  Trans.  1860,  p.  113. 


238 


THE  NON-METALLIC  ELEMENTS 


tube  heated  by  the  flame  of  a  Bunsen-lamp.  Every  trace  of 
heightened  oxidizing  action  will  have  disappeared  and  the  blue 
iodide  of  starch  will  not  be  formed  ;  whilst  on  removing  the 
hot  tube  an  immediate  liberation  of  iodine  is  observed,  if  the 
prepared  paper  is  again  brought  into  contact  with  the  issuing 

gas.  In  order  to  gain  a  knowledge  of 
the  composition  of  ozone,  Andrews 
introduced  into  his  ozone  tube  a  sealed 
glass  bulb  containing  substances  able 
to  destroy  the  ozone,  such  as  iodide 
of  potassium  solution,  or  metallic  mer- 
cury. After  transforming  into  ozone 
as  much  as  possible  of  the  oxygen  con- 
tained in  this  tube,  the  bulb  filled  with 
the  iodide  of  potassium  solution  was 
broken  and  the  iodine  liberated  by 
the  ozone.  On  observing  the  column 
of  sulphuric  acid  in  the  syphon  tube 
it  was  found  to  have  remained  un- 
altered after  the  ozone  had  reacted, 
showing  that  the  change  had  not  been 
attended  with  any  alteration  in  volume, 
whilst  on  afterwards  heating  up  to 
300°  C.  no  further  increase  in  the 
volume  occurred,  proving  that  all  the 
ozone  had  been  decomposed. 

These  facts  are  explained  by  the  sup- 
position that,  in  the  formation  of  ozone, 
three  volumes  of  oxygen  condense  to 
form  two  volumes  of  ozone 

300  =  2(X 


EIG.  71. 


vols.     4  vols. 


which,  when  heated,  increase  m  bulk 
again  to  form  the  original  three  volumes 

of  oxygen,  whilst,  when  acted  upon  by  potassium  iodide,  one- 
third  of  the  ozone  is  spent  in  liberating  the  iodine,  and  the 
other  two-thirds  go  to  form  ordinary  oxygen  thus  :-— 


0  +  2KI 


H2O  =  O 


I    +2KOH. 


This  supposition  has  been  proved  to  be  correct  by  Soret,as  follows  : 
Many  essential  oils,  such  as  turpentine  and  oil  of  thyme,  had 


FORMULA  OF  OZONE  239 

been  observed  by  Schonbein  to  possess  the  property  of  absorbing 
ozone  without  decomposing  it,  and  Soret1  showed  that  the 
diminution  in  volume  which  takes  place  on  the  absorption  of 
the  ozone  from  a  measured  quantity  of  ozonised  oxygen  by 
these  oils  is  exactly  twice  as  great  as  the  increase  of  volume 
observed  when  the  ozone  is  decomposed  by  heating  the  gas. 
A  series  of  three  experiments  proved  that  for  every  19*3  cc.  of 
ozone  absorbed  by  the  oil,  9 '47  cc.,  instead  of  the  exact  number 
9'(55  cc.,  of  common  oxygen  were  formed  on  heating.  Hence 
ozone  possesses  the  molecular  formula  O3,  three  volumes  of 
common  oxygen  having  been  condensed  to  two  volumes  by  the 
formation  of  ozone. 

Soret  obtained  a  confirmation  of  his  results  from  a  totally 
different  point  of  view.2  If  the  density  of  ozone  is  one-and-a- 
half  times  as  great  as  that  of  common  oxygen,  the  rate  of  diffusion 
(p.  65)  will  be  inversely  as  the  square  roots  of  these  numbers ; 
if,  therefore,  we  know  the  rate  at  which  ozone  diffuses,  compared 
with  the  ra-te  of  diffusion  of  another  gas  whose  density  is  also 
known,  we  can  draw  conclusions  respecting  the  density  of  ozone. 
The  gas  chosen  for  experiment  was  chlorine,  and  it  was  found 
by  experiment  that  227  volumes  of  chlorine  diffused  in  the  same 
time  as  271  volumes  of  ozone,  or  for  one  volume  of  ozone  there 
diffused  0*8376  volumes  of  chlorine  ;  hence,  according  to  the  law 
of  inverse  squares  of  the  densities,  the  density  of  ozone  is  24*8> 

for  

1  :  0-8376  :  :  \/3519  :  V24.8 

whereas  from  the  formula  03  it  should  be  the  half  of  3  X  15 '88 
that  is  23-82. 

Brodie  3  arrived,  by  a  long  series  of  most  exact  determinations, 
at  the  same  result,  inasmuch  as  he  obtained  the  ratio  of  1  to  2  be- 
tween the  volume  of  the  oxygen  used  in  liberating  iodine  from 
potassium  iodide  and  that  of  the  ozone  absorbed  by  turpentine, 
and  also  showed  that  all  the  oxidising  effects  of  ozone  upon  the 
most  various  substances  can  be  explained  upon  this  basis. 

These  experiments  prove  conclusively  that  dry  oxygen  is  con- 
verted by  the  action  of  the  silent  electric  discharge  into  an  allo- 
tropic  modification.  But  they  do  not  decide  the  question  whether 
the  strongly-smelling  body  obtained  in  the  electrolysis  of  water 
has  an  analogous  constitution,  or  whether  it  may  not  be  an  oxide 

1  Ann.  Chim.  Phys.  [4],  8,  113  ;  Phil.  Mag.  [4]  31,  82,  and  34,  26. 

2  Ann.  Chim.  Phys.  [4]  13,  257.  3  Phil.  Trans.  1872,  Part  ii.  435. 


240  THE  NON-METALLIC  ELEMENTS 

of  hydrogen.  Andrews,  however,  proved  that  if  such  electrolytic 
oxygen  is  perfectly  dried,  it  does  not  lose  its  powerful  smell,  and 
that  if  the  dried  gas  be  then  passed  through  a  hot  glass  tube,  the 
smell,  as  well  as  the  oxidizing  power,  altogether  disappeared  with- 
out the  smallest  trace  of  moisture  being  formed,  and  this  must 
have  been  deposited  if  the  electrolytic  oxygen  had  contained 
an  oxide  of  hydrogen. 

124  Atmospheric  Ozone. — The  difficult  question  as  to  whether 
ozone  exists  in  the  atmosphere  can  scarcely  be  regarded  as  settled 
in  the  affirmative  ;  it  is,  however,  certain  that  hydrogen 
peroxide  (p.  311)  is  present,  and  it  is  very  probable  that  it  is 
accompanied  by  ozone.  The  higher  oxides  of  nitrogen,  amongst 
other  substances,  possess  the  same,  power  as  ozone  of  liberating 
iodine  from  potassium  iodide,  and  these  oxides  are  certainly 
formed  in  the  atmosphere  by  electrical  discharges,  so  that  if  the 
ozone  be  measured,  as  is  usually  the  case,  by  the  amount  of 
iodine  liberated,  by  observing  the  variation  in  tint  of  the  so- 
called  ozone  papers,  we  measure,  along  with  the  ozone,  the  higher 
oxides  of  nitrogen. 

The  experiments  of  Andrews ]  have,  however,  decisively  proved 
that  an  oxidizing  substance  does  occur  in  the  atmosphere  which 
agrees  in  many  of  its  properties  with  ozone.  Thus  when  air  at 
the  ordinary  temperature  was  passed  over  ozone  test-papers 
contained  in  a  glass  tube,  an  indication  of  ozone  was  seen  in  two 
or  three  minutes.  When  the  air  before  passing  over  the  test- 
paper  was  heated  to  260°  C.  not  the  slightest  action  occurred  on 
the  test-paper,  however  long  the  current  was  allowed  to  pass. 
Similar  experiments  made  with  an  artificial  atmosphere  of  ozone, 
that  is,  with  the  air  of  a  large  chamber  containing  a  little 
electrolytic  ozone,  gave  precisely  the  same  results.  On  the 
other  hand,  when  air  mixed  with  very  small  quantities  of  chlorine 
or  the  higher  oxides  of  nitrogen  was  drawn  over  the  papers, 
they  were  generally  affected  whether  the  air  had  been  previously 
heated  or  not.  Houzeau  has  shown  that  a  neutral  solution  of 
iodide  of  potassium  on  exposure  to  air  becomes  alkaline  with 
the  liberation  of  iodine,  an  effect  which  would  not  be  produced 
by  the  oxides  of  nitrogen,  and  which  he  believed  to  be  due  to 
the  presence  of  ozone  in  the  air. 

These  effects  may,  however,  possibly  be   due   to   hydrogen 
peroxide.     The  only  perfectly  satisfactory  distinction  between 
ozone  and  hydrogen  peroxide  is  that  the  former   produces  a 
1  Proc.  Eoy.  Soc.  16,  63. 


ATMOSPHERIC  OZONE  241 

deposit  of  peroxide  of  silver  on  a  piece  of  silver  foil,  and  this 
has  never  been  obtained  with  atmospheric  air.1 

In  spite  of  the  uncertainty  as  to  whether  ozone  really  exists 
in  the  air,  many  methods  have  been  given  for  its  determination. 
Thus  Zenger  passed  100  litres  of  air  through  a  dilute  solution 
of  hydriodic  acid  and  obtained  iodine  liberated,  which  corre- 
sponded to  0*001  or  0'002  milligram  of  ozone  ;  and  it  is  doubt- 
ful how  far  the  iodine  was  really  liberated  by  ozone.  Certain 
other  observers 2  state  that  the  proportion  of  ozone  in  the  air 
stands  in  a  direct  relation  to  the  amount  of  atmospheric  elec- 
tricity present,  whilst  others3  again  conclude  from  their  ob- 
servations that  under  the  normal  atmospheric  conditions  the 
amount  of  ozone  in  the  air  is  absolutely  constant. 

The  usual  method  of  estimating  the  amount  of  ozone  present 
in  the  air  is  a  very  rough  one.  It  consists  in  exposing  to  the 
air  papers  which  have  been  impregnated  with  a  solution  of  starch 
and  iodide  of  potassium,  for  a  given  time  (and  best  in  the  dark), 
and  noting  the  tint  which  they  assume  compared  with  certain 
standard  tints.  The  papers  prepared  according  to  the  directions 
of  Dr.  Moffat  are  those  on  which  most  reliance  is  placed.  It 
has  indeed  been  proposed  by  Bottger  to  use  papers  impregnated 
with  thallious  oxide  as  a  test  for  ozone,  as  this  substance  is 
not  permanently  changed  in  tint  by  the  nitrogen  oxides,  but 
this  suggestion  has  not  been  generally  adopted,  and  doubt  has 
been  thrown  by  Lamy  on  the  use  of  this  reagent,  as  anything 
more  than  a  qualitative  test  of  the  presence  of  ozone. 

It  is  scarcely  necessary  to  remark  that  in  thickly-inhabited 
districts,  especially  in  towns  where  much  coal  is  burnt,  ozone 
is  almost  always  absent,  as  it  is  reduced  to  ordinary  oxygen 
by  the  organic  emanations  as  well  as  by  the  sulphurous  acid 
constantly  present  in  such  air. 

Some  observers  state  that  in  the  air  of  the  country,  and 
especially  in  sea  air,  the  presence  of  ozone  can  almost  always 
be  recognized,  often  indeed  by  its  peculiar  smell,  this  being  said 
by  them  to  be  the  most  reliable  test  for  its  presence.  Respect- 
ing the  variations  in  the  amount  of  atmospheric  ozone  in  different 
localities  or  in  different  seasons  we  possess  at  present  no  reliable 
information.4 

1  Fremy,  Compt.  Rend.  61,  939  ;  Schone,  Ber.  13,  1503. 
-  Neumann,  Pogg.  Ann,  102,  614,  and  Poey,  Compt.  Rend.  65,  708. 
:1  Smyth,  Proc.  Meteoro.  Soc.  June  16,  1869. 
4  Compare  Ilosva,  Bull.  Soc.  Chim.  3,  2,377. 
17 


242  THE  NON-METALLIC  ELEMENTS 

A  possible  cause  of  the  formation  of  ozone  in  the  air  has 
been  pointed  out  by  Gorup  v.  Besanez,1  inasmuch  as  he 
has  shown  that  an  oxidizing  substance  is  invariably  formed 
when  water  evaporates.  This  substance  is  however  probably 
hydrogen  peroxide.  The  production  of  ozone  by  the  slow 
oxidation  of  phosphorus  has  already  been  mentioned.  Several 
other  substances  on  oxidation  also  give  rise  to  a  formation  of 
ozone ;  thus  turpentine  and  several  other  essential  oils  when 
acted  upon  by  atmospheric  oxygen  transform  a  portion  of  it 
into  ozone.  This  may  be  seen  by  shaking  turpentine  in  a  flask 
containing  air  or  oxygen,  when  the  liquid  will  exhibit  the 
properties  of  ozone.  Another  method  by  which  the  active 
variety  of  oxygen  may  be  obtained  is  by  acting  with  strong 
sulphuric  acid  upon  dry  barium  dioxide,  when  oxygen  is  given 
off,  which  is  found  to  contain  considerable  quantities  of  ozone. 
It  is  also  stated  that  ozone  is  formed  during  combustion,  and 
can  be  recognized  by  its  smell  when  a  current  of  air  is  blown 
through  the  upper  portion  of  a  flame. 

Properties. — Ozone  prepared  by  any  of  the  above  methods 
is  a  gas  possessing  a  peculiar  odour,  somewhat  resembling 
that  of  very  diluted  chlorine.  It  has  a  faint  blue  colour, 
which  is  rendered  more  evident  by  compression.  Ozone,  when 
dry,  may  be  preserved  in  sealed  glass  tubes  at  the  ordinary 
atmospheric  temperature  for  a  very  long  time,  but  it  changes 
gradually  into  common  oxygen.  Not  only  is  ozone  destroyed 
by  heat,  but  also  when  agitated  strongly  with  glass  in  fine 
fragments  (Andrews).  It  is  one  of  the  most  powerful  oxidiz- 
ing agents  known ;  it  attacks  and  at  once  destroys  organic 
substances  such  as  caoutchouc,  paper,  &c.  One  of  the  most 
characteristic  actions  of  ozone  is  its  effect  on  mercury.  The 
metal  at  once  loses  its  mobility  and  adheres  to  the  surface  of  the 
glass  in  a  thin  mirror,  and  so  delicate  is  this  reaction,  that  a 
single  bubble  of  oxygen  containing  -gVth  of  its  bulk  of  ozone  will 
alter  the  physical  characters  of  several  pounds  of  mercury, 
taking  away  its  lustre  and  the  convexity  of  its  surface. 
In  many  of  its  oxidising  actions  the  volume  of  ozone  does 
not  undergo  any  alteration,  one  molecule  of  ozone,  O3,  yielding 
one  molecule  of  ordinary  oxygen,  O2,  and  one  atom  of  oxygen 
being  employed  for  the  oxidation.  Ozone  is  converted  into 
ordinary  oxygen  by  contact  with  certain  metallic  oxides, 
such  as  oxide  of  silver  and  manganese  dioxide  or  with 
1  Annalen,  HI,  232. 


PROPERTIES  OF  OZONE  243 

platinum  black.  These  substances  are  not  permanently  altered 
by  the  reaction,  which  is  probably  of  somewhat  the  same  nature 
as  that  by  which  potassium  chlorate  is  decomposed  at  low  tem- 
peratures in  the  presence  of  certain  bodies  (p.  217).  Some  non- 
metals  as  well  as  most  metals  are  at  once  oxidized  in  presence 
of  moist  ozone :  phosphorus  to  phosphoric  acid,  sulphides  to 
sulphates,  ferrocyanides  to  ferricyanides,  whilst  blood  is  com- 
pletely decolorized,  the  albumen  being  entirely,  and  the  other 
organic  matters  being  nearly  all,  destroyed.  Ozone  has  not, 
however,  according  to  the  experiments  of  Carius,1  the  power  it 
was  formerly  supposed  to  possess,  of  oxidizing  nitrogen  to  nitric 
acid  in  presence  of  water. 

Ozone  is  somewhat  soluble  in  water,  imparting  to  water  its 
peculiar  odour  as  well  as  its  oxidizing  powers.  According  to 
Carius,  1,000  volumes  of  water  dissolve  4*5  volumes  of  ozone, 
and  it  is  much  more  soluble  in  certain  ethereal  oils. 

Ozone  forms  on  condensation  an  indigo-coloured  liquid 
(Hautefeuille  and  Chappuis),  which  boils  at— 106°  (Olszewski), 
and  like  liquid  oxygen  is  strongly  magnetic.2  The  compression 
of  the  gas  must  be  effected  slowly,  or  the  heat  produced  raises 
its  temperature  to  such  a  point  that  the  gas  is  suddenly  con- 
verted into  ordinary  oxygen  with  explosion.  A  considerable 
amount  of  heat  is  evolved  in  this  change,  ozone  being  formed 
from  oxygen  with  absorption  of  29,378  cal.  (Berthelot). 

302  =  203  -  2  x  29378. 

It  is  found  that  the  change  of  one  allotropic  form  of  a  substance 
into  another  is  always  accompanied  by  either  an  evolution  or  an 
absorption  of  heat. 

Schonbein,  and  certain  other  chemists,  believed  that  another 
modification  of  oxygen  besides  ozone  exists,  to  which  they  gave 
the  name  of  ant-ozone  ;  the  chief  peculiarity  of  this  body  being 
its  power  of  combining  with  ozone  to  form  ordinary  oxygen. 
Further  experiments  have,  however,  proved  that  ant-ozone  is 
nothing  more  than  hydrogen  dioxide.3 

1  Liebig,  Annalen,  179,  1. 

2  Dewar,  Proc.  Roy.  Soc.  50,  261. 

3  See  Brodie,  Phil.  Trans,  1862,  837. 


244 


THE  NON-METALLIC  ELEMENTS 


HYDROGEN   AND  OXYGEN 

These  elements  form  two  compounds, 

(1)  HYDROGEN  MONOXIDE  or  WATER,  H2O,  and 

(2)  HYDROGEN  DIOXIDE,  H2O2. 


WATER. 


H20 


17-88. 


125  The  question  of  the  discovery  of  the  composition  of  water, 
a  substance  which  up  to  nearly  the  end  of  the  last  century  was 
considered  to  be  a  simple  body,  has  been  fully  discussed  in  the 


FIG.  72. 

historical  introduction.  We  there  learned  that  Cavendish  first 
ascertained  that  by  the  combustion  of  two  volumes  of  hydro- 
gen and  one  volume  of  oxygen,  pure  water  and  nothing  else  is 
produced.  Warped,  however,  as  his  mind  was  with  the  phlogistic 
theory,  he  did  not  fully  understand  these  results,  and  the  true 
explanation  of  the  composition  of  water  was  first  given  by 
Lavoisier  in  1783,  when  the  French  chemist  repeated  and 
confirmed  the  experiments  of  Cavendish.  The  apparatus,  of 
much  historical  interest,  used  by  him  for  proving  that  hydrogen 
gas  is  really  contained  in  water,  is  seen  in  facsimile  in  Fig.  72. 


WATER 


245 


The  water  contained  in  the  vessel  a  was  allowed  to  drop  slowly 
into  the  tube,  e  d,  from  which  it  flowed  into  the  gunbarrel,  df, 
heated  to  redness  in  the  furnace.  Here  part  of  the  water  is 
decomposed,  the  oxygen  entering  into  combination  with  the 
metallic  iron,  whilst  the  hydrogen  and  some  undecomposed 
steam  passed  through  the  worm,  s,  where  the  steam  was  con- 
densed and  the  hydrogen  was  collected  and  measured  in  the 
glass  bell-jar,  m.  The  result  of  these  experiments  was  found 
to  be  that  13*13  parts  by  weight  of  hydrogen  united  to  86'87 
parts  by  weight  of  oxygen,  or  12  volumes  of  oxygen  with  22'9 
volumes  of  hydrogen.1 

Cavendish,  by  exploding  air  with  hydrogen  by  means  of  the 


FIG.  73. 

electric  spark  had,  on  the  other  hand,  come  to  the  conclusion 
that  the  relation  by  volume  of  the  two  gases  combining  to 
form  water  was  1  of  oxygen  to  2  of  hydrogen,  and  this  was 
confirmed  in  1805  by  the  more  exact  experiments  of  Gay- 
Lussac  and  Humboldt.2 

The  formation  of  water  by  the  combustion  of  hydrogen  in  the 
air  can  be  readily  observed  by  means  of  the  arrangement  shown  in 
Fig.  73.  The  hydrogen  is  dried  by  passing  through  the  horizontal 
tube  filled  with  pieces  of  chloride  of  calcium,  then  ignited  at  the 
end  of  the  tube,  and  the  flame  allowed  to  burn  under  the  bell- 
jar.  By  degrees  drops  of  water  form,  these  collect  on  the  sides 
of  the  glass,  and  drop  down  into  the  small  basin  placed  beneath. 


1  Memoire  par  MM.   Meusnier  et  Lavoisier, 
annee  1781,  p.  269,  In  le  21  Avril,  1784. 


Mem.    de  VAcad.   de  Sciences, 
2  Journ.  de  Phys.  60,  129. 


246 


THE  NON-METALLIC  ELEMENTS 


Another  apparatus  for  exhibiting  the  same  fact  is  seen  in  Fig. 
74.  It  consists  of  a  glass  gasholder  filled  with  hydrogen, 
which  is  dried  by  passing  through  the  chloride  of  calcium  tube 
(b),  and  then  burns  under  the  glass  funnel  (c).  The  water 
formed  collects  in  the  tube  (e),  an  aspirator  (/)  drawing  the 
steam  formed  by  the  combustion  through  the  tube  (d). 

126  Eudiometric  Synthesis  of  Water. — The  method  which 
Cavendish  employed  for  the  purpose  of  ascertaining  the  com- 
position of  water  is  still  employed  in  principle,  although  the 
modern  processes  are  much  superior  in  accuracy  to  the  older 
ones.  Bunsen's  modification  of  the  method  consists  in  bringing 
known  volumes  of  the  constituent  gases  successively  into  a  eudio- 


FIG.  74. 

meter  and  allowing  these  gases  to  combine  under  the  influence 
of  the  electric  spark,  carefully  observing  the  consequent  change 
of  volume.  The  eudiometer  employed  is  a  strong  glass  tube  (c), 
Fig.  75,  one  metre  in  length  and  0'025  m.  in  breadth,  closed 
at  the  top  and  open  at  the  bottom,  having  platinum  wires 
sealed  through  the  glass  near  the  closed  end.  The  tube  is  accu- 
rately divided  into  divisions  of  length  by  etching  a  millimetre 
scale  on  the  glass,  and  the  capacity  of  each  division  of  length  on 
the  scale  is  ascertained  by  a  process  of  calibration,  consisting  in 
pouring  successively  exactly  the  same  volume  of  mercury  into 
the  tube,  until  the  whole  is  filled  with  the  metal,  the  height  to 


EUDIOMETRIC  SYNTHESIS  OF  WATER 


247 


which  each  volume  of  mercury  reaches  being  carefully  read  off 
on  the  millimetre  scale  etched  on  the  glass. 

The  eudiometer  containing  at  the  top  one  drop  of  water  to 
render  the  gases  moist,  is  first  completely  filled  with  mercury  and 
inverted  in  the  pneumatic  trough  (d)  containing  the  same  metal. 


FIG.  75. 


Then  a  certain  volume  of  perfectly  pure  oxygen  gas,  prepared 
from  pure  potassium  chlorate,  is  introduced,  the  volume  is  read 
off,  and  the  necessary  reductions  for  temperature  and  pressure  are 
made.  For  this  purpose  a  thermometer  (6)  is  hung  up  near  the 
eudiometer,  and  the  temperature  as  well  as  the  level  of  the 
meniscus  of  mercury  in  the  tube  read  off  by  means  of  a  tele- 


248  THE  NON-METALLIC  ELEMENTS 

scope  placed  in  a  horizontal  position  at  such  a  distance  that  the 
radiation  from  the  observer  does  not  produce  any  sensible  effect 
on  the  reading.  The  pressure  to  which  the  gas  is  subjected  is 
then  ascertained  by  reading  off  the  height  of  the  barometer  (a), 
also  placed  near  the  eudiometer,  and  subtracting  from  this  the 
height  of  the  column  of  mercury  in  the  eudiometer  above  the 
level  of  the  mercury  in  the  trough,  this  height  being  obtained  by 
reading  the  millimetre  divisions  at  the  upper  and  lower  levels  of 
the  mercury.  The  temperature  of  the  mercurial  columns  in  the 
barometer  and  eudiometer  must  also  be  observed,  so  that  cor- 
rection may  be  made  for  the  expansion  of  the  mercurial  column 
the  height  of  which  must  be  reduced  to  that  of  a  column  at. 
0°C. 

We  have  now,  taking  an  actual  example : — 

(1)  The  observed  volume  of  moist  oxygen  taken  from  the 
reading  of  the  upper  level  of  mercury  and  from  the  calibration- 
table  of  the  eudiometer  =  399'1. 

(2)  The  temperature  of  the  gas  =  15°  C. 

(3)  The  height  of  the  barometer  (corrected  to  0°  C.)  =  765  mm. 

(4)  The  height  of  the  mercury  column   in  the  eudiometer 
(corrected  to  0°  C.)  =  500  mm. 

From  these  data  it  is  easy  to  obtain  the  volume  of  the  gas  at 
the  normal  temperature  (0°)  and  under  some  standard  pressure 
(either  1  m.  or  760  mm.  of  mercury  at  0°).  The  gas  has, 
however,  been  measured  in  the  moist  state  ;  the  water  vapour 
present  exerts  a  certain  pressure,  and  thus  depresses  the  column 
of  mercury  in  the  eudiometer  and  increases  the  apparent  pres- 
sure of  the  gas.  In  order,  therefore,  to  obtain  the  actual 
pressure  of  the  dry  gas  it  is  necessary  to  subtract  from  the- 
height  of  the  barometer,  the  column  of  mercury  in  the  eudio- 
meter and  the  vapour  pressure  of  the  water  at  the  temperature 
of  the  experiment,  which  may  be  found  in  a  table  of  vapour 
pressures  (p.  277).  This  amounts  at  15°  to  127  mm.  of  mercury, 
so  that  the  true  pressure  of  the  gas  considered  dry  is : 

765  -  500  -  127  =  252-3  mm. 

Applying  the  laws  of  Boyle  and  Dalton  it  appears  that  399'1 
vols.  of  gas  at  15°  and  252'3  mm.  pressure  will,  at  0°  and  a 
pressure  of  1  metre  of  mercury,  occupy  a  volume  of : 

3991  x  273  x  252-3  _  Q5.45 
288     x      1000 


EUDIOMETRIC  SYNTHESIS  OF  WATEft  249 

The  second  part  of  the  process  consists  in  adding  a  volume 
of  pure  hydrogen,  care  being  taken  not  to  allow  any  bubbles 
of  gas  to  remain  attached  to  the  sides  of  the  tube.  The 
volume  of  hydrogen  added  must  be  such  that  the  inflammable 
mixture  of  two  volumes  of  hydrogen  and  one  volume  of  oxygen 
shall  make  up  not  more  than  from  30  to  40  per  cent,  by  volume 
of  the  whole  gas,  otherwise  the  mercury  is  apt  to  be  oxidized  by 
the  high  temperature  of  the  explosion.  Thus  supposing  we  had 
five  volumes  of  oxygen,  we  must  add  ten  volumes  of  hydrogen 

to  combine  with  this,  and  —~= —  =  28  volumes  for  the  purpose 

oo 

of  dilution. 

As  soon  as  the  temperature  equilibrium  has  been  established, 
the  volume  of  the  mixed  gases  contained  in  the  eudiometer  is 
again  read  off  with  the  same  precautions,  and  the  temperature 
and  pressure  again  ascertained  as  before.  This  having  been 
accomplished,  the  open  end  of  the  eudiometer  is  firmly  pressed 
down  below  the  mercury  in  the  trough  upon  a  plate  of  caout- 
chouc, previously  moistened  with  corrosive  sublimate  solution, 
and  held  firmly  in  this  position  by  a  stout  clamp.  By  means 
of  an  induction  coil  an  electric  spark  is  then  passed  from  one 
platinum-wire  through  the  gas  to  the  other  wire  ;  the  mixed 
gases  are  thereby  ignited,  and  a  flame  is  seen  to  pass  down  the 
tube.  On  allowing  the  mercury  from  the  trough  again  to  enter 
freely  at  the  bottom  of  the  tube  a  considerable  diminution  of 
bulk  is  observed.  The  eudiometer  is  then  allowed  to  remain 
untouched  until  the  temperature  of  the  gas  has  again  attained 
that  of  the  surrounding  air,  and  the  volume,  pressure,  tension, 
and  temperature  are  ascertained  as  before.  The  volume  which 
has  disappeared  does  not,  however,  exactly  correspond  to  the 
true  volume  of  gases  which  have  united,  inasmuch  as  the  water 
formed,  occupies  a  certain  although  a  very  small,  space.  In 
order  to  obtain  the  exact  volume  of  the  combined  gases,  the 
volume  of  the  explosive  gases  before  the  explosion  must  be 
multiplied  by  the  number  O'OOOo,  which  represents  the  frac- 
tion of  the  total  bulk  of  the  component  gases  which  is  occu- 
pied by  the  liquid  water  formed,  and  this  volume  must  then 
be  added  to  the  observed  contraction.  For  other  corrections 
the  article  on  this  subject  in  Bunsen's  Gasometry  must  be 
consulted. 

The  following  numbers  illustrate  the  course  of  such  an  ex- 
periment : — 


250 


THE  NON-METALLIC  ELEMENTS 


Synthesis  of  water  by  volume. 


Reduced  to  0°  and  1  m.  of  mercury. 

Volume  of  oxygen  taken 95 '45 

Volume  of  oxygen  and  hydrogen  .    .     557*26 
Volume  after  the  explosion   ....     27l'06 

Hence  286*2  volumes  disappeared,  or  95*45  volumes  of  oxygen 
have  combined  with  190*75  of  hydrogen.  Consequently  1*0000 
volume  of  oxygen  combines  with  1*9963  volumes  of  hydrogen 
to  form  water. 


FIG.  76. 

By  careful  repetition  of  the  above  experiments  the  com- 
position of  water  by  volume  has  been  ascertained,  within  very 
narrow  limits,  to  be  in  the  proportions  of  one  of  oxygen  to  two 
of  hydrogen. 

Since  the  knowledge  of  the  exact  composition  of  water  by 
volume  is  of  the  greatest  importance  for  the  determination 
of  the  atomic  weight  of  oxygen,  many  attempts  have  been 
made  to  devise  more  accurate  methods  than  that  above  de- 
scribed, for  ascertaining  the  exact  ratio  in  which  hydrogen  and 
oxygen  combine  by  volume. 


ELECTROLYSIS  OF  WATER  251 

The  most  accurate  of  these  is  due  to  Morley,1  who  has  re- 
tained the  principle  of  the  eudiometric  method,  but  greatly 
improved  its  details.  The  greatest  care  was  expended  in  purify- 
ing the  hydrogen  gas  employed,  which  was  prepared  by  the 
electrolysis  of  pure  dilute  sulphuric  acid.  Special  precau- 
tions were  taken  to  free  the  gas  from  any  traces  of  oxygen 
by  passing  it  over  heated  copper,  after  which  it  was  care- 
fully dried  and  used  for  the  experiment.  Portions  of  each 
of  the  pure  gases  were  then  brought  into  the  eudiometer 
and  exploded,  the  oxygen  being  in  excess  in  some  and  the 
hydrogen  in  other  experiments.  As  soon  as  a  large  volume 
of  the  gases  had  been  brought  into  combination  by  successive 
introductions  followed  by  explosions,  the  residue  remaining  in 
the  eudiometer  was  analysed  and  the  amount  of  residual  hydro- 
gen or  oxygen,  and  impurity  consisting  of  atmospheric  nitrogen 
from  which  it  is  impossible  to  free  the  hydrogen,  ascertained. 
This  was  then  subtracted  and  the  volumes  of  the  two  gases 
thus  determined.  The  average  of  twenty  experiments  con- 
ducted in  this  manner  gave  the  ratio  of  1  :  2'0002.  It  is  clear 
that  if  Gay-Lussac's  law  were  strictly  applicable,  the  volumes 
must  be  in  the  simple  ratio  .of  1:2.  The  slight  deviation  from 
this  proportion,  which  seems  to  be  well  established,  serves  to 
show  that  this  law,  like  those  of  Boyle  and  Dalton,  is  only 
approximately  true,  and  that  Avogadro's  theory  which  is  founded 
upon  it,  is  also  only  an  approximate  representation  of  the  truth. 
This  deviation  must  of  course  vary  with  the  pressure  at  which 
the  comparison  is  made,  since  the  two  gases  deviate  to  different 
extents  from  Boyle's  law. 

Electrolytic  Analysis  of  Water. 

127  A  convenient  form  of  voltameter  for  demonstrating  the  com- 
position of  water  by  volume  is  shown  in  Fig.  76.  On  passing  a 
current  of  electricity  through  the  water,  acidified  with  sulphuric 
.acid,  which  fills  the  U-shaped  tube,  bubbles  of  oxygen  rise  from 
the  surface  of  the  platinum  plate  forming  the  positive  pole, 
whilst  bubbles  of  hydrogen  are  disengaged  from  the  negative 
pole.  The  gases  from  each  pole  are  collected  separately,  and 
the  volume  which  collects  in  the  tube  containing  the  negative 
pole  is  seen  to  be  a  little  more  than  double  that  which  collects 
from  the  positive  pole.  On  trial  the  latter  is  found  to  be 
1  Chem.  News,  63,  218. 


252 


THE  NON-METALLIC  ELEMENTS 


oxygen,  and  the  former  hydrogen.  In  this  experiment  the 
volume  of  the  oxygen  gas  is  found  to  be  rather  less  than  half 
that  of  the  hydrogen,  because,  in  the  first  place,  it  is  more 
soluble  in  water  than  hydrogen,  and,  secondly,  because  a  portion 
of  the  oxygen  is  converted  into  ozone,  which,  being  condensed 
oxygen,  occupies  a  less  volume  than  oxygen  in  the  ordinary 
form.  The  fact  that  ozone  is  thus  produced  may  be  shown  by 
bringing  some  iodized  starch  paper  in  contact  with  the  electro- 
lytic gas,  when  iodine  will  be  liberated,  and  the  paper  will 


FIG.  77. 

at  once  be  turned  blue.  By  raising  the  temperature  of  the 
acidulated  water  to  100°,  the  solution  of  the  gases  is  prevented, 
and  at  the  same  time  the  formation  of  ozone  is  avoided,  so 
that  the  true  volume  relation  is  thus  much  more  closely  attained. 
It  must  be  remembered  that  the  decomposition  of  the  water 
is  not  directly  caused  by  the  passage  of  the  electric  current. 
The  sulphuric  acid  which  is  present  yields  the  ions  H  and  SO4 
(p.  116),  the  former  of  which  give  up  their  charges  of  electricity 
at  the  negative  pole  and  escape  from  the  solution,  the  atoms 


VOLUMETRIC  COMPOSITION  OF  STEAM  253 

combining  to  form  molecules  of  hydrogen.  The  ions  SO4,  on 
the  other  hand,  give  up  their  charges  at  the  positive  pole  and  at 
once  decompose,  oxygen  being  liberated  and  a  fresh  quantity 
of  sulphuric  acid  formed  : — 

S04  +  H20  =  H2S04  +  O. 

The  atoms  of  oxygen  then  unite  to  form  molecules  which 
escape  from  the  solution,  whilst  the  regenerated  sulphuric  acid 
again  undergoes  a  similar  series  of  changes. 

The  apparatus,  the  construction  of  which  is  plainly  shown  in 
Fig.  77,  is  used  for  collecting  the  mixed  gases  evolved  by  the 
electrolysis  of  water.  The  mixed  gases,  thus  prepared,  combine 
with  explosive  violence  when  a  flame  is  brought  in  contact  with 
them,  or  when  an  electric  spark  is  passed  through  the  mixture. 
In  this  act  of  combination,  the  whole  of  the  hydrogen  and  the 
whole  of  the  oxygen  unite  to  form  water :  in  other  words,  sub- 
ject to  the  correction  above  referred  to  respecting  the  volume  of 
water  formed,  the  total  volume  of  the  detonating  gas  disappears. 
That  this  is  the  case  is  seen  from  the  following  experiments 
made  by  Bunsen,  in  which  air  was  mixed  with  the  electrolytic 
gas,  the  mixture  exploded,  and  then  the  volume  of  air  deter- 
mined. A  second  addition  of  the  explosive  gas  was  next  made 
and  the  volume  of  air  again  read  off,  and  the  operation  repeated 
a  third  time.1 

Original  volume  of  air,  in  which  detonating  \ 
gas  had  been  once  exploded j 

After  explosion,  with  55*1.9  vols.  detonating  ) 
gas f 

Ditto,  measured  again  after  24  hours    .    .    .        112*57 

After  second  explosion  with  71 '23  vols.  deto- ") 

,.  I  112'66 

nating  gas J 

128  Volumetric  Composition  of  Steam. — Gay-Lussac  not  only 
determined  the  composition  of  water  by  volume,  but  was  the 
first  to  ascertain  that  three  volumes  of  the  mixed  gases  com- 
bine to  form  two  volumes  of  gaseous  steam ;  inasmuch  as  he 
found  the  specific  gravity  of  steam  to  be  O6235,  the  number 
deduced  from  the  above  composition  being  0'6221. 

This  fact  can  be  readily  shown  by  exploding  some  of  the 
1  Gasometry,  p.  65. 


254 


THE  NON-METALLIC  ELEMENTS 


electrolytic  detonating  gas  evolved  from  the  voltameter,  Fig.  77 
in  the  eudiometer  E  Fig.  78  which  is  so  arranged  that  the 
pressure  on  the  gas  can  be  altered  at  pleasure.  Surrounding 


FIG.  78. 


the  eudiometer  is  a  glass  tube  (T),  and  between  the  two  tubes  a 
current  of  the  vapour  of  amyl  alcohol,  which  boils  at  132°,  can 
be  passed  from  the  flask  (F),  and  the  vapour,  after  passing 


COMPOSITION  OF  WATER  255 

through  the  tube,  condensed  in  the  flask  cooled  in  the  trough  of 
water  (H).  When  the  temperature  of  the  tube  and  of  the  gas 
has  risen  to  132°,  the  volume  of  the  gas  is  exactly  read  off  on 
the  divided  scale  of  the  eudiometer,  the  height  of  the  mercury 
in  the  two  limbs  having  been  brought  up  to  the  same  level  by 
means  of  the  reservoir  of  mercury  (M)  attached  to  the  iron  foot 
of  the  eudiometer  by  the  caoutchouc  tube  (G).  The  pressure  on 
the  gas  is  now  reduced  by  lowering  the  level  of  the  mercury 
and,  by  means  of  the  induction  coil  (c),  a  spark  is  passed.  As 
soon  as  combination  has  taken  place,  the  level  of  mercury  in  the 
two  tubes  is  brought  to  the  same  height,  and  the  volume  of  the 
gaseous  water  is  accurately  read  off,  the  temperature  of  the  whole 
being  still  kept  up  to  132°  by  the  current  of  amyl  alcohol  vapour. 
This  volume  is  found  to  be  almost  exactly  two-thirds  of  that  of 
the  original  mixed  gases,  and  hence  we  conclude  that  2  vols. 
of  hydrogen  and  1  vol.  of  oxygen  unite  together  to  form  2  vols. 
of  steam  or  gaseous  water. 


The  Composition  of  Water  and  the  Atomic  Weight  of 
Oxygen, 

129  A  knowledge  of  the  composition  of  water  by  weight  is  of 
the  utmost  importance,  because  the  ratio  in  which  hydrogen  and 
oxygen  combine  must  be  known  before  the  exact  atomic  weight 
of  the  latter  (hydrogen  being  taken  as  the  unit)  can  be  deter- 
mined. The  earlier  experimenters  adopted  a  very  simple  method 
for  this  purpose. 

Many  metallic  oxides  such  as  copper  oxide,  CuO,  when  heated 
in  a  current  of  hydrogen  lose  their  oxygen  which  combines  with 
the  hydrogen  to  form  water,  the  metal  being  produced.  By 
ascertaining  the  loss  of  weight  which  the  oxide  thus  suffers,  and 
by  weighing  the  water  formed,  we  obtain  all  the  data  required 
for  determining  the  ratio  by  weight  in  which  the  two  gases  are 
present  in  water,  inasmuch  as  water  contains  no  other  constituent 
besides  oxygen  and  hydrogen. 

This  method  of  determining  the  synthesis  of  water  by  weight 
was  first  proposed  and  carried  out  in  1820  by  Berzelius  and 
Dulong,1  with  the  following  results  : — 

1  Ann.  Chim.  Phijs.  15,  386. 


256  THE  NON-METALLIC  ELEMENTS 


Synthesis   of  Water  ly    Weight. 


No. 
1     . 

Loss  of  weight 
of  copper  oxide. 

.      8-051     .    . 

Weight  of 
water  obtained. 

9-052 

2     . 

.     10-832    .    . 

12-197 

3 

8-246 

9-270 

Hence  we  have  the  following  numbers  as  representing  the  per- 
centage composition  of  water  according  to  these  experiments  : — 

Percentage  Composition  of  Water  ly    Weight  (Berzelius  and 

Didong}. 


No.  1. 

No.  2. 

No.  3. 

Mean. 

Oxygen     .    . 

88-942 

88-809 

88-954 

88-90 

Hydrogen 

11-058 

11-191 

11-046 

11-10 

100-000  100-000  100-000  100-00 

It  is  thus  seen  that  the  separate  experiments  do  not  agree  very 
closely  amongst  themselves,  and  do  not  therefore  yield  us  cer- 
tain information  as  to  the  exact  proportions  by  weight  in  which 
the  gases  combine  to  form  water. 

In  the  year  1843,  Dumas1  undertook  in  conjunction  with 
Stas,  a  most  careful  repetition  of  these  experiments  ;  pointing 
out  the  following  probable  sources  of  error  in  Berzelius's 
experiments  : — 

(1)  The  weight  of  water  formed  ought  either  to  be  ascertained 
in  vacuo  or  reduced  to  a  vacuum  ;  this  reduction  would  increase 
the  quantity  of  water  by  about  10  to  12  milligrams. 

(2)  The  weight  of  oxygen  ought  also  to  be  reduced  to  a  vacuum. 

(3)  The  hydrogen  ought  to  be  much  more  carefully  dried  than 
was  the  case  in  the  older  experiments. 

(4)  Lastly,  even  supposing  that  the   weights  had  thus  been 
adjusted,  and  if  the  hydrogen  had  been  properly  dried,  Berze- 
lius's determinations  were  made  upon  too  small  a  scale  to  ensure 
the  necessary  degree  of  accuracy. 

A  facsimile  of  the  apparatus  as  used  by  Dumas  is  shown  in 
Fig.  79. 

F  is  the  vessel  in  which  the  hydrogen  is  evolved. 

E  is  a  funnel  with  stop-cock,  containing  sulphuric  acid. 

1  Ann.  Chim.  Phys.  [3],  8,  189 


COMPOSITION  OF  WATER 


257 


18 


258  THE  NON-METALLIC  ELEMENTS 

A  is  a  cylinder  filled  with  mercury  under  the  surface  of  which 
dips  a  safety  tube. 

The  first  U-tube  contains  pieces  of  glass  moistened  with 
nitrate  of  lead. 

The  second  U-tube  contains  glass  moistened  with  silver 
sulphate. 

The  third  U-tube  contains  in  the  first  limb  pumice  moist- 
ened with  potash,  and  in  the  second  limb  pieces  of  solid  caustic 
potash. 

The  fourth  and  fifth  U-tubes  contain  fused  solid  caustic 
potash. 

The  sixth  and  seventh  U-tubes  contain  fragments  of  pumice 
powdered  over  with  phosphorus  pentoxide,  and  are  immersed  in 
a  freezing  mixture. 

The  eighth  is  a  small  weighed  tube  containing  phosphorus 
pentoxide. 

B  is  a  bulb  blown  on  hard  glass  containing  the  dry  oxide  of 
copper,  furnished  with  a  stop-cock  (r)  at  its  upper  end,  and 
drawn  out  so  as  to  pass  into  the  narrow  neck  of  the  vessel  Bt 
at  the  lower  end. 

The  bulb  B  can  be  heated  by  the  Bunsen  lamp  placed  on  the 
sliding  holder  of  the  retort-stand. 

B:  is  the  bulb  in  which  the  water,  formed  by  the  decom- 
position, collects. 

The  U-tube  placed  next  to  the  bulb  B,  contains  pieces  of 
fused  caustic  potash. 

The  next  U-tube  contains  phosphorus  pentoxide  and  is  sur- 
rounded by  a  freezing  mixture.  Next  to  this  is  placed  a  small 
weighed  tube  containing  phosphorus  pentoxide,  whilst  at  the 
end  we  find  another  tube  like  the  last,  but  not  weighed.  A 
cylinder  A2  filled  with  sulphuric  acid,  through  which  the  excess 
of  hydrogen  gas  escapes,  completes  the  arrangement. 

With  .this  apparatus  Dumas  made  no  less  than  19  separate 
experiments  carried  out  with  very  great  care.  The  hydrogen 
evolved  from  zinc  arid  dilute  sulphuric  acid,  might  contain  oxides 
of  nitrogen,  sulphur  dioxide,  arseniuretted  hydrogen  and  sul- 
phuretted hydrogen.  These  impurities  are  eliminated,  and 
the  gas  at  the  same  time  completely  dried  by  passing  over 
the  substances  contained  in  the  U-tubes ;  the  nitrate  of  lead 
absorbs  the  sulphuretted  hydrogen  ;  the  sulphate  of  silver  de- 
composes any  trace  of  arseniuretted  hydrogen,  and  the  rest  of 


COMPOSITION  OF  WATER  259 

the  tubes  serve  to  arrest  every  trace  of  carbonic  acid  and  mois- 
ture, so  that  the  gas  passing  through  the  stop-cock  r  into  the 
bulb  B  consists  of  perfectly  dry  and  pure  hydrogen.  In  order  to 
render  this  certain,  the  small  tube  next  to  the  bulb  is  weighed 
before  and  after  the  experiment,  and  if  its  weight  remain  con- 
stant we  have  proof  that  the  gas  has  been  properly  dried.  A 
similar  small  weighed  tube  serves  a  like  purpose  at  the  other 
end  of  the  apparatus. 

Great  care  must  be  taken  that  the  oxide  of  copper  contained 
in  the  bulb  B  is  perfectly  dry,  for  this  oxide  being  a  hygroscopic 
substance  is  liable  to  absorb  water  from  the  atmosphere.  The 
weight  of  the  bulb  containing  the  oxide  is  then  accurately  deter- 
mined, and  after  all  the  air  has  been  driven  out  of  the  U-tubes 
by  the  dry  hydrogen,  the  bulb  is  fixed  in  its  place.  The  bulb 
destined  to  receive  the  water  is  also  carefully  weighed  before  the 
experiment,  together  with  the  3  drying-tubes  placed  beyond  it 
for  the  purpose  of  absorbing  every  trace  of  aqueous  vapour  car- 
ried over  by  the  hydrogen.  Then  the  oxide  of  copper  is  heated 
to  dull  redness  ;  the  reduction  commences,  and  the  formation  of 
water  continues  for  from  10  to  12  hours.  After  this,  the  bulb  B 
is  allowed  to  cool  in  a  current  of  hydrogen  ;  the  apparatus 
is  then  taken  to  pieces,  the  bulb  rendered  vacuous  and  weighed, 
whilst  the  hydrogen  contained  in  the  bulb  and  tubes  serving  to 
collect  the  water  is  displaced  by  dry  air  before  this  portion  of 
the  apparatus  is  weighed.  It  is  clear  the  weight  of  hydrogen  is 
not  directly  determined  by  this  method  but  that  it  is  obtained 
as  the  difference  between  the  weight  of  water  produced  and  that 
of  the  oxygen  consumed.  As,  however,  the  weight  of  the 
hydrogen  is  only  £  of  that  of  the  water  formed,  it  is  evident  that 
a  percentage  error  of  a  given  amount  on  the  weight  of  water  will 
represent  a  much  larger  percentage  error  on  the  smaller  weight 
of  hydrogen.  The  simplest  way  of  reducing  such  errors  is  to 
arrange  the  experiment  so  that  a  large  quantity  of  water  is 
obtained,  for  the  experimental  errors  remain,  for  the  most  part, 
constant,  and  by  increasing  the  quantity  of  substance  experi- 
mented upon,  the  percentage  error  is  kept  down.  For  this  pur- 
pose Dumas  took  such  weights  of  copper  oxide  as  would  produce 
in  general  about  50  grams  of  water,  so  that  the  experimental 
error,  on  hydrogen  taken  as  the  unit,  is  reduced  to  O'OOo  of  its 
weight.  In  the  19  experiments  Dumas  found  that  840*161 
grains  of  oxygen  were  consumed  in  the  production  of  945*439 


260  THE  NON-METALLIC  ELEMENTS 

grams  of  water ;   or  the  percentage   composition  of  water  by 
weight  is  as  follows : — 

Percentage  Composition  of  Water  ly  Weight  (Dumas). 

Oxygen 88'864 

Hydrogen 11136 


100-000 

In  other  words  two  parts  by  weight  of  hydrogen  combine  with 
15 '9 6 08  parts  by  weight  of  oxygen  to  form  water. 

This  number  is  in  almost  exact  accordance  with  the  results 
of  the  volumetric  analysis  of  Bunsen,  and  the  density  deter- 
minations of  Regnault.  According  to  the  former,  the  ratio 
of  the  volumes  in  which  the  two  gases  combine  is  exactly 
1 : 2,  whilst  the  latter  .found  that  oxygen  is  15 '96  times  as 
heavy  as  hydrogen.  Dumas'  results  represented  the  most  ac- 
curate determinations  made  up  to  the  year  1888.  but  since 
that  time  a  number  of  investigations  on  the  subject  has  been 
made  with  the  utmost  care  and  accuracy,  the  result  of  which 
has  been  to  show  that  the  number  15 '96  is  undoubtedly  too 
high.  The  chief  sources  of  error  in  the  experiments  of  Dumas 
are — (1)  the  presence  of  impurities  in  the  hydrogen;  (2)  the 
fact  that  heated  copper  takes  up  a  certain  amount  of  hydro- 
gen, forming  a  hydride  of  this  metal;  (3)  the  action  of  the 
hydrogen  upon  sulphuric  acid,  used  by  Dumas  in  some  of  his 
experiments  as  a  drying  agent,  by  which  sulphur  dioxide  is 
liberated. 

In  the  more  recent  determinations,  the  results  of  which  are 
given  in  the  accompanying  table,  these  errors  have  as  far  as 
possible  been  avoided.  Thus  Cooke  and  Richards,  Noyes  and 
Reiser,  weighed  the  hydrogen  employed,  the  last-named  in  the 
form  of  palladium  hydride,  the  others  as  the  free  gas,  together  with 
the  water  produced  by  the  reduction  of  copper  oxide.  Dittmar 
and  Henderson,  and  Leduc  made  use  of  Dumas'  original  method, 
whilst  Rayleigh  weighed  both  the  hydrogen  and  the  oxygen, 
but  not  the  water.  The  atomic  weight  calculated  from  the 
volumetric  determination  of  Morley  and  the  density  as  found  by 
Rayleigh  leads  to  a  number  which  is  identical  with  the  mean  of 
those  directly  obtained  (if  the  result  of  Reiser's  experiments, 
which  seems  to  be  undoubtedly  too  high,  be  excluded).  It  may, 
therefore,  be  concluded  that  two  parts  by  weight  of  hydrogen 


PKOPERTIES  OF  ELECTROLYTIC  GAS 


261 


combine  with  15 '88  of  oxygen  to  form  17*88  parts  of  water,  the 
percentage  composition  of  which  is  the  following : — 


Hydrogen 
Oxygen   . 


11186 

88-814 

100-000 


The  following  table  contains  a  summary  of  the  results  obtained 
by  the  different  investigators  who  have  determined  the  relative 
atomic  weights,  density  and  combining  volumes  of  these  two 
elements : — 


Name. 

Date. 

Atomic 
Weight. 

Density. 

Combining  Volumes. 

Gay-Lussac  and  Humboldt 
Dumas 

1805 
1842 

15-96 

... 

1  :2 

Regnault       

1845 

15-96 

Regnault1  corrected     .    .    . 
Ravleiffh  2 

1888 

... 

15-91 

15-884 

Cooke  and  Richards  3  .    .    . 
Keiser4  . 

1888 
1888 

15-869 
15-949 

Scott^     
Rayleigh  •* 

1888-93 
1889 

15-89 

... 

1  .  1-995—2-0025 

Noyes  6   
Dittmar  and  Henderson  7    . 
Morley  8 

1890 
1890 
1891 

15-896 
15-866 
15-879 

1  •  2-00023 

Leduc9   . 

1891 

15-905 

1  :  2-0037 

Rayleigh  2 

1892 

15-882 

f      *i 

Leduc  y   

1892 

15-880 

Experiments  with  the  Detonating  Mixture  of  Oxygen  and 
Hydrogen. 

130  In  order  to  exhibit  the  explosive  force  of  this  detonating 
gas  a  thin  bulb  (B),  Fig.  80,  of  a  capacity  from  70  to  100  cubic 
centimetres  is  blown  on  a  glass  tube.  This  is  filled  with  the  gas 
evolved  from  the  voltameter  (A)  as  shown  in  the  figure,  and,  when 

1  Regnault  (corrected  by  Crafts),  Compt.  Rend.  106,  1662. 

2  Rayleigh,  Proc.  Eoy.  Soc.  43,  356  ;  45,  425  ;  50,  448. 

3  Cooke  and  Richards,  Amer.  Chem,.  Journ.  10,  81,  191. 

4  Keiser,  Amer.  Chem.  Journ.  10,  249. 

5  Scott,  Proc.  Eoy.  Soc.  42,  396  ;  Brit.  Assoc.  Reports,  1888,  631. 

6  Noyes,  Amer.  Chem.  Journ.  12,  441. 

7  Dittmar  and  Henderson,  Proc.  of  Glasgow  Phil.  Soc.  1890—91  ;  Chem.  News 
(1893),  54. 

8  Morley,  Amer.  Journ.  of  Science  [3],  41,  220  ;  Chem  News,  63,  218,  &c. 

9  Leduc,  Compt.  Rend.  113,  186  ;  115,  41,  311  ;  116,  1248. 


262 


THE  NON-METALLIC  ELEMENTS 


full,  is  placed  over  the  perforated  cork  (c),  through  which  two 
insulated  copper  wires  are  inserted,  these  being  connected  at 
the  extremity  by  a  fine  platinum  wire.  The  bulb  is  then  sur- 
rounded with  a  protecting  cover  of  wire  gauze  (G),  and  a 
current  of  electricity  passed  through  the  platinum  wire,  which 
soon  becomes  heated  to  a  temperature  high  enough  to  cause  an 
instantaneous  combination  of  the  oxygen  and  hydrogen  to  occur ; 
a  sharp  explosion  is  heard,  and  the  bulb  is  shattered  to  fine 
dust. 

The  amount  of  the  energy  thus  generated  can  be  easily  calcu- 
lated when  the  quantity  of  heat  developed  by  the  combination  is 
known.  Thus  1  grm.  of  hydrogen  on  burning  to  form  water 


FIG.  80. 

evolves  33,970  thermal  units,  or  heat  sufficient  to  raise  33,970 
grms.  of  water  from  0°  to  1°.  But  the  mechanical  equivalent  of 
heat  is  423,  that  is,  a  weight  of  423  grams  falling  through  the 
space  of  1  metre  is  capable  of  evolving  heat  enough  to  raise  1 
gram  of  water  from  0°  to  1°.  Hence  1  gram  of  hydrogen  on 
burning  to  form  water,  sets  free  an  amount  of  energy  represented 
by  that  required  to  raise  a  weight  of  33,970  x  423  grams  or 
14,309  kilograms  through  the  space  of  1  metre. 

The  gases  may,  however,  be  made  to  combine  not  only 
rapidly,  as  we  have  seen,  but  also  slowly  and  quietly.  High  tem- 
perature, the  passage  of  the  electric  spark,  and  the  presence  of 
platinum  and  other  bodies  effect  the  change  in  the  first  of  these 
ways.  The  smallest  electric  spark  suffices  to  cause  the  combina- 
tion of  the  largest  masses  of  pure  detonating  gas,  because  the 


PHENOMENA  OF  EXPLOSION  263 

heat  which  is  evolved  by  the  union  of  those  particles  in  whose 
neighbourhood  the  spark  passes  is  sufficient  to  cause  the  com- 
bination of  the  adjacent  particles,  and  so  on.  In  every  case  a 
certain  minimum  temperature,  termed  the  temperature  of  igni- 
tion, differing  for  each  gas,  must  be  reached  in  order  that  the 
union  shall  take  place,  and  the  temperature  may  be  so  lowered 
by  mixing  the  detonating  gas  in  certain  proportions  with  inactive 
gases  that  the  explosive  mixture  cannot  inflame.  Thus  one 
volume  of  detonating  gas  explodes  when  mixed  with  2'82  vols. 
of  carbon  dioxide,  with  3*37  vols.  of  hydrogen,  or  with  9*35  vols. 
of  oxygen  ;  but  it  does  not  explode  when  mixed  with  2*89  vols. 
of  carbon  dioxide,  with  3'93  vols.  of  hydrogen,  or  with  10*68 
vols.  of  oxygen.1 

When  the  mixture  of  electrolytic  gases  is  sealed  up  in  a  glass 
bulb  (which  can  be  done  without  explosion  occurring  if  the 
side  tubes  are  of  capillary  bore)  and  heated  in  the  vapour  of 
boiling  sulphur  (448°)  combination  begins  to  take  place  slowly, 
the  whole  of  the  mixed  gases  being  at  length  converted  into 
water  without  any  explosion  occurring.  If  on  the  other  hand 
the  bulb  be  immersed  in  the  vapour  of  boiling  zinc  chloride 
(606°)  an  explosion  invariably  takes  place.  This  does  not  occur, 
however,  until  the  temperature  reaches  650—730°  if  the  gas 
is  passed  through  the  bulb  in  a  slow  stream.  The  temperature 
at  which  combination  commences  is  much  influenced  by  the 
nature  of  the  surface  with  which  the  gas  is  in  contact.  Thus 
if  the  interior  of  the  bulb  be  coated  with  silver,  combination 
begins  at  as  low  a  temperature  as  1550.2 

The  following  experiments  indicate  the  slow  combination 
of  oxygen  and  hydrogen.  If  a  spiral  of  clean  platinum  wire  is 
held  for  a  few  seconds  in  the  flame  of  a  Bunsen  burner,  and 
then  the  flame  extinguished  and  the  gas  still  allowed  to  stream 
out  round  the  spiral,  it  will  be  seen  that  the  spiral  soon  becomes 
red  hot,  either  continuing  to  glow  as  long  as  the  supply  of  gas 
is  kept  up,  or  rising  to  a  temperature  sufficient  to  ignite  the 
gas  (Davy).  A  palladium  wire  acts  in  a  similar  way,  but 
wires  of  gold,  silver,  copper,  iron  and  zinc  produce  no  action  of 
this  kind. 

A  perfectly  clean  surface  of  platinum  plate  also  first  effects  a 

1  Bunsen,  Gasometry,  p.  248. 

2  V.  Meyer  and  Krause,  Annalen,  264,  85  ;  V.  Meyer  and  Askenasy,  Annalen, 
269,  49  ;  V.  Meyer  and  Freyer,  Ber.  25,  622  ;  Zeit.  Phys.  Chem.  H,  28  ;  V. 
Meyer,  Ber.  24,  4233.     See  also  Mitscherlich,  Ber.  26,  160. 


264  THE  NON-METALLIC  ELEMENTS 

slow,  but  after  a  time  even  an  explosive  combination  of  the 
detonating  gas  (Faraday).  The  finely-divided  metal  (spongy 
platinum)  which  exposes  a  great  surface  to  the  action  of  the  gas, 
also  induces,  at  the  ordinary  temperature,  the  combination  of 
hydrogen  with  air  or  oxygen ;  at  first  a  slow  combustion  takes 
place,  but  when  the  metal  becomes  red  hot,  a  sudden  explosion 
occurs  (Dobereiner). 

Small  traces  of  certain  absorbable  gases,  such  as  ammonia, 
destroy  the  inflaming  power  of  the  spongy  platinum,  but  this 
power  is  regained  on  ignition.  The  most  probable  explanation 
of  this  property  of  platinum  is  that  this  metal  possesses  the 
power  of  condensing  on  to  its  surface  a  film  of  hydrogen  and 
oxygen,  which  gases,  when  brought  under  these  circumstances 
into  intimate  contact,  are  able  to  combine  at  the  ordinary 
atmospheric  temperature,  and  by  the  heat  which  their  com- 
bination evolves,  to  excite  the  union  of  the  remaining  gaseous 
mixture. 

The  following  experiment  strikingly  shows  that  a  mixture 
of  hydrogen  and  air  becomes  inflammable  only  when  a  definite 
proportion  between  the  two  gases  has  been  reached.  Fig.  81 
represents  a  suspended  glass  bell-jar  closed  at  the  top,  and 
covered  at  its  mouth  by  a  sheet  of  paper  gummed  on  to  the 
glass.  A  glass  syphon  passing  through  the  paper  cover  is 
fastened  by  copper  wires  to  the  bell-jar  with  the  longer  limb  on 
the  outside.  By  means  of  a  gas-generating  apparatus  the  bell- 
jar  is  filled  with  hydrogen  by  displacement,  a  rapid  current  of 
the  gas  being  made  to  pass  in  through  the  syphon,  the  air  finding 
its  way  out  through  the  pores  of  the  paper.  When  the  bell- 
jar  is  full  of  hydrogen,  the  vulcanized  tube  is  removed  from  the 
end  of  the  long  limb  of  the  syphon,  and  the  stream  of  hydrogen 
gas  which  issues  from  the  end  (hydrogen  being  lighter  than  the 
air,  can  be  syphoned  upwards)  is  then  lighted  and  is  seen  to 
burn  with  its  usual  quiet  non-luminous  flame.  After  a  short 
time,  however,  this  flame  may  be  seen  to  flicker,  and  is  heard  to 
emit  a  musical  note  which  begins  by  being  shrill,  but  gradually 
deepens  to  a  bass  sound,  until,  after  a  time,  distinct  and  separate 
impulses  or  beats  are  heard,  and  at  last,  when  the  requisite  pro- 
portion between  the  hydrogen  and  the  air  which  enters  through 
the  pores  of  the  paper  has  been  reached,  the  flame  is  seen  to 
pass  down  the  syphon  and  enter  the  bell-jar,  when  the  whole 
mass  ignites  with  a  sudden  and  violent  detonation. 

131   The  Phenomena  of  Explosion  in  Gases. — The  phenomena 


PHENOMENA  OF  EXPLOSION 


265 


which  accompany  the  ignition  of  a  detonating  gas  are  of  a  very 
interesting  and  important  character.  Bunsen,  who  was  the  first 
to  investigate  this  subject,  directed  his  attention  mainly  to  two 
points,  the  rate  of  propagation  of  the  explosion  and  the  pressure 
produced,  both  of  which  have  been  more  fully  studied  by  later 
investigators.1 

Bunsen,2  in  1857,  attempted  to  determine  the  rate  at  which 
the  explosion  is  propagated  by  igniting  the  gas  as  it  issued  from 


FIG.  81. 

the  end  of  a  narrow  tube  and  measuring  the  rate  at  which  the 
detonating  mixture  had  to  be  supplied  to  prevent  the  flame 
passing  back  along  the  tube.  In  this  way  he  found  for  hydrogen 
and  oxygen  the  rate  of  34  metres  per  second. 

1  Berthelot,  "Sur  la  force  des  Matieres  Explosives  (Paris),"  Ann.  Chim.  Phys. 
[5],  28,  289  ;  Mallard  and  Le  Chatelier,  Compt.  Rend.  1881,  145,  "Combustions 
des  Melanges  Gazeux  "  ;  Dixon,  Phil.  Trans.  184.  97. 
Phil.  Mag.  [4],  34,  493. 


266  THE  NON-METALLIC  ELEMENTS 

In  1881,  however,  it  was  found  that  when  the  explosive  gas 
is  fired  in  a  tube,  the  rate  of  explosion  increases  from  its  origin 
until  it  reaches  a  certain  maximum,  after  which  it  remains 
constant  whatever  the  length  of  the  column  of  gas  may  be 
(Berthelot,  Mallard  and  Le  Chatelier).  The  disturbance  by  which 
the  ignition  is  propagated  throughout  the  gas  is  known  as  the 
"  explosion  wave  " ;  the  maximum  rate  attained  is  exceedingly 
high  and  has  a  perfectly  definite  and  constant  value  for  each 
explosive  mixture.  The  experiment  of  Bunsen,  as  will  be  seen, 
referred  to  the  initial  period  of  the  combination  before  the 
explosion  wave  had  attained  its  characteristic  velocity. 

In  order  to  determine  the  rate,  the  detonating  gas  is  brought 
into  a  leaden  tube,  about  9mm.  in  diameter  and  100  metres  in 
length,  which  is  closed  at  either  end  by  steel  stop-cocks.  Near 
one  end  the  gas  can  be  fired  by  means  of  an  electric  spark,  and 
at  about  four  feet  from  this  point  an  insulated  bridge  of  silver 
foil  is  placed  across  the  interior  of  the  tube,  a  second  similar 
bridge  being  placed  near  the  second  stop-cock.  These  bridges 
convey  electric  currents,  and  are  connected  with  a  delicate 
chronograph.  The  gas  is  fired  by  a  spark,  and  the  explosion 
after  attaining  its  maximum  rate  in  the  first  four  feet  of  the 
tube  breaks  the  first  bridge,  passes  throughout  the  length  of 
the  tube  and  finally  breaks  the  second  bridge,  the  time  which 
elapses  between  the  two  ruptures  being  recorded  by  the  chrono- 
graph. In  the  case  of  hydrogen  and  oxygen  the  enormous 
velocity  of  2,821  metres  per  second  has  been  found,  and  the 
rates  in  other  gases  are  of  the  same  order  of  magnitude. 

The  addition  of  an  excess  of  one  or  other  of  the  gases  or  of  an 
inert  gas  is  found  to  modify  the  rate  of  explosion  by  altering  the 
temperature  which  is  attained  and  the  density  of  the  gases. 
Thus,  in  a  mixture  of  8  vols.  of  hydrogen  with  1  of  oxygen  the 
rate  is  3,532m.  whilst  in  one  containing  1  vol.  of  hydrogen  with 
3  vols.  of  oxygen  it  is  1,707m.  per  second.  These  velocities 
correspond  closely  with  those  which  would  be  attained  by 
sound  in  the  gases  concerned  at  the  high  temperature  produced 
by  the  combustion  under  the  circumstances  of  the  experiment 
(Dixon).  The  ignition  seems  indeed  to  be  propagated  in 
somewhat  the  same  manner  as  a  sound-wave.  The  numbers 
calculated  for  hydrogen  and  oxygen  on  this  supposition  agree 
well  with  the  experimental  results  obtained  with  the  diluted 
gases.  When  the  pure  detonating  gas  is  employed,  however, 
the  calculated  results  are  invariably  higher  than  the  experi- 


OXYHYDROGEN  FLAME  267 

mental.  This  is  probably  due  to  the  fact,  that  a  certain  fraction 
of  the  gas  escapes  combustion  in  the  explosion  wave,  the 
temperature  of  which  is  probably  above  that  at  which  the 
dissociation  of  steam  begins.  The  greater  part  of  this  uncom- 
bined  gas  undergoes  combustion  as  a  secondary  reaction  after  the 
passage  of  the  wave,  but  about  1  per  cent,  entirely  escapes  and 
is  found  in  the  tube  at  the  close  of  the  experiment  (Dixon). 

According  to  Bunsen's  experiments  the  pressure  produced 
when  electrolytic  gas  is  exploded  is  equal  to  9*5  atmospheres, 
and  a  similar  result  was  subsequently  obtained  by  Berthelot. 
By  comparing  this  result  with  the  pressure  as  calculated  from 
the  heat  of  combination,  Bunsen  concluded  that  only  one-third 
of  the  total  volume  of  gas  is  burnt  at  the  highest  temperature 
of  the  explosion.  Later  experiments  made  with  more  delicate 
appliances  have  shown  that  the  pressures  produced  in  the 
explosion  wave,  although  of  exceedingly  short  duration,  are 
considerably  higher  in  value  than  those  measured  by  Bunsen ; 
the  pressure  in  the  case  of  electrolytic  gas  for  example  probably 
exceeds  20  atmospheres. 

132  The  Oxyhydrogen  Flame. — By  bringing  a  jet  of  oxygen 
gas  within  a  flame  of  hydrogen  gas,  burning  from  a  platinum 
nozzle,  a  flame  of  the  mixed  gases  is  obtained  which  evolves 
but  very  little  light,  although  it  possesses  a  very  high  tempera- 
ture. A  watch-spring  held  in  the  flame  quickly  bums  with 
bright  scintillations.  Platinum,  one  of  the  most  infusible  of 
the  metals,  can  be  readily  melted  and  even  boiled,  whilst  silver 
can  thus  be  distilled  without  difficulty. 

The  arrangement  of  such  an  oxyhydrogen  blowpipe  is  seen  in 
Fig.  82,  the  gases  being  collected  separately  in  the  two  gas- 
holders. The  nozzle  at  s  (Fig.  83)  is  screwed  on  to  the  tap  of 
the  oxygen  gasholder,  the  points  a  and  b  serving  to  keep  the 
oxygen  tube  in  the  centre,  whilst  the  hydrogen  enters  the  tube 
by  the  opening  w,  which  is  connected  with  the  supply  of  this 
gas  by  a  caoutchouc  tube.  The  hydrogen  is  first  turned  on  and 
ignited  where  it  issues  from  the  point  of  the  nozzle ;  the 
oxygen  tap  is  then  gently  turned  on  so  that  the  flame  burns 
quietly.  No  backward  rush  of  gas  or  explosion  can  here  occur, 
for  the  gases  only  mix  at  the  point  where  combustion  takes 
place. 

If  any  solid,  infusible  and  non-volatile  substance,  such  as  a 
piece  of  quick-lime,  be  held  in  the  flame,  the  temperature  of  the 
surface  of  the  solid  is  raised  to  a  very  high  point  and  an  intense 


THE  NON-METALLIC  ELEMENTS 


white  light  is  emitted,  which  is  frequently  used,  under  the  name 
of  the  Drummond  light,  for  illuminating  purposes. 

In  certain  metallurgical  processes,  especially  in  working  the 
platinum  metals,   this   high  temperature   of  the  oxyhydrogen 


FIG  82. 


flame  is  turned  to  useful  account.  One  of  the  forms  of  furnace 
used  for  this  purpose  is  shown  in  Fig.  84.  It  is  built  from  a 
block  of  very  carefully  burnt  lime  A,  A,  which  has  been  cut  in 
half  and  then  each  piece  hollowed  out,  so  that  when  brought 


FIG.  83. 


together  they  form  a  chamber  into  which  the  substance  to 
be  melted  is  placed.  The  upper  block  is  perforated  to 
allow  the  nozzle  (c,  Q)  of  the  blowpipe  to  fit  in,  and  the 
gases  pass  from  the  separate  gasholders,  into  two  concentric 


OXIDATION  AND  REDUCTION 


269 


tubes  c  c  and  Ef  E',  each  provided  with  a  stopcock  (o  and  H), 
the  hydrogen  being  delivered  by  the  outer  and  the  oxygen  by 
the  inner  tube.  MM.  Deville  and  Debray  l  have  in  this  way 
melted  50  kilos,  of  platinum  in  one  operation,  and  Messrs. 
Johnson,  Matthey,  &  Co.  melted,  by  this  process,  a  mass  of  pure 
platinum  weighing  100  kilos,  which  was  shown  at  the  Exhi- 
bition of  1862.  Since  that  time  the  same  firm  has  melted  no 
less  than  250  kilos,  of  an  alloy  of  platinum  and  iridium  for  the 
International  Metrical  Commission. 

133    Alternate    Oxidation    and    Reduction. — An    interesting 
experiment  exhibiting  the  increase  and  loss  of  weight  on  the 


FIG.  84. 

oxidation  of  metallic  copper,  and  the  subsequent  reduction  of 
the  copper  oxide  is  carried  out  as  follows  :  Copper  oxide  is 
rubbed  up  into  a  stiff  paste  with  gum-water,  and  the  mass 
rolled  into  the  form  of  a  cylinder  about  1  cm.  broad,  and  3  cm. 
long,  which  is  then  dried  and  ignited  in  the  air.  On  heating 
this  in  a  current  of  hydrogen,  the  oxide  is  reduced  to  the  metal, 
and  a  cylinder  of  porous  copper  is  obtained.  A  platinum  wire 
is  then  wound  round  it,  and  the  cylinder  held  in  the  flame  of  a 
Bunsen  burner,  so  as  to  warm  it,  but  not  to  bring  it  to  a  red 
heat.  On  now  plunging  it  into  a  jar  of  oxygen  gas,  it  is  seen  to 
glow  from  combination  with  oxygen,  and  this  continues  until 

1  Ann.  Chim.  Phys.  [3],  56,  385. 


270  THE  NON-METALLIC  ELEMENTS 

the  whole  is  converted  into  oxide.  When  removed  from  the 
oxygen,  the  colour  of  the  metallic  cylinder  will  be  seen  to  be 
changed  from  a  bright  red  to  the  black  colour  of  the  oxide. 
Next  let  this  oxidised  cylinder  be  suspended  from  one  pan  of  a 
balance,  and  a  counterpoise  placed  in  the  other  pan.  The 
cylinder  is  then  removed  from  the  balance,  gently  heated,  and 
whilst  warm  pushed  up  into  a  wide  tube,  placed  mouth  down- 
wards, through  which  a  current  of  hydrogen  is  passing  ;  the 
copper  oxide  is  at  once  seen  to  glow,  water  being  formed,  which 
condenses  and  drops  down  from  the  sides  of  the  tube,  whilst  the 
colour  of  the  cylinder  changes  from  black  to  the  brilliant  red  of 
the  reduced  metal.  As  soon  as  the  reduction  is  complete  the 
cylinder  is  removed  from  the  hydrogen  and  again  hung  on  to  the 
pan  of  the  balance,  when  a  considerable  loss  of  weight,  due  to 
the  loss  of  the  oxygen,  will  be  perceived. 


PKOPERTIES  OF  WATER. 

134  Pure  water  is  a  clear,  tasteless  liquid,  colourless  when 
seen  in  moderate  quantity,  but  when  viewed  in  bulk  possessing 
a  bluish  green  colour,  well  seen  in  the  water  of  certain  springs, 
especially  those  in  Iceland,  and  in  certain  lakes,  particularly 
those  of  Switzerland,  which  are  fed  by  glacier  streams  This 
blue  colour  is  also  observed  if  a  bright  white  object  be  viewed 
through  a  column  of  distilled  water  about  six  to  eight  metres  in 
length,  contained  in  a  tube  with  blackened  sides  and  plate-glass 
ends.  Water  is  an  almost  incompressible  fluid,  one  million 
volumes  becoming  less  by  fifty  volumes  when  the  atmospheric 
pressure  is  doubled  ;  it  is  a  bad  conductor  of  heat,  and  probably 
a  non-conductor  of  electricity. 

Expansion  and  Contraction  of  Water. — When  heated  from 
0°  to  4°,  water  is  found  to  contract,  thus  forming  a  striking 
exception  to  the  general  law,  that  bodies  expand  when  heated 
and  contract  on  cooling;  on  cooling  from  4°  to  0°  it  expands 
again.  Above  4°,  however,  it  follows  the  ordinary  law,  expand- 
ing when  heated,  and  contracting  when  cooled.  This  pecu- 
liarity in  the  expansion  and  contraction  of  water  may  be 
expressed  by  saying  that  the  point  of  maximum  density  of  water 
is  4°  C. ;  or  according  to  the  exact  determinations  of  Joule, 
3°'945 ;  that  is,  a  given  bulk  of  water  will  at  this  temperature 


PROPERTIES  OF  WATER  271 

weigh  more  than  at  any  other.  Although  the  amount  of  con- 
traction on  heating  from  0°  to  4°  is  but  small,  yet  it  exerts  a 
most  important  influence  upon  the  economy  of  nature.  If  it 
were  not  for  this  apparently  unimportant  property,  our  climate 
would  be  perfectly  Arctic,  and  Europe  would  in  all  probability 
be  as  uninhabitable  as  Melville  Island.  In  order  better  to 
understand  what  the  state  of  things  would  be  if  water  followed 
the  ordinary  laws  of  expansion  by  heat,  we  may  perform  the 
following  experiment,  first  made  by  Dr.  Hope.  Take  a  jar  con- 
taining water  at  a  temperature  above  4°,  place  one  thermometer 
at  the  top  and  another  at  the  bottom  of  the  liquid.  Now  bring 
the  jar  into  a  place  where  the  temperature  is  below  the  freezing 
point,  and  observe  the  temperature  at  the  top  and  bottom  of  the 
liquid  as  it  cools.  It  will  be  seen  that  at  first  the  upper  ther- 
mometer always  indicates  a  higher  temperature  than  the  lower 
one  ;  after  a  short  time  both  thermometers  mark  4° ;  and,  as  the 
water  cools  still  further,  it  will  be  seen  that  the  thermometer  at 
the  top  always  indicates  a  lower  temperature  than  that  shown 
by  the  one  at  the  bottom  :  hence  we  conclude  that  water  above 
or  below  4°  is  lighter  than  water  at  4°.  This  cooling  goes  on 
till  the  temperature  of  the  top  layer  of  water  sinks  to  0°,  after 
which  a  crust  of  ice  is  formed ;  and  if  the  mass  of  the  water  be 
sufficiently  large,  the  temperature  of  the  water  at  the  bottom  is 
never  reduced  below  4°.  In  nature  precisely  the  same  phe- 
nomenon occurs  in  the  freezing  of  lakes  and  rivers  ; *  the  surface- 
water  is  gradually  cooled  by  cold  winds,  and  thus  becoming 
heavier,  sinks,  whilst  lighter  and  warmer  water  rises  to  supply 
its  place  :  this  goes  on  till  the  temperature  of  the  whole  mass  is 
reduced  to  4°,  after  which  the  surface-water  never  sinks,  however 
much  it  be  cooled,  as  it  is  always  lighter  than  the  deeper  water 
at  4°.  Hence  ice  is  formed  only  at  the  top,  the  mass  of  water 
retaining  the  temperature  of  4°.  Had  water  become  heavier  as  it 
cooled  down  to  the  freezing  point,  a  continual  circulation  would  be 
kept  up,  until  the  mass  was  cooled  throughout  to  0°,  when  solidi- 
fication of  the  whole  would  ensue.  Thus  our  lakes  and  rivers 
would  be  converted  into  solid  masses  of  ice,  which  the  summer's 
warmth  would  be  quite  insufficient  thoroughly  to  melt ;  and 
hence  the  climate  of  our  now  temperate  zone  might  approach 
in  severity  that  of  the  Arctic  regions. 

The  following  table  gives  the  volume,  and  specific  gravities 

1  The  point  of  maximum  density  of  sea-water  is  considerably  lower  than  that 
of  fresh,  and  is  in  fact  below  0°  C. 


272 


THE  NON-METALLIC  ELEMENTS 


of  water  for  temperatures  varying  from  0°  to  100°,  according  to 
the  most  accurate  experiments.1 


Tempe- 
rature. 

Volume. 

Specific 
Gravity. 

Tempe- 
rature. 

Volume. 

Specific 
Gravity. 

0° 

1-000122 

0-999878 

19° 

1-00153 

0-99847 

1 

1-000067 

0-999933 

20 

1-00173 

0-99827 

2 

1-000028 

0-999972 

21 

1-00194 

0-99806 

3 

1-000007 

0-999993 

22 

1-00216 

0-99785 

4 

1-000000 

1-000000 

23 

1-00238 

0-99762 

5 

1-000008 

0-999992 

24 

1-00262 

0-99739 

6 

1-000031 

0-999969 

25 

1-00287 

0-99714 

7 

1-000067 

0-999933 

30 

1-00425 

0-99577 

8 

1-000118 

0-999882 

35 

1-00586 

0-99417 

9 

1-000181 

0-999819 

40 

1-00770 

0-99236 

10 

1-000261 

0-999739 

45 

1-00974 

0-29035 

11 

1-000350 

0-999650 

50 

1-01197 

0-98817 

12 

1-000456 

0-999544 

55 

1-01436 

0-98584 

13 

1-000570 

0-999430 

60 

1-01694 

0-98334 

14 

1-000703 

0-999297 

65 

1-01967 

0-98071 

15 

1-000847 

0-999154 

70 

1-02261 

0-97789  , 

16 

1-000997 

0-999004 

80 

1-02891 

0-97190 

17 

1-001162 

0-998839 

90 

1-03574 

0-96549 

18 

1-001339 

0-998663 

100 

1-04323 

0-95856 

, 

135  Latent  Heat  of  Water. — In  the  passage  from  solid  ice  to 
liquid  water,  we  notice  that  a  very  remarkable  absorption  or 
disappearance  of  heat  occurs.  This  is  rendered  plain  by  the 
following  simple  experiment : — Let  us  take  a  kilogram  of  water 
at  the  temperature  0°,  and  another  kilogram  of  water  at  79°.  If 
we  mix  these,  the  temperature  of  the  mixture  will  be  the  mean, 
or  39°'5  ;  if,  however,  we  take  one  kilogram  of  ice  at  0°  and  mix 
it  with  a  kilogram  of  water  at  79°,  we  shall  find  that  the  whole 
of  the  ice  is  melted,  but  that  the  temperature  of  the  resulting 
2  kilograms  of  water  is  exactly  0°.  In  other  words,  the  whole 
of  the  heat  lost  by  the  hot  water  has  just  sufficed  to  melt 
the  ice,  but  has  not  raised  the  temperature  of  the  water  thus 
produced.  Hence  we  see  that  in  passing  from  the  solid  to  the 
liquid  state  a  given  weight  of  water  takes  up  or  renders  latent 
just  so  much  heat  as  would  suffice  to  raise  the  temperature  of 
the  same  weight  of  water  through  79°  C. ;  the  latent  heat  of  water 
1  Volkmann,  Wied.  Ann.  14,  260. 


PROPERTIES  OF  WATER  273 

is,  therefore,  said  to  be  79  thermal  units — a  thermal  unit  mean- 
ing the  amount  of  heat  required  to  raise  a  unit  weight  of  water 
through  1°  C.  When  water  freezes,  or  becomes  solid,  this 
amount  of  heat,  which  is  necessary  to  keep  the  water  in  the 
liquid  form,  and  is,  therefore,  well  termed  the  heat  of  liquidity, 
is  evolved,  or  rendered  sensible.  A  similar  disappearance  of 
heat  on  passing  from  the  solid  to  the  liquid  state,  and  a  similar 
evolution  of  heat  on  passing  from  the  liquid  to  the  solid  form, 
occurs  with  all  substances ;  the  amount  of  heat  thus  evolved  or 
rendered  latent  varies,  however,  with  the  nature  of  the  substance. 
A  simple  means  of  showing  that  heat  is  evolved  on  solidification 
consists  in  obtaining  a  saturated  hot  solution  of  acetate  of  soda, 
and  allowing  it  to  cool.  Whilst  it  remains  undisturbed,  it 
retains  the  liquid  form,  but  if  agitated,  it  at  once  begins  to 
crystallize,  and  in  a  few  moments  becomes  a  solid  mass.  If  a 
delicate  thermometer  be  now  plunged  into  the  salt  while  solidi- 
fying, a  rapid  rise  of  temperature  will  be  noticed. 

136  Freezing  Point  of  Water  and  Melting  Point  of  Ice. — 
Although  water  usually  freezes  at  0°  it  was  observed  so  long  ago 
as  1714  by  Fahrenheit  that  under  certain  circumstances  water 
may  remain  liquid  at  temperatures  much  below  this  point. 
Thus  when  brought  under  a  diminished  atmospheric  pressure, 
water  may  be  cooled  to  —  12°  without  freezing  ;  or  if  water  be 
boiled  in  a  glass  flask,  and  the  neck  of  the  flask  be  plugged 
whilst  it  is  hot  with  cotton  wool,  the  flask  and  its  contents  may 
be  cooled  to  —  9°  without  the  water  freezing,  but  when  the 
cotton  wool  is  taken  out,  particles  of  dust  fall  into  the  water, 
and  these  bring  about  an  immediate  crystallisation,  the  tem- 
perature of  the  mass  quickly  rising  to  0°.  Sorby1  has  shown 
that  when  contained  in  thin  capillary  glass  tubes,  water  may  be 
cooled  to  —  15°  without  freezing,  whilst  Boussingault 2  has 
exposed  water  contained  in  a  closed  steel  cylinder  to  a  tem- 
perature of  —  24°  for  several  days  in  succession  without  its 
freezing.  The  melting  point  of  ice  under  the  ordinary  atmo- 
spheric pressure  is  0°,  but  this  point  is  lowered  by  increase  of 
pressure  ;  thus  under  a  pressure  of  81  atmospheres,  ice  melts  at 
—  0°'059  and  under  16'8  atmospheres,  at  —  0°'129  or  the  melting 
point  is  lowered  by  about  0°'0075  3  for  every  additional  atmo- 
sphere. This  peculiarity  of  a  lowering  of  the  melting  point 
under  pressure  is  common  to  all  substances  which,  like  water 

1  Phil.  Mag.  [4],  18,  105.  2  Compt.  Rend.  73,  77. 

3  James  Thomson,  Edin.  Roy.  Soc.  Trans,  vol.  16,  575,  1849. 
19 


274  THE  NON-METALLIC  ELEMENTS 

expand  in  passing  from  the  liquid  to  the  solid  state,  whilst 
in  the  case  of  bodies  which  contract  under  like  circumstances, 
the  melting  point  is  raised  by  increase  of  pressure.  Thus 
Bunsen  in  the  case  of  paraffin,1  and  Hopkins  in  the  case  of 
sulphur,  obtained  the  following  results  : — 

Under  a  pressure  of  1  atmosphere,  paraffin  melts  at  46°*3 

85  „  „  48°-9 

100  „  „  49°-9 

„  „  1  atmosphere,  sulphur  melts  at  1 07°'0 

519          „  „  135°-2 

792  „  140°- 5 

From  what  has  been  stated  we  should  expect  that  by  increas- 
ing the  pressure  upon  ice  it  could  be  melted,  and  Mousson 2  has 
shown  that  this  is  the  case,  for  by  exposing  it  to  a  pressure  of 
13,000  atmospheres  he  has  converted  ice  into  water  at  a  tem- 
perature of  —18°.  This  lowering  of  the  melting  point  of  ice  with 
pressure  explains  the  fact  that  when  two  pieces  of  ice  are  rubbed 
together  the  pressure  causes  the  ice  to  melt  at  the  portions  of 
the  surface  in  contact,  the  water  thus  formed  running  away,  and 
the  temperature  being  lowered  ;  then  as  soon  as  the  excess  of 
pressure  is  taken  away  the  two  surfaces  freeze  together  at  a 
temperature  below  0°,  one  mass  of  solid  ice  being  produced. 
This  phenomenon,  termed  regelation,  was  first  observed  by  Fara- 
day in  1850,  and  was  afterwards  applied  by  Tyndall  to  explain 
glacier  motion. 

137  The  crystalline  form  of  ice  is  hexagonal,  being  that 
of  a  rhombohedron.  Snow  crystals  exhibit  this  hexagonal  form 
very  clearly ;  they  usually  consist  of  crystals  which  have  grown 
on  to  another  crystal  in  the  direction  of  the  three  horizontal 
axes,  that  so  the  snow  crystal  clearly  exhibits  these  three 
directions,  as  shown  in  Figs.  85,  86. 

Ice  is  transparent,  and  when  seen  in  small  quantities  it  ap- 
pears to  be  colourless,  though  large  masses  of  ice,  such  as 
icebergs  or  glaciers,  possess  a  deep  blue  colour ;  like  water 
it  is  also  a  bad  conductor  of  heat  and  a  non-conductor  of 
electricity,  and  becomes  electrical  when  rubbed. 

Water  on  freezing  increases  nearly  y1^-  of  its  bulk,  or,  according 
to  the  exact  experiments  of  Bunsen,3  the  specific  gravity  of  ice 

1  Ann.  Chim.  Phys.  [3],  35,  383.  -  Pogg.  Ann.  105,  161. 

3  Phil.  Mag.  [4],  41,  165. 


PROPERTIES  OF  WATE11 


275 


at  0°  is  0*91674,  that  of  water  at  0°  being  taken  as  the  unit ; 
or  one  volume  of  water  at  0°  becomes  1*09082  volumes  of  ice  at 
the  same  temperature.  This  expansion  plays  an  important 
part  in  the  disintegration  and  splitting  of  rocks  during  the 
winter.  Water  penetrates  into  the  cracks  and  crevices  of  the  rocks 
and  on  freezing  widens  these  openings;  this  process  being 
repeated  over  and  over  again,  the  rock  is  ultimately  split  into 
fragments.  Hollow  balls  of  thick  cast-iron  can  thus  easily  be 
split  in  two  by  filling  them  with  water  and  closing  by  a 
tightly  fitting  screw,  and  then  exposing  them  to  a  temperature 
below  0°. 


FIG.  85. 


FIG.  86. 


138  Latent  Heat  of  Steam. — Under  the  normal  barometric 
pressure  of  760rom  water  boils  in  a  metal  vessel  at  100°  C. 
When  liquid  water  is  converted  ir  to  gaseous  steam,  a  large 
quantity  of  heat  becomes  latent,  the  temperature  of  the  steam 
given  off  being  the  same  as  that  of  the  boiling  water,  as,  like  all 
other  bodies,  water  requires  more  heat  for  its  existence  as  a  gas 
than  as  a  liquid.  The  amount  of  heat  latent  in  steam,  is  roughly 
ascertained  by  the  following  experiment.  Into  1  kilogram  of 
water  at  0°,  steam  from  boiling  water,  having  the  temperature 
of  100°,  is  passed  until  the  water  boils  :  it  is  then  found  that 
the  whole  weighs  T187  kilos.,  or  0'187  kilo,  of  water  in  the  form 
of  steam  at  100°  has  raised  1  kilo,  of  water  from  0°  to  100°  ;  or  1 
kilo,  of  steam  at  100°  would  raise  5 '36  kilos,  of  ice-cold  water 


276  THE  NON-METALLIC  ELEMENTS 

through  100°,  or  536  kilos,  through  1°.    Hence  the  latent  heat  of 
steam  is  said  to  be  536  thermal  units. 

Whenever  water  evaporates  or  passes  into  the  gaseous  state, 
heat  is  absorbed,  and  so  much  heat  may  be  thus  abstracted  from 
water  that  it  may  be  made  to  freeze  by  its  own  evaporation. 
A  beautiful  illustration  of  this  is  found  in  an  instrument  called 
Wollaston's  Cryophorus,  Fig.  87  ;  it  consists  of  a  bent  tube, 
having  a  bulb  on  each  end,  and  containing  water  and  vapour  of 
water,  but  no  air.  On  placing  all  the  water  in  one  bulb,  and 
plunging  the  empty  bulb  into  a  freezing  mixture,  a  condensation 
of  the  vapour  of  water  in  this  empty  bulb  occurs,  and  a  corre- 
sponding quantity  of  water  evaporates  from  the  other  bulb  to 
supply  the  place  of  the  condensed  vapour ;  this  condensation 
and  evaporation  go  on  so  rapidly  that  in  a  short  time  the  water 
cools  down  below  0°,  and  a  solid  mass  of  ice  is  left  in  the  bulb. 
By  a  very  ingenious  arrangement  this  plan  of  freezing  water  by 
its  own  evaporation  has  been  practically  carried  out  on  a  large 


FIG.  87. 

scale  by  M.  Carre",  by  means  of  which  ice  can  be  most  easily 
prepared.  This  arrangement  consists  simply  of  a  powerful 
air-pump  (A,  Fig.  88),  and  a  reservoir  (B),  of  a  hygroscopic 
substance,  such  as  strong  sulphuric  acid.  On  placing  a 
bottle  of  water  (c)  in  connection  with  this  apparatus,  and{  on 
pumping  for  a  few  minutes,  the  water  begins  to  boil  rapidly, 
and  its  temperature  is  so  lowered  by  the  evaporation  that  the 
water  freezes  to  a  mass  of  ice. 

139  Tension  of  Aqueous  Vapour. — Water,  and  even  ice,  con- 
stantly give  off  steam  or  aqueous  vapour  at  all  temperatures, 
when  exposed  to  the  air.  Thus  we  know  that  if  a  glass 
of  water  be  left  in  a  room  for  some  days,  the  whole  of  the 
water  will  gradually  evaporate.  This  power  of  water  to  rise  in 
vapour  at  all  temperatures  is  due  to  the  elastic  force  or  tension 
of  aqueous  vapour ;  it  may  be  measured,  when  a  small  quantity 
of  water  is  placed  above  the  mercury  in  a  barometer,  by  the 
extent  to  which  the  vapour  thus  given  off  is  capable .  of 


PKOPERTIES  OF  WATER 


277 


depressing  the  mercurial  column.  If  we  gradually  heat 
the  drops  of  water  thus  placed  in  the  barometer,  we  shall 
notice  that  the  column  of  mercury  gradually  sinks ;  and  when 
the  water  is  heated  up  to  100°  C.,  the  mercury  in  the  barometer 
tube  is  found  to  stand  at  the  same  level  as  that  in  the  trough, 
showing  that  the  elastic  force  of  the  vapour  at  that  tem- 
perature is  equal  to  the  atmospheric  pressure.  Water  is  said  to 
boil  in  the  air  when  the  tension  of  its  vapour  is  equal  to  the 
superincumbent  atmospheric  pressure.  On  the  tops  of  mountains, 
where  the  atmospheric  pressure  is  less  than  at  the  sea's  level, 
water  boils  at  a  temperature  below  100° :  thus  at  Quito,  at  a 
height  of  2914  metres  above  the  sea's  level,  the  mean  height  of 


FIG.  88. 


the  barometer  is  523  mm.,  and  the  boiling  point  of  water  is 
90°'l  ;  that  is,  the  tension  of  aqueous  vapour  at  90°'l  is  equal  to 
the  pressure  exerted  by  a  column  of  mercury  523  mm.  high. 
Founded  on  this  principle,  an  instrument  has  been  constructed 
for  determining  heights  by  noticing  the  temperatures  at  which 
water  boils.  A  simple  experiment  to  illustrate  this  fact  consists 
in  boiling  water  in  a  globular  flask,  into  the  neck  of  which  a 
stopcock  is  fitted  :  as  soon  as  the  air  is  expelled,  the  stopcock  is 
closed,  and  the  flask  removed  from  the  source  of  heat ;  the 
boiling  then  ceases ;  but  on  immersing  the  flask  in  cold  water, 
the  ebullition  recommences  briskly,  owing  to  the  reduction  of 
the  pressure  consequent  upon  the  condensation  of  the  steam  ; 
the  tension  of  the  vapour  at  the  temperature  of  the  water  in 


278 


THE  NON-METALLIC  ELEMENTS 


the  flask  being  greater  than  the  diminished  pressure.  All  other 
liquids  follow  a  similar  law  respecting  ebullition  ;  but  as  the 
tensions  of  their  vapours  are  very  different,  their  boiling  points 
vary  considerably. 

When  steam  is  heated  alone,  it  expands  according  to  the 
law  previously  given  for  permanent  gases  ;  but  when  water  is 
present,  and  the  experiment  is  performed  in  a  closed  vessel, 
the  elastic  force  of  the  steam  increases  in  a  far  more  rapid 
ratio  than  the  increase  of  temperature.  The  following  table 
gives  the  tension  of  aqueous  vapour,  as  determined  by  experi- 
ment, at  different  temperatures  measured  on  the  air  ther- 
mometer. 

Table  of  the  Tension  of  the  Vapour  of  Water. 


Temperature 
Centigrade. 

Tension  in  millime- 
tres of  mercury. 

Temperature 
Centigrade. 

Tension  in  atmosphs., 
1  atmosphere  =  760 
mm.  of  mercury. 

-20° 

0-927 

100° 

1 

-  10 

2-093 

111-7 

1-5 

0 

4-600 

120-6 

2 

+  5 

6-534 

127-8 

2-5 

10 

9165 

133-9 

3 

15 

12-699 

144-0 

4 

20 

17-391 

159-2 

6 

30 

31-548 

170-8 

8 

40 

54-906 

180-3 

10 

50 

91-982 

188-4 

12 

60 

148-791 

195-5 

14 

70 

233-093 

201-9 

16 

80 

354-280 

207-7 

18 

90 

525-450 

213-0 

20 

100 

760-000 

224-7 

25 

140  Water  as  a  Solvent. — Water  is  the  most  generally  valuable 
of  known  solvents.  Not  only  do  many  solids,  such  as  sugar  and 
salt,  dissolve  in  water,  but  certain  liquids,  such  as  alcohol  and 
acetic  acid,  mix  with  it  completely.  Other  liquids  again,  such 
as  ether,  dissolve  to  a  certain  extent  in  water,  although  they  do 
not  mix  with  it  in  all  proportions.  Gases  also  dissolve  in  water, 
some,  such  as  ammonia  and  hydrochloric  acid,  in  very  large 
quantities,  exceeding  more  than  one  hundred  times  the  bulk 
of  the  water ;  others,  again,  such  as  hydrogen  and  nitrogen, 


WATER  OF  CRYSTALLIZATION  279 

are  but  very  slightly   soluble,  while  carbon  dioxide  and  some 
other  gases  stand,  as  regards  solubility,  between  these  extremes. 

Concerning  the  nature  of  solution,  whether  of  solids,  liquids, 
or  gases,  we  know  at  present  but  little.  The  phenomena  of 
solution  differ,  however,  essentially  from  those  of  chemical 
combination,  inasmuch  as  in  the  former  we  have  to  do  with 
gradual  increase  up  to  a  given  limit,  termed  the  point  of 
saturation,  whereas  in  the  latter  we  observe  the  occurrence  of 
constant  proportions  in  which,  and  in  no  others,  combination 
occurs.  Solution  follows  a  law  of  continuity,  chemical  com- 
bination one  of  sudden  change  or  discontinuity. 

The  solubility  of  solids  varies  with  the  essential  nature  of  the 
solid,  with  that  of  the  liquid,  and  with  the  temperature  at 
which  they  are  brought  together ;  the  same  may  be  said  of  the 
solvent  action  of  water  upon  liquids  and  upon  gases,  except  that 
the  solubility  of  gases  is  also  influenced  by  the  pressure  to  which 
the  gas  and  the  water  are  subjected.  The  quantity  of  any  solid, 
liquid,  or  gas  which  dissolves  in  a  solvent,  such  as  water,  must 
be  ascertained  empirically  in  every  case,  as  we  are  unacquainted 
with  any  law  according  to  which  such  solvent  action  takes  place, 
and  we,  therefore,  cannot  calculate  the  amount.  The  effect  of 
change  of  temperature  on  the  solubility  on  a  substance,  whether 
solid,  liquid,  or  gaseous  must  likewise  be  determined  by  experi- 
ment, but  the  effect  of  pressure  upon  the  solubility  of  gases 
is  given  by  a  simple  law,  known  as  the  law  of  Dalton  and 
Henry  (p.  284). 

The  solubility  of  most  solids  increases  with  the  temperature, 
a  •  limit  being  reached  at  each  temperature,  beyond  which  no 
further  quantity  of  the  solid  dissolves.  When  the  temperature 
of  such  saturated  solutions  falls,  or  when  the  solvent  is  allowed  to 
evaporate,  a  portion  of  the  dissolved  substance  is  deposited  from 
solution  usually  in  the  form  of  a  solid  possessing  some  definite 
geometrical  form,  and  termed  a  crystal,  whilst  the  substance  is 
said  to  crystallize. 

Water  of  Crystallization. — Many  salts  owe  their  crystalline 
character  to  the  presence  in  a  solid  state  of  a  certain  definite 
number  of  molecules  of  water.  When  this  chemically  combined 
water  is  driven  off  by  heat,  the  crystal  falls  to  powder,  and 
hence  it  has  been  termed  water  of  crystallization.  Some  salts 
contain  a  large  quantity  of  water  definitely  combined  in  this 
form ;  thus  the  opaque  white  powder  of  anhydrous  alum, 
K2A12(SO4)4,  unites  with  no  less  than  twenty-four  molecules  of 


280  THE  NON-METALLIC  ELEMENTS 

water  to  form  the  well-known  transparent  octohedral  crystals  of 
common  alum,  K2AL,(SO4)4  +  24H2O  ;  in  like  manner  anhydrous 
and  powdery  carbonate  of  soda,  Na2CO3,  when  dissolved  in 
water  deposits  large  monosymmetric  crystals  of  common  wash- 
ing-soda, having  the  composition  Na.2CO3  +10  H2O.  The  tem- 
perature of  the  solution  from  which  such  crystals  are  deposited 
materially  affects  the  quantity  of  water  with  which  the  salt 
combines ;  thus,  in  the  case  of  carbonate  of  soda,  whilst  mono- 
symmetric  crystals  of  the  ten-molecule  hydrate  are  deposited  at 
the  ordinary  temperature,  other  crystals,  having  the  composition 
Na2CO3  +  8H2O,  or  again  others  represented  by  the  formula 
Na2CO3  +  5H2O,  are  deposited,  when  crystallization  is  allowed 
to  take  place  at  higher  temperatures. 

The  water  in  these  crystals  has  a  definite  vapour  tension  and  is, 
therefore,  given  off  when  the  temperature  becomes  so  high  that 
the  tension  of  the  water  of  crystallization  is  greater  than  that  in 
the  surrounding  atmosphere.  Different  substances  lose  their 
water  in  the  air  at  very  different  temperatures,  and  even  the 
molecules  of  water  combined  with  a  single  molecule  of  salt 
behave  differently  in  this  respect.  Thus  potash  alum  loses  ten 
molecules  of  water  at  100°,  but  it  needs  to  be  heated  to  120°  in 
order  to  drive  off  a  second  ten  molecules  of  water,  and  retains  the 
last  four  molecules  until  the  temperature  rises  to  200°.  Copper 
sulphate  in  a  similar  manner  loses  four  of  its  molecules  of  water 
below  110°,  whilst  the  fifth  is  only  driven  off  at  200°.  Sodium 
carbonate,  Na2CO3  +  10H2O,  loses  water  on  simple  exposure  to 
the  air,  the  tension  of  its  combined  water  being  greater  than  that 
of  the  aqueous  vapour  in  the  atmosphere,  and  the  salt  becomes 
covered  with  a  white  powder.  Crystals  which  behave  in  this 
manner  are  said  to  effloresce.  Many  salts  which  do  not  lose 
water  in  the  atmosphere  do  so  when  placed  in  dry  air.  Other 
solid  salts,  such  as  calcium  chloride,  and  potassium  acetate, 
combine  with  water  with  such  avidity  that  when  left  exposed 
to  the  air  they  begin  to  liquefy  from  absorption  of  the  atmo- 
spheric moisture  ;  the  salts  are  then  said  to  deliquesce. 

In  the  year  1840  Dalton  observed  that  different  salts,  whose 
water  of  crystallization  has  been  driven  off  by  heat,  dissolve  in 
water  without  increasing  the  volume  of  the  liquid,  whereas  if 
the  hydrated  salt  is  dissolved,  an  increase  of  volume  occurs  which 
is  exactly  that  due  to  the  water  which  is  combined  in  the  salt. 
Playfair  and  Joule l  extended  these  observations,  showing 

1  Chem  Soc.  Mem.  2,  477  ;  3,  54,  199  ;  Chem.  Soc.  Quart.  Journ.  1,  121. 


CRYOHYDRATES  281 


for  instance,  that  in  the  case  of  carbonate  of  soda,  crystallizing 
with  ten  molecules  of  water,  and  in  that  of  the  phosphates  and 
arsenates  crystallizing  with  twelve  molecules,  the  volume  of  the 
whole  molecule  of  hydrated  salt  is  the  same  as  that  of  its  water 
of  crystallization  would  be  if  frozen  to  ice.  The  particles  of 
anhydrous  salt  would  hence  appear  to  occupy  the  spaces  inter- 
vening between  those  of  the  water  without  increasing  its  volume. 
Thus  the  crystals  of  common  washing  soda  have  the  following 
composition : 

Na2CO3 =  104-29 

10H26 =  178-80 

283-09 

and  283'09  grams,  of  these  crystals  occupy  exactly  the  space  of 
178'8  grams,  of  ice. 

The  following  table  gives  the  specific  gravities  of  the  above 
mentioned  salts,  first  as  observed  by  experiment,  and  secondly 
as  calculated  upon  the  above  hypothesis,  and  shows  the  close 

agreement  of  the  two  sets  of  numbers. 

Specific  Gravity. 

Observed.  Calculated. 

Sodium  Carbonate  .     .     .     .     Na2  C03  +  10  H20      .     .     1'454     .  .     1-463 

Hydrogen  Sodium  Phosphate     Na2  HP04  +  12  H20  .     .     1'525     .  .     1'527 

Normal  Sodium  Phosphate   .     Na3  P04  +  12  H20     .     .     1'622     .  .     1'622 

Hydrogen  Sodium  Arsenate  .     Na2  H  As04  +  12  H20     .     1736     .  .     1736 

Normal  Sodium  Arsenate      .     Na3As04  +  12  H20     .     .     1'804     .  .     1*834 

In  the  case  of  certain  other  salts  the  volume  of  the  crystal 
was  found  to  be  equal  to  the  sum  of  the  volume  of  the  water 
when  frozen  and  that  of  the  anhydrous  salt. 

141  Cryohydrates. — It  has  been  already  mentioned  that  when 
a  dilute  solution  is  cooled,  ice  separates  out  (p.  11 4).  If  the  ice  be 
removed  and  the  cooling  continued,  a  temperature  is  at  length 
reached  at  which  the  whole  solution  becomes  solid.  Thus 
Guthrie,1  who  has  investigated  this  subject,  found  that  when  a 
dilute  solution  of  common  salt  is  cooled  down  to  —  1°'5,  ice 
begins  to  separate  out,  and  this  formation  of  ice  continues  until 
the  temperature  sinks  to  —  23°,  at  which  the  whole  mass 
becomes  solid.  A  concentrated  salt  solution,  on  the  other  hand, 
deposits  at  —  7°  crystals  having  the  composition  NaCl  +  2H2O, 
and  the  separation  of  this  compound,  in  the  form  of  iridescent 
scales,  also  goes  on  until  the  liquid  has  cooled  down  to  —  23° 
when,  as  before,  it  freezes  en  masse. 

1  Phil.  Mag.  [4],  49,  1,  206,  266. 


282  THE  NON-METALLIC  ELEMENTS 

The  solid  mass  thus  formed  has  been  called  a  cryohydrate, 
and  resembles  a  chemical  compound  inasmuch  as  it  has  a 
definite  composition  and  a  definite  melting-point.  It  has, 
however,  been  shown  that  these  so-called  cryohydrates  are 
simply  mixtures  of  ice  and  a  solid  salt,  and  that  their  properties 
are  in  all  cases  the  mean  of  those  of  their  constituents.1 

142  Freezing  Mixtures. — The  solution  of  a  solid  in  water  is 
generally  accompanied  by  a  lowering  of  temperature,  caused  by 
the  conversion  of  sensible  into  latent  heat  by  the  liquefaction 
of  the  solid.  In  the  case,  however,  of 'many  anhydrous  salts, 
solution  is  accompanied  by  a  rise  in  temperature,  which  may 
possibly  be  caused  by  the  production  of  a  definite  chemical 
compound  between  the  solid  and  the  solvent.  By  the  solution 
of  many  salts  such  a  diminution  of  temperature  is  effected  that 
this  process  may  be  used  for  obtaining  ice  ;  thus  when  500 
grams,  of  potassium  thiocyanate  are  dissolved  in  400  grams, 
of  cold  water,  the  temperature  of  the  solution  sinks  to  —  20°. 
When  common  salt  is  mixed  with  snow  or  pounded  ice  a  con- 
siderable reduction  of  the  temperature  of  the  mass  occurs,  the 
two  solid  bodies  becoming  liquid  and  forming  a  concentrated 
brine  whose  freezing-point  lies  at  —  23°.  This  solution  contains 
thirty-two  parts  by  weight  of  salt  to  100  parts  of  water,  and  in 
order  to  bring  about  the  greatest  possible  reduction  in  tempera- 
ture the  salt  and  snow  must  be  mixed  in  the  above  proportions. 
Equal  weights  of  crystallized  calcium  chloride  and  snow  when 
mixed  together  give  a  freezing  mixture  whose  temperature  sinks 
from  0°  to  -  45°. 

The  temperatures  produced  in  this  way  are  those  at  which 
the  so-called  cryohydrates  are  formed,  since  these  are  the  lowest 
temperatures  at  which  the  mass  can  remain  liquid. 

The  Properties  of  Solutions. — Concerning  the  relation  between 
the  dissolved  substance  and  the  solvent  in  a  concentrated  solu- 
tion, but  little  is  known.  Many  such  solutions,  when  cooled, 
deposit  crystals  which  contain  a  portion  of  the  solvent  combined 
with  the  substance  which  has  been  dissolved.  It  has,  therefore, 
been  supposed  that  the  solutions  in  these  cases  actually  contain 
these  compounds,  and  this  seems  to  be  probable  at  all  events  at 
low  temperatures.  On  the  other  hand,  it  has  been  held  that 
no  compounds  are  present  in  the  solutions,  but  that  the  com- 
bination takes  place  at  the  moment  of  separation  of  the  solid 
substance. 

1  Offer,   Wien.  Akad.  Ber.  81,  ii.  1058. 


PROPERTIES  OF  SOLUTIONS  283 

The  properties  of  dilute  solutions  can  be  to  a  large  extent 
explained,  as  already  described  (p.  Ill),  on  the  supposition  that 
the  dissolved  substance  is  not  combined  with  the  solvent. 

In  the  case  of  solutions  of  salts  (electrolytes)  in  water  a 
special  hypothesis,  that  of  electrolytic  dissociation,  has  been 
introduced,  according  to  which  the  salt  is  to  a  large  extent  split 
up  in  the  solution  into  its  electrolytic  ions.  This  view  is 
rendered  probable  not  only  by  the  facts  already  adduced  (p.  116), 
but  by  the  circumstance  that  almost  all  the  properties  of  such 
solutions  have  been  shown  to  be  equal  to  the  sum  of  the 
properties  of  the  ions.  Each  ion  has  a  definite  weight,  volume, 
colour,  &c.,  and  the  effect  produced  by  a  salt  is  found  to  be 
equal  to  the  sum  of  the  effects  which  would  be  independently 
produced  by  the  ions  into  which  it  is  capable  of  being  decom- 
posed. This  has  already  been  explained  for  the  osmotic 
pressure,  freezing-point  and  vapour  pressure  of  solutions,  and 
also  holds  for  the  colour,  specific  gravity,  refractive  index,  &c. 

This  theory  is,  therefore,  in  accordance  with  the  physical 
properties  of  such  solutions,  and  it  moreover  throws  great  light 
upon  many  chemical  reactions.  Chemical  action  occurring  in 
dilute  solution,  according  to  this  view,  takes  place  between  the 
ions  of  the  substances  concerned,  so  that  the  tests  usually 
applied  for  the  metals  and  acids  are  in  reality  tests  for  the 
corresponding  ions.  Thus,  for  example,  the  tests  for  ferrous 
salts  fail  to  detect  iron  in  potassium  ferrocyanide,  K4FeC6N6, 
although  they  are  given  by  ferrous  sulphate,  FeSO4.  This  is 
due  to  the  fact  that  in  the  latter  case  the  ion  of  ferrous  iron  is 
present  in  the  solution,  whilst  in  the  former  the  ions  are  K  and 
the  complex  group  FeC6N6. 

The  fact  that  the  same  amount  of  heat  is  evolved  by  the 
neutralisation  of  any  of  the  strong  acids  by  any  of  the  strong 
bases  also  receives  a  new  and  interesting  interpretation  in  the 
light  of  this  theory.  Taking  the  case  of  the  action  of  hydro- 
chloric acid  on  caustic  soda,  which  is  usually  represented  by  the 
equation, 

NaHO  +  HC1  =  NaCl  +  H2O, 

we  see  that  the  action  takes  place  between  the  ions  in  the 
following  way : 

Na  +  HO  +  H  +  Cl  =  Na  +  Cl  +  H2O. 


284  THE  NON-METALLIC  ELEMENTS 

The  result  of  the  reaction  is  simply  the  formation  of  water, 
since  the  Na  and  Cl  ions  remain  dissociated.  Exactly  the  same 
reaction  takes  place  if  another  acid  and  a  different  base  be 
employed : 

+  -  +  -  + 

K  +  HO  +  H  +  N03  =  K  +  N03  +  H2O. 

The  amount  of  heat  evolved  is,  therefore,  also  the  same. 

143  Absorption  of  Gases  by  Water. — All  gases  are  soluble  to  a 
greater  or  less  degree  in  water,  the  extent  of  this  solubility 
depending  upon  (1)  the  nature  of  the  gas,  (2)  the  temperature  of 
the  gas  and  water,  (3)  the  pressure  under  which  the  absorption 
occurs.  No  simple  law  is  known  expressing  the  relation  between 
the  amount  of  gas  absorbed  and  the  temperature.  Usually  the 
solubility  of  a  gas  diminishes  as  the  temperature  increases,  but 
the  rate  of  diminution  varies  with  each  gas,  so  that  the  amount 
of  gas  dissolved  in  water  at  a  given  temperature  can  be  ascer- 
tained only  by  experiment.  A  simple  relation  has  however 
been  found  to  exist  between  the  quantity  of  gas  absorbed  under 
varying  conditions  of  pressure,  the  temperature  remaining 
constant. 

In  the  year  1803  William  Henry  *  proved  that  the  amount  of 
gas  absorbed  by  water  varies  directly  as  the  pressure,  or,  in  the 
words  of  the  discoverer  of  the  law,  "  under  equal  circumstances 
of  temperature,  water  takes  up  in  all  cases  the  same  volume  of 
condensed  gas  as  of  gas  under  ordinary  pressure.  But  as  the 
spaces  occupied  by  every  gas  are  inversely  as  the  compressing 
force,  it  follows  that  water  takes  up  of  gas  condensed  by  one, 
two,  or  more  additional  atmospheres,  a  quantity  which  ordinarily 
compressed,  would  be  equal  to  twice,  thrice,  and  so  on,  the 
volume  absorbed  under  the  common  pressure  of  the  atmosphere." 
Two  years  after  Henry  had  enunciated  this  law,  Dalton2 
extended  the  law  to  the  case  of  mixed  gases,  proving  that  when 
a  mixture  of  two  or  more  gases  in  given  proportions  is  shaken 
up  with  water,  the  volume  of  the  gas  having  a  finite  relation  to 
that  of  the  liquid,  the  absorptiometric  equilibrium  occurs  when 
the  pressure  of  each  gas  dissolved  in  the  liquid  is  equal  to  that 
of  the  portion  of  the  gas  which  remains  unabsorbed  by  the 
liquid ;  the  amount  of  each  gas  absorbed  by  water  from  such 
a  mixture  being  solely  dependent  on  the  pressure  exerted  by  the 
particular  gas.  This  law,  termed  Dalton 's  law  of  partial  pressures. 
1  Phil.  Trans.  1803,  29,  274.  2  Mane.  Memoirs,  1805. 


SOLUTION  OF  GASES  IN  WATER  285 

may  be  illustrated  by  the  following  example  :  if  two  or  more  gases 
which  do  not  act  chemically  upon  each  other  be  mixed  together, 
and  the  mixture  of  gases  brought  into  contact  with  water  until 
the  absorptiometric  equilibrium  is  established,  the  quantity  of 
each  gas  which  dissolves  is  exactly  what  it  would  have  been  if 
only  the  one  gas  had  been  present  in  the  space.  Thus,  for 
instance,  the  absorption  co-efficient  of  oxygen  at  0°  is  0*04890, 
that  of  nitrogen  at  the  same  temperature  being  0*023481.  Now 
100  volumes  of  air  contain  on  an  average  79*04  volumes  of 
nitrogen  and  20'96  volumes  of  oxygen,  hence  the  partial  pressure 
of  the  oxygen  is  0*2096  of  an  atmosphere,  whilst  that  of  the 
nitrogen  is  07904,  and  as  the  solubility  of  each  gas  is  propor- 
tioned to  its  partial  pressure, 

0*2096  X  0*04890  =  0*010244 
will  be  the  proportion  of  oxygen  dissolved,  and 
0*7904  x  0*023481  =  0*018559 

will  be  the  proportion  of  the  nitrogen  dissolved,  or  the  per- 
centage composition  of  the  air  dissolved  in  water  will  be  : — 

Calculated.  Found. 

Nitrogen  .    .    .     64*4    .    .    .    64*9 
Oxygen     .    .    .     35*6     .    .    .     35*1 

100*0  lOO'O 

Thus  the  relation  between  the  dissolved  gases  as  found  by  ex- 
periment agrees  closely  with  that  calculated  on  the  above 
assumption,  and  the  law  of  partial  pressures  is  verified. 

Every  absorbed  gas  which  follows  the  law  of  pressure  will  of 
course  be  driven  out  of  solution  when  the  pressure  on  the  gas  is 
reduced  to  zero.  This  can  be  effected  by  removing  the  super- 
incumbent pressure  by  means  of  an  air-pump,  by  allowing 
the  liquid  to  come  in  contact  with  a  very  large  volume  of  some 
other  indifferent  gas,  or,  lastly,  by  boiling  the  liquid  when 
all  the  dissolved  gas  will  be  driven  off  with  the  issuing  steam, 
except  where  a  chemical  combination  or  attraction  exists  between 
the  gas  and  the  water. 

Some  gases  dissolve  in  water  in  very  large  quantities,  whereas 
<others  are  only  slightly  soluble.  Two  distinct  methods  of 


286  THE  NON-METALLIC  ELEMENTS 

experimentation  are  needed  for  ascertaining  the  co-efficients  of 
solubility  of  these  two  classes.  On  the  one  hand,  the  amount 
of  the  absorbed  gas  is  determined  chemically ;  on  the  other 
the  volume  of  gas  absorbed  by  a  known  volume  of  water  is 
ascertained  either  by  measuring  the  diminution  in  bulk  of  a 
given  volume  of  the  gas  when  agitated  with  water,  or  by 
saturating  water  with  the  gas  and  driving  out  the  latter  by 
heat  and  then  measuring  it.  The  first  of  these  methods  was 
that  adopted  by  Bunsen l  to  whom  we  are  indebted  for  the  first 
exact  and  extended  experimental  investigation  of  this  subject. 
Among  the  gases  whose  solubility  has  been  determined  by 
chemical  methods  are  oxygen,  sulphuretted  hydrogen,  sulphur 
dioxide,  ammonia,  carbon  dioxide,  hydrochloric  acid,  and 
chlorine.  These  gases,  evolved  in  a  state  of  purity,  were 
passed  for  a  long  time  through  a  large  volume  of  water,  which 
had  been  freed  from  air  by  continued  boiling,  and  was  kept  at 
a  constant  temperature  during  the  experiment.  After  the  gas 
had  passed  so  long  through  the  water  that  the  latter  was  com- 
pletely saturated,  the  barometric  pressure  was  read  off,  and  a 
known  volume  of  the  water  withdrawn,  special  precautions  to 
avoid  possible  loss  of  the  gas  being  observed.  The  gas  con- 
tained in  this  liquid  was  then  quantitatively  determined  either 
by  means  of  volumetric  analysis,  or  by  the  ordinary  processes  of 
analytical  chemistry.  If  the  volume  of  the  liquid  does  not 
undergo  any  appreciable  alteration  in  bulk  owing  to  the  ab- 
sorption of  the  gas,  we  are  easily  able  to  calculate  the  co-efficients 
of  absorption  from  the  data  obtained  by  this  process.  If, 
however,  the  volume  of  the  saturated  liquid  is  considerably 
larger,  as  is  usually  the  case,  than  that  of  the  liquid  before 
saturation,  either  the  specific  gravity  of  the  saturated  liquid 
must  be  ascertained,  or  only  a  small  volume  of  water  must  be 
saturated,  and  the  absolute  quantity  of  absorbed  gas  ascertained 
by  weighing  before  and  after  the  experiment. 

144  Bunsen's  Absorptiometer,  as  shown  in  Fig.  89 2  consists 
essentially  of  two  parts  :  (1)  a  eudiometric  tube,  e,.  in  which 
a  measured  volume  of  the  gas  to  be  experimented  upon  is 
brought  in  contact  with  a  given  volume  of  water ;  (2)  an  outer 
vessel,  consisting  of  a  glass  cylinder,  fitting  at  the  lower  end 
into  a  wooden  stand,  f,  and  having  a  water-tight  lid  at  the 
upper  end.  The  eudiometer  tube,  which  is  divided  and  accu- 

1  Gfasometry,  p.  129,  or  Watts's  Dictionary  (1st  Edition),  article  "Gases,  Absorp- 
tion of  by  Liquids."  2  Bumen's  Gasom.  43  and  44. 


SOLUTION  OF  GASES  IN  WATER 


287 


rately  calibrated,  is  partially  filled  with  the  given  gas  in  the 
usual  way  over  a  mercurial  trough,  and  the  volume  of  this  gas 


FIG.  89. 


read  off  with  all  due  precautions ;  a  measured  volume  of  water 
perfectly  free  from  air  is  next  admitted  under  the  mercury  into 


288  THE  NON-METALLIC  ELEMENTS 

the  tube,  and  the  open  end  of  the  tube  then  closed  by  screwing 
it  tightly  against  the  caoutchouc  plate  of  the  small  iron  foot,  a, 
fixed  on  to  its  lower  end,  as  shown  in  Fig.  90.  The  tube  by 
this  means  can  be  removed  from  the  mercurial  trough,  without 
any  danger  of  losing  gas  or  water,  and  placed  in  the  glass  cylin- 
der, which  contains  mercury  a  in  its  lower  part,  and  water 
above,  in  which  it  can  be  safely  shaken  to  ensure  the  establish- 
ment of  the  proper  absorptiometric  equilibrium  between  gas  and 
water.  The  pressure  in  the  tube  can  be  readily  adjusted  from 
time  to  time  by  unscrewing  the  open  ends  of  the  tube  from  the 
caoutchouc  plate,  and  thus  placing  the  mercury  inside  in  con- 
nection with  that  outside  the  tube.  The  heights  of  the  two 
levels  of  mercury  and  the  level  of  the  water  in  the  tube,  as  well 
as  the  temperature  (indicated  by  the  thermometer  &),  can  then 
be  read  off  through  the  glass  cylinder,  and  thus  all  the  data 
are  obtained  for  ascertaining  exactly  the  volume  of  gas  absorbed 
by  a  given  volume  of  water  under  given  conditions  of  tempera- 
ture and  pressure. 

Still  more  accurate  results  have  been  obtained  by  the  use 
of  various  modified  forms  of  this  apparatus,  in  which  larger 
volumes  of  water  are  employed.1 

145  The  truth  of  the  law  of  Dalton  for  pressures  not  greatly 
higher  than  that  of  the  atmosphere  has  been  experimentally  tested 
by  Bunsen,2  who  showed  that  the  results  of  an  absorptiometric 
analysis  of  a  gaseous  mixture — that  is,  of  an  experimental  deter- 
mination of  its  solubility  in  water,  from  which  the  composition  of 
the  orginal  gaseous  mixture  is  calculated,  on  the  supposition  that 
the  law  of  partial  pressures  holds  good — agrees  exactly  with  a 
direct  eudiometric  analysis  of  the  same  mixture.  Thus  it  has 
been  shown  by  the  same  chemist  that  in  mixtures  of  carbon 
dioxide  and  carbon  monoxide,  of  carbon  monoxide  and  marsh 
gas,  of  carbon  dioxide  and  hydrogen,  the  component  gases  are 
absorbed  in  quantities  exactly  corresponding  to  Dalton's  law. 

The  limits  of  pressure  beyond  which  gases  do  not  follow  the 
law  of  pressures  have  not  as  yet  been  experimentally  ascertained 
in  many  cases ;  but,  at  any  rate  in  the  case  of  the  more  soluble 
gases,  the  limits  are  reached  within  ranges  of  pressure  varying 
from  0  to  2  atmospheres.  That  under  high  pressures  deviations 
from  the  law  must  in  many  cases  occur  is  clear,  inasmuch  as  gases 
do  not  conform  to  the  law  of  Boyle  under  greatly  increased 

1  Winkler,  L.  W.  Ber.  24,  89  ;  Timofejew,  Zeit.  Phys.  Chem.  6,  141. 

2  Gasometry,  p.  124. 


NATURAL  WATERS  289 


pressure.  So  too,  it  is  found  that  certain  gases  which  follow 
the  law  of  absorption  at  one  temperature  do  not  conform  at 
another  ;  thus,  for  instance,  ammonia  dissolves  in  water  at  100° 
under  high  pressures,  in  quantities  exactly  proportional  to  the 
pressure,  although  at  lower  temperatures  this  is  not  the  case.1 

Instances  also  occur  in  which  certain  gases,  although  agreeing 
with  the  law  of  pressures  when  in  the  pure  state,  do  not  follow 
it  when  mixed  together  with  other  gases.  Thus  in  mixtures  of 
equal  volumes  of  chlorine  and  hydrogen,  and  mixtures  of  vary- 
ing proportions  of  chlorine  and  carbon  dioxide,  the  carbon 
dioxide  and  hydrogen  do  not  dissolve  in  water  in  quantities 
proportional  to  their  partial  pressures,  although  they  both  follow 
the  law  when  unmixed  with  other  gases.2 

Amongst  the  various  applications  of  the  laws  of  the  absorption 
of  gases  in  water  none  is  more  interesting  than  the  process  pro- 
posed by  Mallet  for  solving  the  difficult  problem  of  separating 
the  atmospheric  oxygen  from  the  nitrogen.  We  have  already 
seen  that  the  percentage  of  oxygen  contained  in  the  air  is  20 '9, 
whereas  the  mixture  of  oxygen  and  nitrogen  dissolved  in  water 
contains  35 '1  per  cent,  of  the  former  gas.  If  the  gas  thus  dis- 
solved be  driven  off  by  boiling,  and  then  this  again  shaken  up 
with  water,  the  dissolved  gases  will  possess  about  the  following 
percentage  composition : — 

After  the  second  absorption. 

Nitrogen 52%5 

Oxygen 47*5 


100-0 

This  again  set  free,  and  again  shaken  up  with  water,  yields  a 
gaseous  mixture,  containing  75  per  cent,  of  oxygen.  Continuing 
this  process  of  alternately  absorbing  and  liberating  the  mixture 
of  gases,  the  percentage  of  oxygen  regularly  rises,  until  after  the 
8th  absorption  the  gas  contains  97*3  per  cent.,  or  is  nearly  pure 
oxygen  gas. 

NATURAL  WATERS. 

146  None  of  the  various  forms  of  water  met  with  in  nature 
are  free  from  certain  impurities.  These  may  be  of  two  kinds ; 
(1)  Mechanically  suspended  impurities ;  (2)  Soluble  impurities. 
The  first  can  be  separated  either  by  subsidence  or  by  mechanical 

1  Roscoe,  Journ.  Chem.  Soc.  1856,  14.  2  Sims,  Journ.  Uhem.  Soc.  1862,  1. 


290  THE  NON-METALLIC  ELEMENTS 

filtration,  the  latter  cannot  be  thus  got  rid  of  from  the  water,  but 
must  be  separated  by  distillation  or  by  some  chemical  reaction. 

Even  rain-  or  snow-water  collected  in  clean  vessels  contains 
in  addition  to  the  dissolved  atmospheric  gases  traces  of  foreign 
bodies  which  are  contained  in  the  air  either  as  dust  or  vapour, 
and  no  sooner  does  rain-water  touch  the  earth  than  it  at  once 
takes  up  into  solution  certain  soluble  constituents  of  the  portion 
of  the  earth's  crust  through  which  it  percolates,  thus  gradually 
becoming  more  and  more  impure  until  it  again  reaches  the 
ocean  from  which  it  had  its  origin. 

147  Purification  of  Water. — The  separation  of  suspended 
matter  is  effected  on  the  small  scale  for  laboratory  purposes  by 
filtration  through  porous  paper  placed  in  glass  funnels,  and  on 
the  large  scale  by  employing  filtering  beds  of  sand  and  gravel. 
In  order  to  separate  suspended  matter  from  water  used  for 
drinking  purposes  it  is  usual  to  filter  it  through  a  layer  of  wood 
charcoal,  which  not  only  holds  back  the  solid  matter  but  also 
acts  in  other  ways,  as  we  shall  hereafter  learn,  in  improving  the 
character  of  the  water. 

The  soluble  constituents  may  be  distinguished  as  (1)  fixed, 
and  (2)  volatile  constituents,  and  water  can  be  obtained  free 
from  the  first  of  these  by  the  process  of  distillation,  whilst  the 
latter  may  come  over  with  the  steam,  and,  therefore,  require  the 
employment  of  other  means.  In  order  to  obtain  pure  distilled 
water,  spring  or  rain-water  is  boiled  in  a  vessel  termed  a  still, 
(B)  Fig.  91  so  arranged  that  the  escaping  steam  is  condensed 
by  passing  through  a  cooled  worm  or  tube  made  of  block  tin, 
platinum,  or  silver,  but  not  of  glass,  for  if  this  substance  be 
used  a  trace  of  its  more  soluble  constituents,  the  alkaline 
silicates,  is  always  dissolved.  This  process  frees  the  water  from 
all  non-volatile  impurities,  provided  care  has  been  taken  to 
prevent  any  mechanical  spirting  of  the  liquid,  but  substances 
which  are  volatile  will  still  be  found  in  the  distillate.  Thus 
ordinary  distilled  water  invariably  contains  ammonia,  as  may 
easily  be  proved  by  adding  a  few  drops  of  Nesslers  reagent. 
This  consists  of  an  alkaline  solution  of  mercuric  iodide  in 
potassium  iodide.  If  a  few  drops  of  this  reagent  be  added  to 
about  100  cc.  of  ordinary  distilled  water  contained  in  a  cylin- 
drical glass  standing  on  a  white  plate,  the  water  will  be  seen  ta 
attain  a  distinct  yellowish  tint  if  small  amounts  of  ammonia  or 
ammoniacal  salts  be  present,  whilst  if  larger  quantities  of 
ammonia  be  present  a  brown  precipitate  will  be  formed. 


GASES  IN  WATER 


291 


In  order  completely  to  free  distilled  water  from  volatile  nitro- 
genous organic  bodies  which  it  is  likewise  apt  to  contain,  it  is 
necessary  to  re-distil  it  after  solutions  of  potassium  perman- 
ganate and  caustic  potash  have  been  added.  These  substances 
oxidize  the  organic  matter  with  formation  of  ammonia,  and 
after  about  one-twentieth  of  the  water  has  come  over,  the 
distillate  is  usually  found  to  be  free  from  ammonia,  and  to  leave 
no  residue  on  evaporation.  If  ammonia  can  be  still  detected 
the  water  must  again  be  distilled  with  the  addition  of  a  small 
quantity  of  acid  sulphate  of  potash  which  fixes  the  ammonia.1 

148  Gases  Dissolved  in  Water. — All  water  contains  in  solution 
the  gases  of  the  atmosphere,  oxygen,  nitrogen,  and  carbon 


FIG.  91. 


dioxide.  In  order  to  obtain  water  free  from  these  dissolved 
gases  the  water  is  well  boiled  and  the  glass  vessel  in  which  it  is 
boiled  is  then  sealed  hermetically.  The  arrangement  Fig.  92 
shows  how  this  may  be  conveniently  accomplished.  After  the 
water  has  been  quickly  boiling  for  half  an  hour  the  caoutchouc 
tube  (a)  from  which  the  steam  issues  is  closed  by  a  clamp,  the 
lamp  is  removed,  and  the  drawn  out  neck  of  the  flask  melted 
off  before  the  blow-pipe  at  (6). 

Even  when  boiled  for  many  hours  a  small  residue  of  nitrogen 
gas  is  left  behind,  and  on  condensing,  the  steam  coming  off 
from  such  water  leaves  a  minute  bubble  of  nitrogen  so  that  it 

1  Stas,  Eecherches,  p.  109. 


292  THE  NON-METALLIC  ELEMENTS 

appears  impossible  to  obtain  water  quite  free  from  nitrogen. 
All  water  which  is  exposed  to  the  air  dissolves  a  certain  quantity 
of  oxygen  and  nitrogen,  a  quantity  which  is  determined  by  the 
laws  of  gas  absorption.  It  is  indeed  upon  this  dissolved  oxygen 
that  the  life  of  water-breathing  animals  depends.  In  every 
pure  water  the  proportion  between  the  dissolved  nitrogen  and 
oxygen  is  found  to  be  constant,  and  it  is  represented  by  the 
following  numbers  : — 

Percentage  Composition  of  Air  Dissolved  in   Water. 

Oxygen     .     .     .     .     35 '1 
Nitrogen  .... 


FIG.  92. 

1,000  cc.  of  pure  water,  such  as  rain-water,  when  saturated,  dis- 
solves 17'95  cc.  of  air.  If  the  water  is  rendered  impure  by  the 
introduction  of  organic  matter  undergoing  oxidation,  the  pro- 
portion between  the  dissolved  oxygen  and  nitrogen  becomes 
different  owing  to  the  oxygen  having  been  partly  or  wholly  used 
for  the  oxidation  of  this  material.  This  is  clearly  shown  in  the 

1  Grove,  Journ.  Chem.  Soc.  1863,  263. 


GASES  IN  WATER 


293 


following  analyses,  made  by  Miller,  of  the  dissolved  gases  con- 
tained in  Thames  water  collected  at  various  points  above  and 
below  London. 


— 

Thames  Water  taken  at 

Kings- 
ton. 

Ham-     Somerset 
mersmith    House. 

i 

Green- 
wich. 

Wool- 
wich. 

Erith. 

Total  volume  of 
gas  per  litre  . 

cc. 

527 

cc. 

cc. 

62-9 

cc. 

71-25 

cc. 

63-05 

cc. 

74-3 

Carbon  dioxide  . 
Oxygen    .     .     . 
Nitrogen  .     .     . 

Ratio     of    oxy- 
gen to  nitrogen 

30-3 
7"4 
150 

4-1 
lo'l 

45-2 
1-5 
16-2 

55-6 
0-25 
15-4 

48-3 
0-25 
14-5 

57 
1-8 
15-5 

1  :  8-1 

1:2 

1:37 

1:10-5 

1  :60 

1  :52 

This  table  shows  that  whereas  the  pure  water  at  Kingston 
contains  the  normal  quantity  of  dissolved  oxygen,  the  ratio  of 
oxygen  to  nitrogen  decreases  at  a  very  rapid  rate  as  the  river 
water  becomes  contaminated  with  London  sewage,  but  that 
this  ratio  again  shows  signs  of  a  return  to  the  normal  at  Erith. 

Hence  it  is  clear  that  an  analysis  of  the  gases  dissolved  in 
water  may  prove  of  some  help  in  ascertaining  whether  the  water 
is  pure,  or  whether  it  has  been  contaminated  with  putrescent 
organic  matter.  Indeed  Miller  concludes  that  whenever  the 
proportion  between  dissolved  oxygen  and  nitrogen  falls  to  less 
than  1  to  2  the  water  is  unfit  for  drinking  purposes.  It  is, 
however,  found  that  oxygen  is  almost  entirely  absent  from 
certain  deep  spring  waters  of  great  purity,  although  they 
contain  their  full  complement  of  nitrogen. 

In  order  to  collect  the  gases  dissolved  in  water,  it  is  only 
necessary  to  boil  the  water  and  to  collect  in  a  suitable  measur- 
ing apparatus  the  gases  which  thus  become  free.  A  simple  form 
of  apparatus  used  for  this  purpose  is  shown  in  Fig.  93.  It  con- 
sists of  a  globular  flask,  capable  of  holding  from  500  to  1,000 
cc.  of  water.  This  flask,  connected  with  a  bulb  and  long  tube 
by  a  strong  piece  of  caoutchouc  tubing,  is  filled  with  the  water, 
the  tubing  being  closed  by  a  screw-clamp.  The  bulb,  also  con- 
taining water,  is  next  heated  so  as  to  make  the  water  boil 


294 


THE  NON-METALLIC  ELEMENTS 


briskly,  and  thus  the  air  contained  in  the  bulb  and  tube  is 
driven  out  at  the  open  end  of  the  tube  which  dips  under 
mercury.  As  soon  as  all  the  air  is  driven  out,  the  screw-clamp 
is  opened,  and  heat  applied  to  the  flask  until  the  water  boils, 
which  under  the  diminished  pressure  it  will  soon  do.  The  dis- 
solved gases  then  begin  to  come  off,  and  are  collected  and 
measured  in  the  eudiometer  filled  with  mercury,  the  operation 
being  continued  for  not  less  than  an  hour,  until  the  last  trace 


FIG.  93. 


of  air  has  been  expelled.  A  Geissler's  mercury  pump  may  also 
be  employed  for  this  same  purpose,  and  the  apparatus  thus 
modified  has  been  used  for  determining  the  amount  of  dissolved 
oxygen  in  water.1 

The  amount  of  oxygen  dissolved  in  water  may  also  be  deter- 
mined chemically  by  several  methods,2  the  best  of  which  depends 

1  Koscoe  and  Lunt,  Journ.  Chem.  Soc.  1889,  i.  563. 

2  Roscoe  and  Lunt,  Journ.   Chem.   Soc.   1889,   i.  565  ;  Thresh,  Journ.  Chem. 
Soc.  1890,  i.  185. 


RAIN-WATER  295 


upon  a  measurement  of  the  amount  of  sodium  hyposulphite 
which  can  be  oxidised  by  a  given  volume  of  the  water 
(Schiitzenberger). 

The  several  kinds  of  naturally  occurring  waters  may  be  classed 
as  rain-water,  spring-water,  river-water,  and  sea-water. 

149  Rain-  Water. — Although  this  is  the  purest  form  of  natural 
water  inasmuch  as  it  has  not  come  into  contact  with  the  solid 
crust  of  the  earth,  it  still  contains  certain  impurities  which  are 
washed  out  by  it  from  the  atmosphere.  Thus  rain-water 
invariably  contains  ammoniacal  salts,  chloride  of  sodium,  and 
organic  matter  of  various  kinds  in  the  state  of  minute  suspended 
particles  which  we  see  when  a  glass  full  of  such  water  is  held 
up  to  the  light.  The  amount  of  the  constituents  thus  taken 
out  of  the  air  by  the  falling  rain  may  serve  as  a  means  of 
ascertaining  the  chemical  climate  of  the  locality,  that  is  the 
amount  of  those  varying  chemical  constituents  of  the  atmo- 
sphere which  are  brought  in  by  local  causes.  Thus,  for  instance, 
the  rain  collected  in  towns  where  much  coal  is  burnt  is  generally 
found  to  have  an  acid  reaction,  owing  to  the  presence  of  free 
sulphuric  acid  derived  from  the  oxidation  of  the  sulphur  con- 
tained in  the  pyrites  present  in  most  coal.  The  amount  of  this 
acid  may  reach,  under  certain  circumstances,  as  much  as  7 
grains  per  gallon.  In  towns,  the  rain-water  also  contains  a 
larger  proportion  of  ammoniacal  salts  and  nitrates  than  that 
falling  in  the  country,  whilst  it  is  also  found  to  hold  in  suspen- 
sion or  solution  albuminous  matter  derived  from  decomposing 
animal  substances.  An  elaborate  examination  of  the  chemical 
composition  of  a  large  number  of  samples  of  rain-water  has 
been  made  by  Dr.  Angus  Smith,1  and  in  this  work  will  be  found, 
not  only  a  valuable  series  of  original  determinations  of  the 
constituents  of  rain-water  collected  in  various  parts  of  the 
country,  but  a  statement  of  the  results  of  the  labours  of 
other  chemists  on  the  same  subject. 

According  to  the  experiments  of  Lawes  and  Gilbert,2 
the  average  amount  of  nitrogen  contained  in  country  rain- 
water as  ammonia,  organic  nitrogen,  nitrous  and  nitric  acids, 
is  about  07  parts  in  a  million  of  rain-water,  whilst  the 
rain  of  London  (Hyde  Park)  contains  2*2  parts  per  million. 
Boussingault,  on  the  other  hand,  found  in  the  rain  of  Paris 

1  On  Air  and  Rain ;  the  Beginnings  of  a  Chemical  Climatology.  Longmans, 
1872. 

3  Sixth  Report  of  the  River  Commissioners,  1874. 


296  THE  NON-METALLIC  ELEMENTS 


4  parts  of  ammonia  in  one  million,  and  of  nitric  acid  0'2  in  a 
million. 

Spring-  Water. — The  water  flowing  from  springs,  whether  they 
are  surface-  or  deep-springs,  is  always  more  impure  than  rain- 
water owing  to  the  solution  of  certain  portions  of  the  earth's 
crust,  through  which  the  water  has  percolated.  The  nature  arid 
amount  of  the  material  taken  up  by  the  water  must  of  course 
change  with  the  nature  of  the  strata  through  which  it  passes, 
and  we  accordingly  find  that  the  soluble  constituents  of  spring- 
water  vary  most  widely,  some  spring- waters  containing  only  a 
trace  of  soluble  ingredients,  whilst  others  are  highly  charged  with 
mineral  constituents.  Those  waters  in  which  the  soluble  ingre- 
dients are  present  only  in  such  proportion  as  not  sensibly  to 
affect  the  taste,  are  termed  fresh  waters ;  whereas,  those  in  which 
the  saline  or  gaseous  contents  are  present  in  quantity  sufficient 
to  impart  to  the  water  a  peculiar  taste  or  medicinal  qualities  are 
termed  mineral  ivaters.  The  salts  which  most  commonly  occur 
in  solution  in  spring-water  are  :  (1)  The  carbonates  of  calcium, 
magnesium,  iron,  and  manganese,  dissolved  in  an  excess  of  car- 
bonic acid.  (2)  The  sulphates  of  calcium  and  magnesium.  (3) 
Alkaline  carbonates,  chlorides,  sulphates,  nitrates,  or  silicates. 
The  gaseous  constituents  consist  of  oxygen,  nitrogen,  and  carbon 
dioxide  ;  the  latter  gas  being  present  in  varying  amount  though 
always  in  much  larger  quantity  than  we  find  it  in  rain-water. 
The  nature  and  quantity  of  the  inorganic,  as  well  as  of  the 
gaseous  constituents  of  a  fresh  spring,  or  mineral-water  must  be 
ascertained  by  a  complete  chemical  analysis,  frequently  a  long 
and  complicated  operation. 

150  Mineral-  Waters  and  Thermal  Springs. — Spring- waters 
which  issue  from  considerable  depths,  or  which  originate  in 
volcanic  districts,  are  always  hotter  than  the  mean  annual  tem- 
perature of  the  locality  where  they  come  to  the  surface.  In 
many  of  these  springs  the  water  issues  together  with  a  copious 
discharge  of  undissolved  gas,  and  in  some  cases,  as  in  the  cele- 
brated Gey  sirs  of  Iceland,  so  carefully  investigated  by  Bunsen,1 
steam  accompanies  the  water  or  forces  it  out  at  certain  intervals. 
Several  remarkable  hot  springs  of  this  kind  have  been  discovered 
in  New  Zealand,  but  a  still  more  extensive  series  occurs  in  the 
district  of  the  Yellowstone  river  in  the  United  States.  The 
following  is  a  list  of  the  most  important  thermal  springs,  the 

1  On  the  Pseudo-Volcanic  Phenomena  of  Iceland,  Cav.  Soc.  Memoirs,  1848, 
p.  323. 


MINERAL  WATERS  297 


temperature  of  all  of  which  is  much  above  that  of  the  locality 
where  they  occur. 

THERMAL  SPRINGS. 

Temp.  Temp. 

Wildbad  .    .        37°'5  Baden-Baden    ....  67°'5 

Aachen     .    .  44°  to  57°'5  Wiesbaden 70° 

Vichy   ...        45°  Karlsbad 75° 

Bath     ...        47°  Trincheras  (Venezuela)  97° 

The  chief  gases  found  free  in  these  springs  are  carbonic  acid 
and  sulphuretted  hydrogen. 

According  to  the  materials  which  the  water  contains  in 
solution  these  springs  may  be  grouped  as  follows  : — 

(1)  Carbonated,    Waters,   which    are   cold,   and    are   rich   in 
carbonic  acid,  and  contain  small  quantities  of  alkaline  carbon- 
ates, chloride  of  sodium,  and  other  salts.     Amongst  the  best 
known    of  these   are   the    waters   of  Seltzer,   Apollinaris,  and 
Taunus. 

(2)  Alkaline   Waters,  containing  a  larger  quantity  of  bicar- 
bonate   of  soda,  as  well   as   common   salt,   and  Glauber    salt. 
These  are  sometimes  warm,  such  as  the  springs  at  Ems  and 
Vichy,  but  generally  cold.      They  are  often  rich   in  carbonic 
acid. 

(3)  Saline  Waters,  are  those  in  which  the  alkaline  bicarbonate 
is  replaced  by  other  salts,  thus  Glauber-salt  water,  as  Marienbad ; 
Magnesian  water,  such  as  Friedrichshall,  Seidschutz  and  Epsom, 
in  which  the  sulphate  and  chloride  of  magnesium  occur ;  Chaly- 
leate    waters  in  which  ferrous  carbonate  is  found  dissolved  in 
carbonic  acid,  such  as  that  of  Pyrmont  and  Spa ;    Sulphuretted 
water,  containing  sulphuretted  hydrogen  and  the  sulphides  of 
the  alkaline  metals,  as  the   springs  at  Aachen  and  Harrogate. 
Hot-springs    also    occur    in    which    but   very   small   traces   of 
soluble   constituents    are    found,   but   which   from   their   high 
temperature  are  used  for  the  purpose  of  medicinal  bathing ; 
such  springs  are  those  of  Pfafers  44°,  Gastein  35°,  Bath  47°,  and 
Buxton  28°. 

(4)  Silicious   Waters  are  those  in  which  the  saline  contents 
consist  chiefly  of  alkaline  silicates,  such  as  the  hot-spring  waters 
of  Iceland. 

The  following  analysis  by  Bunsen  of  the  mineral  waters 
of  Diirkheim  and  of  Baden-Baden  may  serve  as  examples 


298 


THE  NON-METALLIC  ELEMENTS 


of  the  complexity  in  chemical  composition  of  certain  mineral 
waters : — 


Analyses  of  1,000  parts  of  the  Mineral  Waters  in  which  the  new 
Alkali  Metals  Ccesium    and    Rubidium    were   discovered    by 


Diirkheim.  Baden-Baden. 

Calcium  bicarbonate   ....  0*28350  .  .  T475 

Magnesium  bicarbonate  .    .    .  0*01460  .  .  0712 

Ferrous  bicarbonate     ....  0'00848  .  .  O'OIO 

Manganous  bicarbonate  .    .    .  traces  .  .  traces 

Calcium  sulphate .  .  2*202 

Calcium  chloride 3*03100  .  .  0'463 

Magnesium  chloride    ....  0'39870  .  .  0*126 

Strontium  chloride      ....  0'00818  .  . 

Strontium  sulphate     ....  0*01950  .  .  0  023 

Barium  sulphate .  .  traces 

Sodium  chloride      1271000  .  .  20  834 

Potassium  chloride      ....  0*09660  .  .  1-518 

Potassium  bromide      ....  0  02220  .  .  traces 

Lithium  chloride 0-03910  .  .  0'451 

Kubidium  chloride      ....  0'00021  .  .  0*0013 

Caesium  chloride 0*00017  .  .  traces 

Alumina 0*00020  .  . 

Silica 0-00040  .  .  1-230 

Free  carbonic  acid 1-64300  .  .  0'456 

Nitrogen 0*00460  .  . 

Sulphuretted  hydrogen  .    .    .  traces  .  . 

Combined  nitric  acid  ....  .  .  0*030 

Phosphates traces  .  .  traces 

Arsenic  acid .  .  traces 

Ammoniacal  salts traces  .  .  0*008 

Oxide  of  copper .  .  traces 

Organic  matter traces  .  .  traces 


Total  soluble  constituents  .    .  18*28028 


29*6393 


151  Hard  and  Soft  Water. — Waters  are  familiarly  distinguished 
as  hard  and  soft  according  as  they  contain  large  or  small  quantities 
of  lime  or  magnesia  salts  in  solution.  These  may  exist  either  as 
carbonates  held  in  solution  by  carbonic  acid,  or  as  sulphates. 
In  both  cases  the  water  is  hard,  that  is,  it  requires  much  soap  to 


HARDNESS  OF  WATER  299 

be  used  in  order  to  make  a  lather,  because  an  insoluble  compound 
is  formed  by  the  union  of  the  lime  or  magnesia  with  the  fatty 
acid  of  the  soap  which  consists  of  sodium  or  potassium  salts  of 
the  acids  of  this  series.  But  in  the  first  instance,  the  hardness 
is  said  to  be  temporary  because  it  is  removed  either  by  the  addi- 
tion of  milk  of  lime  or  by  boiling  the  water,  when  the  carbonic 
acid  holding  the  carbonate  of  lime  in  solution  is  either  pre- 
cipitated or  driven  off,  whereas  in  the  second  instance,  it  cannot 
be  thus  removed,  and  is  therefore  termed  permanent  hardness. 
In  order  to  ascertain  the  amount  of  this  hardness  a  simple 
method  was  proposed  by  the  late  Dr.  Clark.  It  consists  in 
ascertaining  how  many  measures  of  a  standard  soap  solution 
are  needed  by  a  gallon  of  water  to  form  a  lather.  Thus  this 
soap  test  serves  as  a  rough  but  convenient  method  of  determin- 
ing the  amount  of  lime  or  magnesia  salts  which  the  water 
contains. 

The  following  is  a  description  of  a  method  employed  for 
determining  the  hardness  of  a  water.  10  grams  of  good  Castile 
soap  are  dissolved  in  one  litre  of  dilute  alcohol  containing  about 
35  per  cent,  of  alcohol,  and  the  strength  is  so  proportioned  that 
1  cc.  of  this  solution  will  precipitate  exactly  1  mgrm.  of  calcium 
carbonate  when  in  solution.  In  order  to  standardise  the  soap- 
solution  1  gram  of  calc-spar  is  dissolved  in  hydrochloric  acid, 
the  solution  evaporated  to  dryness  in  order  to  get  rid  of  the  excess 
of  hydrochloric  acid,  and  the  residue  consisting  of  chloride  of 
calcium  dissolved  in  one  litre  of  distilled  water.  Of  this  solution 
12  cc.  are  brought  into  a  small  stoppered  bottle,  after  being 
diluted  up  to  70  cc.  with  distilled  water.  The  soap-solution  is 
gradually  added  from  a  burette,  until  when  vigorously  shaken  a 
permanent  lather  is  formed.  If  the  solution  has  been  made  of 
the  right  strength  13  cc.  are  needed  for  this  purpose,  inasmuch 
as  70  cc.  of  distilled  water  will  themselves  require  1  cc.  of 
soap-solution  in  order  to  make  a  permanent  lather.  For  the 
purpose  of  determining  the  hardness  of  a  water,  a  measured 
quantity  of  the  water  is  taken,  and  the  standard  soap-solution 
run  in  until  a  permanent  lather  is  obtained.  70  cc.  of  water 
are  usually  employed  for  this  purpose,  because  every  cc.  of  the 
soap-solution  will  then  correspond  to  one  grain  of  calcium 
carbonate  in  70,000,  or  in  a  gallon  of  water.  On  the  Continent 
however,  the  hardness  is  usually  calculated  into  parts  per  100,000 
of  water.  The  hardness  of  a  water  is  expressed  in  degrees,  by 
which  is  understood  the  number  of  parts  of  calcium  carbonate 


300  THE  NON-METALLIC  ELEMENTS 

or  of  the  corresponding  magnesium,  or  other  calcium  salts,  which 
are  contained  in  70,000  or  in  100,000  parts  of  the  water.  Thus 
Thames  water  has  a  hardness  of  15°*0,  or  contains  in  solution 
15  grains  of  carbonate  of  lime  per  gallon,  whilst  in  the  water  of 
Bala  Lake,  only  1-3  grains  per  gallon  are  present.  The  presence 
of  magnesium  salts  interferes  to  some  extent  with  the  accuracy 
of  the  determination  of  hardness. 

152  The  Organic  Constituents  of  Waters. — Spring-water  not 
only  contains  inorganic,  but  also  soluble  organic  constituents,  and 
these  likewise  vary  with  the  constituents  of  the  strata  through 
which  the  water  passes.  This  organic  matter  may  be  dis- 
tinguished as  (a)  that  which  is  derived  from  a  vegetable,  and 
(6)  that  derived  from  an  animal  source.  If  the  water  has  been 
collected  from  moorland  it  will  contain  some  soluble  vegetable 
matter,  if  it  has  come  in  contact  with  any  decomposing  animal 
substances  it  will  have  taken  up  soluble  animal  matter.  These 
two  forms  of  impurity  are  of  a  very  different  degree  of  importance 
as  regards  the  suitability  of  a  water  thus  impregnated  for  drinking 
purposes. 

It  is  now  generally  admitted  that  a  number  of  infectious 
diseases,  especially  cholera  and  typhoid  fever,  are  frequently 
contracted  and  spread  by  means  of  drinking  water  containing 
the  bacteria  of  these  diseases,  derived  from  the  excreta  or  other 
discharges  from  persons  suffering  from  them,  or  from  the  washing 
of  infected  clothing.  Hence  it  becomes  very  important  to  be 
able  to  detect  the  presence  of  these  pathogenic  or  disease  pro- 
ducing organisms.  This  is  however  a  matter  of  considerable 
difficulty,  and  hence  the  bacteriological  examination  of  a  water 
is  mainly  valuable  as  a  test  of  the  efficacy  of  processes  of  purifi- 
cation, since  the  conclusion  may  be  safely  drawn  that  any  mode 
of  treating  the  water  which  proves  fatal  to  the  harmless  bacteria, 
which  are  generally  present  in  considerable  numbers,  will  also 
prove  fatal  to  the  dangerous  organisms.1 

The  actual  determination  of  the  number  of  organisms  of  all 
kinds  present  in  a  sample  of  water,  without  attempting  to  ascer- 
tain their  specific  nature,  is  carried  out  by  mixing  a  known  volume 
of  the  water  with  sterilised  nutrient  gelatine  and  maintaining  the 
whole  at  a  temperature  of  20 — 23°,  which  is  found  to  be  favourable 
to  the  development  of  these  organisms,  for  several  days.  Under 
these  circumstances  each  organism  produces  a  colony  around 
itself,  and  reproduction  proceeds  so  rapidly  that  in  a  few  days 
1  Frankland,  Journ.  Soc.  Chem.  hid.  1887,  6,  319. 


ORGANIC  CONSTITUENTS  OF  WATERS  301 

a  mass  which  is  visible  to  the  naked  eye  is  produced.  The 
colonies  are  then  counted  and  the  number  of  bacteria  which  were 
present  in  the  volume  thus  ascertained.  The  result  of  a 
determination  of  this  character  is  not  absolute  because  many 
organisms  do  not  develop  under  the  conditions  observed.  In 
1886  the  water  of  the  Lea  unfiltered  contained  an  average  of 
19781  organisms  per  cc.,  whilst  after  storage  and  filtration 
the  same  water  as  supplied  to  the  consumers  contained  only  253 
or  about  1*3  per  cent,  of  this  number.  Filtration  through  sand, 
charcoal,  or  spongy  iron,  has  the  effect  of  removing  the  whole  of 
these  bacteria,  but,  unless  the  material  of  the  filter  is  frequently 
renewed,  this  high  state  of  efficiency  is  not  maintained,  and  the 
filter  may  even  serve  as  an  incubating  bed,  so  that  the  water 
passing  through  is  rendered  worse  by  filtration. 

Simply  boiling  the  water  for  a  few  minutes  does  not  entirely 
destroy  the  bacteria  contained  in  it  but  greatly  diminishes  their 
number.  Thus  a  sample  of  Parisian  canal  water  which  con- 
tained 460,800  bacteria  per  cc.  was  found  after  boiling  for  ten 
minutes  only  to  contain  a  single  living  organism  in  every 
Sec.1 

The  chemical  examination  of  a  water  is  a  much  more  rapid 
process,  but  yields  even  less  direct  information  as  to  the  whole- 
someness  of  the  sample  than  the  bacteriological.  It  aims  at 
detecting  and  estimating  in  the  water  such  substances  as  are 
characteristic  of  the  probable  sources  of  pollution. 

Nitrogen  for  example  is  one  of  the  characteristic  constituents 
of  animal  matter,  being  present  in  considerable  quantity  in  a 
state  of  combination  in  every  part  of  the  flesh,  nerves  and  tissues 
of  the  body,  whilst  it  is  contained  in  plants  in  smaller  quantity 
and  only  in  their  fruit  and  seeds.  Hence  if  water  be  impreg- 
nated with  animal  matter  this  will  be  indicated  by  the  presence 
of  nitrogen  in  solution,  either  in  the  form  of  albumin  or 
albuminous  matter,  if  the  animal  matter  be  contained  in  the 
water  unchanged,  or,  if  the  animal  matter  has  undergone  oxida- 
tion, in  the  form  of  ammonia,  or  nitrous  or  nitric  acid.  The 
amount  of  the  nitrogen  which  has  been  oxidized  to  ammonia 
can  be  easily  determined  by  distilling  the  water  with  carbonate 
of  soda  when  the  whole  of  the  ammonia  is  obtained  in  the 
distillate  and  estimated  by  Nessler's  colorimetric  test.  The 
Nessler's  solution  is  prepared  as  follows : — 35  grams  of  potas- 
sium iodide,  and  13  grams  of  mercuric  chloride  (corrosive  subli- 

1  Miguel  quoted  in  article  Water,  Thorpe's  Technical  Dictionary. 


302 


THE  NON-METALLIC  ELEMENTS 


mate)  are  dissolved  in  about  800  cc.  of  hot  water  and  then  a 
saturated  solution  of  mercuric  chloride  is  gradually  added  until 
the  precipitate  formed  ceases  to  re-dissolve.  100  grams  of 
caustic  potash  are  then  dissolved  in  the  liquid  and  the  cold 
solution  is  diluted  to  one  litre,  and  is  allowed  to  deposit  an}7 
undissolved  matter.  Half  a  litre  of  the  water  under  examina- 
tion must  be  distilled  in  a  glass  retort  as  shown  in  Fig.  94, 
carbonate  of  soda  having  been  previously  added,  and  care 
must  be  taken  to  free  the  apparatus  from  ammonia  by  a  previ- 
ous process  of  distillation.  The  distillate  is  collected  successively 
in  volumes  of  50  cc.  and  the  ammount  of  ammonia  in  each  of 
these  separate  distillates  determined.  For  this  purpose  the 
distillate  is  collected  in  a  high  cylinder  of  white  glass,  2  cc.  of 


FIG.  94. 


Nessler's  solution  is  added,  and  the  mixture  well  stirred.  If 
the  smallest  quantity  of  ammonia  be  present,  a  yellow  colour  is 
noticed,  and  a  corresponding  degree  of  tint  is  obtained  in  a 
second  cylinder  by  gradually  adding  a  standard  solution  of  sal- 
ammoniac  to  50  cc.  of  water  perfectly  free  from  ammonia, 
and  containing  some  of  the  Nessler's  reagent  until  the  tint  is 
reached.  This  standard  solution  is  prepared  by  dissolving  315 
grams  of  ammonium  chloride  in  one  litre  of  water  and  diluting 
10  cc.  of  this  solution  to  one  litre,  so  that  each  cc.  will  therefore 
correspond  to  O'Ol  mgrm.  of  ammonia. 

The  nitrates  and  nitrites  present  can  be  estimated  in  another 
portion  of  the  water  by  reducing  these  acids  to  ammonia  by  means 
of  the  hydrogen  evolved  by  aluminium  in  presence  of  pure 


ORGANIC  CONSTITUENTS  OF  WATERS  303 

caustic    alkalis.      For   other   methods   the   treatises  on  water 
analysis  must  be  consulted. 

In  order  to  estimate  the  quantity  of  unaltered  albuminous 
matter  which  may  possibly  be  contained  in  the  water,  two  pro- 
cesses have  been  proposed.  The  first  of  these,  proposed  by 
Wanklyn  and  Chapman,  depends  upon  the  fact  that  these 
albuminous  bodies  are  either  wholly  or  in  part  decomposed  on 
distillation  with  an  alkaline  solution  of  potassium  permangan- 
ate, the  nitrogen  being  in  this  case  again  evolved  as  ammonia 
which  is  determined  as  above.  In  the  second  process, 
described  by  Frankland  and  Armstrong,  the  nitrogen  gas  con- 
tained combined  in  albuminous  matter  in  the  water  is  liberated 
as  such  by  a  combustion  analysis  performed  on  the  dry  residue 
of  the  water,  the  volume  of  the  free  nitrogen  being  afterwards 
carefully  measured.  In  this  latter  process  not  only  the  organic 
nitrogen  but  also  the  organic  carbon,  that  is,  the  carbon  de- 
rived from  animal  and  vegetable  sources,  can  be  quantitatively 
determined. 

If,  now,  by  means  of  either  of  these  processes,  a  water  is 
found  to  contain  more  than  015  parts  of  albuminoid  nitrogen 
in  one  million  parts  of  water,  it  may  be  considered  as  unfit  for 
drinking  purposes  ;  many  surface  well-waters  occur  in  large 
towns  in  which  the  amount  of  albuminoid  nitrogen  reaches  0'3 
to  0'8  parts  per  million,  and  such  waters  must  be  regarded  as 
little  better  than  sewage,  and.  therefore,  as  absolutely  poisonous. 
But  water  in  which  no  albuminous  matter  has  been  found  may 
also  be  largely  impregnated  with  sewage  or  infiltrated  animal 
impurity,  the  greater  part  of  which  has  undergone  oxidation. 
Thus  when  the  amount  of  free  ammonia  exceeds  0*08  parts  per 
million,  it  almost  invariably  proceeds  from  the  decomposition  of 
urea  into  carbonate  of  ammonia,  and  shows  that  the  water  consists 
of  diluted  urine.  In  like  manner,  when  the  oxidation  has  pro- 
ceeded further,  the  nitrogen  will  be  found  as  nitrates  and 
nitrites,  and  should  any  considerable  quantity  of  these  sub- 
stances be  found  in  surface  well-  or  river-water,  the  previous 
admixture  of  animal  impurity  may  be  inferred.  In  some 
instances,  however,  water  from  deep  wells  in  the  chalk  has  been 
found  to  contain  nitrates,  which  in  such  cases  cannot  as  a  rule 
be  supposed  to  indicate  dangerous  impurity. 

153  The  water  analyst  is  also  assisted  in  his  attempts  to 
indicate  the  limits  of  wholesomeness  in  a  water  by  the  determina- 
tion of  the  amount  of  chlorine  present  as  chloride  of  sodium,  &c. 
which  the  water  contains.  Not  that  chlorides  are  in  themselves 


304 


THE  NON-METALLIC  ELEMENTS 


of  importance,  but  because  their  presence  serves  as  an  indication 
of  sewage-contamination,  for  pure  natural  waters  are  almost  free 
from  chloride  of  sodium,  whilst  urine  and  sewage  are  highly 
charged  with  this  substance  So  that  if  we  meet  with  a  water 
almost  free  from  chlorine,  it  cannot  have  come  into  contact  with 
sewage.  Thus  the  water  of  Ullswater  and  that  of  the  Thames 
at  Kew  contain  from  0'7  to  0'8  grains  of  chlorine  in  the  gallon, 
whilst  many  surface  wells  in  large  towns  may  be  found  which 
contain  from  10  to  above  30  grains  of  chlorine  per  gallon. 
Taken  alone,  the  chlorine  test  cannot  be  relied  upon,  as  man} 
pure  well-waters  occur,  such  as  those  in  Cheshire,  in  the  neigh- 
bourhood of  the  salt  beds,  or  near  the  sea,  which  contain  com- 
mon salt.  If,  however,  this  test  be  employed  in  conjunction 
with  those  previously  mentioned,  the  evidence  for  or  against  a 
water  is  rendered  much  more  cogent.  As  a  rule  it  may  be 
said  that  waters  containing  more  than  two  grains  of  chlorine  per 
gallon  must  be  looked  upon  with  suspicion,  unless  indeed  some 
good  reason  for  the  presence  of  common  salt  can  be  assigned. 

The  following  analyses  serve  to  show  the  difference  between 
a  good  potable  water  and  one  which  is  totally  unfit  for  drinking 
purposes.  No  1,  the  water  supplied  by  the  Manchester  Cor- 
poration from  the  Derbyshire  hills  ;  No.  2  is  a  surface-well 
water,  at  one  time  used  for  drinking  purposes  in  a  manufactur- 
ing town,  although  little  better  than  effluent  sewage.1 


No.  1.  G 

sod  Water. 

No.  2.  E 

>ad  Water. 

Parts  per 
Million. 

Grains  pel- 
Gallon. 

Parts  per 
Million. 

Grains  per 
Gallon. 

Total  solids      .... 
Nitrogen  as  nitrites  and 
nitrates       

63-0 
0*25 

4-4 

0-017 

530 

7'8 

37-1 
0-546 

Free  ammonia  .... 
Albuminoid  ammonia    . 
Chlorine 

0-03 
0-07 
11-4 

0-002 
0-005 

0-8 

4-32 
0-9 
69 

0-303 
0-063 

4-8 

Temporary  hardness  . 
Permanent  hardness. 
Total  hardness 

o-i 

2-4 
2-5 

7-2 
14-4 
21-6 

1  For  the  special  details  of  the  processes  of  water  analysis,  the  following  works 
or  memoirs  may  be  consulted  : —  Water  Analysis,  by  Wanklyn  and  Chapman. 
1889.  Triibner,  London.  Frankland  and  Armstrong,  Journ.  Chem.  Soc.  1868. 
p.  77  ;  Frankland,  ibid.  109  ;  also  Journ.  Chem.  Soc.  June,  1876.  Percy  Frank- 
land,  Agricultural  Chemical  Analysis,  p.  257.  Macmillan,  1883. 


RIVER-WATERS 


305 


154  River-  Waters. — The  composition  of  river- water  varies 
considerably  with  the  nature  of  the  ground  over  which  the 
water  runs;  thus  Thames  water  contains  about  11  grains  per 
gallon  of  carbonate  of  lime ;  the  Trent  21  grains  of  sulphate  of 
lime,  or  they  are  both  hard  waters,  the  first  temporarily  and  the 
second  permanently  hard.  The  waters  of  the  Dee  and  the  Don, 
in  Aberdeenshire,  draining  a  granite  district,  are,  on  the  other 
hand,  soft  waters.  The  composition  of  these  waters  is  shown  in 
the  following  table  : — 

TABLE  GIVING  THE  COMPOSITION  OF  CERTAIN  RIVER- WATERS. 


— 

Grains  per  Gallon. 

Calcium  carbonate  .     .     . 
Calcium  sulphate     .     .     . 
Calcium  nitrate  .... 
Magnesium  carbonate  . 
Sodium  chloride  .... 
Silica      .               .... 

Thames. 

10-80 
3-00 

017 

1-25 

1-80 
0-56 

0-27 

trace 
2-36 

Trent. 

0-32 
21-55 

5-66 
17-63 
0-72 

0-50 
trace 
3-68 

Dee. 

0-85 
0'12 

0-36 
0-72 
0-14 

0-06 
trace 
1-54 

Don. 

2-23 
0-13 

1-07 
1-26 
0-52 

0-27 
trace 
3-06 

Ferric   chloride   and    alu- 
mina 

Calcium  phosphate  .     .     . 
Organic  matter    .... 

Hardness    

20-21 

50-06 

3-89 

8-54 

14-0 

26-5 

1-5 

3-0 

Unfortunately  in  England,  as  in  other  manufacturing  and 
densely  populated  countries,  the  running  water  seldom  reaches 
the  sea  in  its  natural  or  pure  state,  but  is  largely  contaminated 
with  the  sewage  of  towns,  or  the  refuse  from  manufactures  or 
mines.  So  serious  indeed  is  this  state  of  things  that  steps  have 
been  taken  to  prevent  the  further  pollution  of  the  rivers  of  the 
country,  and  a  Royal  Commission  has  already  reported  and 
several  acts  of  Parliament  have  been  passed  with  a  view  of 
preventing  the  evil.  The  following  analyses  of  the  composition 
of  Lancashire  rivers,  taken  from  the  First  Report  of  the  Com- 
missioners l  appointed  in  1868,  show  clearly  the  pollution  which 
the  originally  pure  waters  of  the  Irwell  and  Mersey  undergo  on 
flowing  down  to  the  sea, 

1  P.  15. 

21 


306 


THE  NON-METALLIC  ELEMENTS 


COMPOSITION  OF  LANCASHIRE  RIVERS. 

Parts  in  100,000. 


— 

IRWELL. 

MERSEY. 

n     |      , 

3 

4 

Total  soluble  solids  .     .     . 

7-8 

55-80 

7-62 

39-50 

Organic  carbon    .... 

0-187     1-173 

0-222 

1-231 

Organic  nitrogen 

0-025 

0-332 

0 

0-601 

Ammonia   

0-004 

0-740 

0-002 

0-622 

Nitrogen  as  nitrates   and 

nitrites    

0-021 

0-707 

0-021 

0 

Total   combined   nitrogen 

0-049 

1-648 

0-023 

1-113 

Chlorine      

115 

9-63 

0-94 



Hardness  temporary     .     . 

3-72 

15-04 

4-61 

10-18 

Total  hardness     .... 

3-72 

15-04 

4-61 

10-18 

SUSPENDED  MATTER— 

Organic  

0 

2-71 

0 

Mineral  

0 

2-71 

0 

Total      

0 

5-42 

0 

— 

From  these  numbers  it  is  seen  that  the  quantities  of  free 
ammonia  and  nitric  acid  become  increased  300  or  400-fold  in 
the  river  below  Manchester,  whilst  the  total  combined  nitrogen 
is  increased  from  0'049  to  1"648. 

155  Sea-  Water. — The  amount  of  solid  matter  contained  in  the 
waters  of  the  ocean  is  remarkably  constant  when  collected  far 
from  land.  The  mean  quantity  is  about  35*976  grams  in 
1000  grams  of  sea- water;  the  average  specific  gravity  of  sea- 
water  is  1-02975  at  0°. 


hl.     The  Irwell  near  its  source. 
2.     The  Irwell  below  Manchester. 


3.  The  Mersey,  one  of  its  sources. 

4.  The  Mersey  below  Stockport. 


SEA-WATER 


307 


COMPOSITION  OF  THE  WATER  OF  THE  IRISH  SEA  IN  THE 
SUMMER  OF  1870.1 


One  thousand  grams  of  sea-water  contain 

Grams. 

Sodium  chloride  
Potassium  chloride  
Magnesium  chloride  

26-43918 
0-74619 
3-15083 

Magnesium  bromide  

0-07052 

Magnesium  sulphate  
Magnesium  carbonate  .... 
Magnesium  nitrate  
Calcium  sulphate  

2  06608 
traces 
0-00207 
1-33158 

Calcium  carbonate  
Lithium  chloride  . 

0-04754 
traces 

Ammonium  chloride  
Ferrous  carbonate  
Silicic  acid  

0-00044 
0-00503 
traces 

33-85946 

For  the  purpose  of  controlling  the  analysis,  1,000  grams  of 
water  were  evaporated  to  dryness,  and  the  dry  residue  weighed. 
Its  weight  was  found  to  be  33"83855  grams.  The  specific 
gravity  of  the  water  at  0°  C.  was  1*02721,  whilst  that  at  15°  C. 
was  1-02484. 

Forchhammer  found  that  1,000  parts  by  weight  of  the  water 
of  the  mid-Atlantic  Ocean  contained  35*976  parts  of  dissolved 
salts,  whilst  the  mean,  of  analyses  of  sea-water  from  different 
localities  gave  34*082  for  the  total  salts  in  summer  and  33*838 
in  winter.  Dittmar,2  on  the  other  hand,  from  77  specimens 
of  sea-water  collected  on  board  the  Challenger  in  various  parts 
of  the  world,  concludes  that  the  maximum  quantity  of  salt 
contained  in  the  water  of  the  Indian  Ocean  is  33'01,  and  in 
that  of  the  North  Atlantic  37 '37.  In  the  neighbourhood  of  the 
shore  or  in  narrow  straits  the  quantity  of  saline  matter  is  often 
much  smaller. 

According  to  Forchhammer  the  relation  in  which  the  several 

1  Thorpe  and  Morton,  Journ.  Chem.  Soc.  24,  506. 

~  Challenger  Reports,  "On  the  Composition  of  Ocean  Water."  By  Professor 
Dittmar,  F.R.S.  London,  1884. 


308  THE  NON-METALLIC  ELEMENTS 


salts  stand  to  one  another  is  a  constant  one ;  and  in  this 
conclusion  Dittmar  agrees.  The  former  calculated  the  quantity 
of  lime,  magnesia,  potash,  and  sulphuric  acid  present  with  100 
parts  of  chlorine,  including  bromine.  Dittmar  estimated  this 
element  and  also  soda  and  carbon  dioxide,  and  then  calculated 
as  Forchhammer,  reckoning  the  bromine  as  its  equivalent  of 
chlorine.  Their  results  are  : — 

Forchhammer. 
Cl  .     .     .     100-00     . 
Br  .     .     .         — 
S03      .     .       11-88     . 
C02     .     .        -        . 

Dittmar  calculated  the  following  average  composition  of  the 
total  saline  constituents  of  sea- water,  giving  a  somewhat  different 
arrangement  to  the  acids  and  bases  from  that  adopted  by  Thorpe 
and  Morton. 


Dittmar. 

Forchhammer. 

Dittmar. 

99-848 

CaO. 

.      .        2-93     . 

.        3-026 

0-3 

MgO 

.     .     11-03     . 

.      11-221 

11-576 

K20  . 

1-93     . 

.       2-405 

0-276 

Na20 

.     .         — 

.     74-462 

Sodium  chloride 
Magnesium  chloride 
Magnesium  sulphate 
Calcium  sulphate     . 
Potassium  sulphate 


77*758  Magnesium  bromide     .     .     0'217 

10-878  Calcium  carbonate  .     .     .     0*345 
4-737 

3-600  100-000 

2'465  


All  the  elements  are  doubtless  contained  in  sea-water. 
In  addition  to  those  named  above  the  following  have  been 
detected  : — 

Iodine,  fluorine,  nitrogen,  phosphorus,  silicon,  carbon,  boron, 
zinc,  cobalt,  nickel,  copper,  strontium,  barium,  manganese, 
aluminium,  iron,  lithium,  caesium,  rubidium  (these  three  detected 
spectroscopically),  silver,  lead,  and  lastly  arsenic,  making  a  total 
of  thirty  elements. 


HYDROGEN  DIOXIDE,  OR  HYDROGEN  PEROXIDE.    H2O2  =  3376. 

156  This  body  was  discovered  in  1818  by  Thenard,1  who 
prepared  it,  by  the  action  of  dilute  hydrochloric  acid  on  barium 
dioxide,  thus : — 

Ba02  +  2HC1  =  H2O2  +  BaCl2 

The  compound  is  also  easily  formed  by  passing  a  current  of 
carbon  dioxide  through  water,  and  gradually  adding  barium 
dioxide  in  very  small  quantities,2  thus : — 

Ba02  +  C02  +  H20  =  H20,  +  BaCO3. 

1  Ann.  Ghem.  Phys.  8,  306. 

2  Duprey,  Compt.  Rend.  55,  736  ;  Balard,  Compt.  Rend.  55,  758. 


HYDROGEN  PEROXIDE  309 


Preparation. — Hydrogen  dioxide  is,  however,  most  generally 
obtained  by  decomposing  pure  barium  dioxide  with  dilute 
sulphuric  acid  ; l  thus  : — 

Ba02  +  H2S04  =  H202  +  BaSO4. 

The  pure  barium  dioxide  needed  for  these  experiments  is 
prepared  as  follows : — commercial  barium  dioxide,  very  finely 
powdered,  is  brought  little  by  little  into  dilute  hydrochloric  acid, 
until  the  acid  is  nearly  neutralised.  The  cooled  and  filtered 
solution  is  then  treated  with  baryta- water,  in  order  to  precipitate 
the  ferric  oxide,  manganic  oxide,  alumina,  and  silica  which  are 
always  contained  in  the  impure  barium  dioxide.  As  soon  as  a 
white  precipitate  of  the  hydrated  barium  dioxide  makes  its 
appearance,  the  solution  is  filtered,  and  to  the  filtrate  concen- 
trated baryta-water  is  added  ;  a  crystalline  precipitate  then  falls 
consisting  of  hydrated  barium  dioxide.  This  is  well  washed 
and  preserved,  in  the  moist  state,  in  stoppered  bottles.  In  order 
to  prepare  hydrogen  dioxide  by  means  of  this  substance,  the 
moist  precipitate  is  gradually  added  to  a  cold  mixture  of  not  less 
than  five  parts  of  water  to  one  part  of  concentrated  sulphuric 
acid,  until  the  mixture  remains  very  slightly  acid.  The  preci- 
pitate of  barium  sulphate  is  allowed  to  settle  and  the  liquid 
filtered.  The  small  trace  of  sulphuric  acid  which  the  filtrate 
contains  can  be  precipitated  by  careful  addition  of  dilute  baryta 
solution.  When  the  aqueous  solution  of  the  hydrogen  dioxide 
thus  prepared  is  brought  over  sulphuric  acid  in  vacua,  water 
evaporates,  and  the  solution  of  the  dioxide  becomes  more  con- 
centrated. If,  during  the  concentration,  an  evolution  of  oxygen 
be  noticed,  a  drop  or  two  of  sulphuric  acid  should  be  added,  as 
the  dioxide  is  more  stable  in  presence  of  free  acid  than  when 
perfectly  pure  (Thenard). 

An  aqueous  solution  of  the  peroxide  can  readily  be  prepared 
for  use  in  cases  in  which  the  presence  of  a  soluble  sodium 
salt  does  not  interfere,  by  dissolving  sodium  peroxide,  Na2O0,  a 
compound  now  prepared  on  a  large  scale,  in  cold  water  and  adding 
a  dilute  acid. 

Properties. — After  the  slow  evaporation  has  been  carried  on 

for  some  time  a  colourless  transparent  oily  liquid  is  obtained, 

having   a   specific  gravity    of   1/452.      This    liquid    evaporates 

slowly  in  vacuo  without   the   residue   undergoing   any  change 

1  Thomsen,  Ber.  7,  74. 


310  THE  NON-METALLIC  ELEMENTS 

(Thenard).  Hydrogen  dioxide  does  not  solidify  at  —  30°  ;  it 
possesses  no  smell,  has  an  astringent,  bitter  taste,  and  brought 
on  to  the  skin  produces  a  white  blister,  which  after  a  time 
produces  great  irritation.  It  bleaches  organic  colouring  matters 
like  chlorine,  although  acting  more  slowly,  and  is  a  very  unstable 
compound,  easily  decomposing  into  oxygen  and  water.  One 
volume  of  the  liquid  at  14°  and  under  a  pressure  of  760mm. 
yields,  according  to  Thenard,  475  volumes  of  oxygen,  whilst 
the  theoretical  amount  is  5  01  '8  volumes.  The  decomposition  of 
hydrogen  dioxide  takes  place  very  slowly  at  a  low  temperature, 
whilst  at  20°  the  evolution  of  gas  becomes  plainly  visible,  and 
if  the  concentrated  solution  is  heated  up  to  100°,  the  separa- 
tion of  oxygen  occurs  so  rapidly  as  sometimes  to  give  rise  to  an 
explosion.  A  dilute  aqueous  solution  of  the  dioxide  is,  how- 
ever, much  more  stable,  and  can  even  be  concentrated  up  to  a 
certain  point  by  ebullition,  a  portion  of  the  dioxide  passing 
over  undecomposed  together  with  the  vapour  of  water.  This 
explains  why  in  the  above  decomposition  the  theoretical  quantity 
of  oxygen  is  not  obtained. 

Hydrogen  dioxide  is  easily  soluble  in  ether,  and  if  an  aqueous 
solution  of  the  compound  be  shaken  up  with  ether,  the 
dioxide  dissolves  in  it.  The  ethereal  is  more  stable  than 
the  aqueous  solution,  and  it  can  be  distilled  without  decom- 
position. 

Hydrogen  dioxide  undergoes  decomposition  in  presence  of 
a  large  number  of  different  solid  substances,  and  with  the 
greater  rapidity  the  more  finely  divided  these  substances  are. 
Some  of  the  phenomena  which  thus  present  themselves  may, 
to  a  certain  extent,  be  accounted  for,  but  for  others  we  still 
need  an  explanation.  Thus,  for  example,  the  anhydrous  com- 
pound is  decomposed  with  almost  explosive  violence  into  oxygen 
and  water,  in  presence  of  finely-divided  silver,  gold,  platinum, 
and  other  metals,  the  metals  themselves,  however,  remaining 
unaltered.  The  oxides  of  these  metals  also  decompose  hydrogen 
dioxide  easily,  being  reduced  to  the  metallic  state.  The  same 
decomposition  also  occurs  with  a  dilute  aqueous  solution  of  the 
dioxide,  and  the  reaction  may  be  represented  by  the  following 
equation  :  — 


Here    we   have   the   remarkable    phenomenon    of    a   powerful 
oxidizing  agent  exerting  a  reducing  action  upon  metallic  oxides, 


HYDROGEN  PEROXIDE  311 

the  metal  being  formed.  The  explanation  of  this  fact  is 
however,  not  far  to  seek.  The  above-named  metals  possess 
only  a  weak  power  of  combination  for  oxygen,  and  their  oxides 
accordingly  decompose  easily  into  their  elements.  When  these 
oxides  are  brought  into  contact  with  hydrogen  dioxide,  which 
itself  contains  one  atom  of  oxygen  but  feebly  united,  mutual 
reduction  takes  place,  the  one  atom  of  oxygen  in  the  dioxide 
combining  with  one  atom  of  oxygen  in  the  metallic  oxide  to 
form  a  molecule  of  free  oxygen.1  In  the  same  way  we  explain 
the  fact  that  common  oxygen  is  formed  when  ozonized  oxygen 
is  brought  into  contact  with  aqueous  hydrogen  dioxide.  Here, 
too,  both  bodies  contain  a  loosely  combined  atom  of  oxygen, 
which  unite  together  to  form  a  molecule  of  free  oxygen  ; 
thus  :  — 


When  baryta-water  (barium  monoxide)  is  mixed  with  hy- 
drogen dioxide  a  precipitate  of  barium  dioxide  separates  out  ; 
thus  :  — 

BaO  +  H2O2  =  Ba02  +  H2O. 

If  the  hydrogen  peroxide  be  left  in  contact  with  the  barium 
peroxide,  oxygen  is  slowly  evolved  until  the  whole  of  the 
hydrogen  peroxide  has  been  decomposed,  the  barium  peroxide 
being  however  left  unaltered.  According  to  Schone  this  is  due 
to  the  formation  of  a  compound,  BaO2.H2O2.  This  is  a  white 
substance,  which  decomposes  in  the  presence  of  an  excess  of 
hydrogen  peroxide  into  barium  peroxide,  water  and  free 
oxygen.  Similar  reactions  occur  with  the  alkaline  hydroxides, 
and  this  may  explain  the  instability  of  hydrogen  peroxide  in 
alkaline  solution. 

Hydrogen  dioxide  also  transforms  many  other  basic  oxides. 
especially  in  presence  of  an  alkali,  to  peroxides.  Thus  manganous 
salts  become  in  this  way  converted  into  manganese  dioxide.  On 
the  other  hand,  these  peroxides  in  presence  of  an  acid  are  again 
reduced  by  hydrogen  dioxide  to  basic  oxide.  Thus,  if  hydrogen 
dioxide  be  brought  into  contact  with  dilute  sulphuric  acid  and 
manganese  dioxide,  oxygen  gas  is  given  off,  and  manganous 
sulphate  is  formed  ;  thus  :  — 

Mn02  +  H202  +  H2S04  =  MnSO4  +  2H20+O2. 

1  Cf.  Berthelot,  Compt.  Rend.  90,  572. 


312  THE  NON-METALLIC  ELEMENTS 


The  decomposition  here  occurring  is  similar  to  that  which  takes 
place  in  the  reduction  of  oxide  of  silver,  and  this  change  is  assisted 
by  the  presence  of  the  acid,  which  then  combines  with  the  basic 
oxide  to  form  a  salt. 

Detection  and  Estimation  of  Hydrogen  Dioxide.  —  In  order 
to  detect  the  presence  of  hydrogen  dioxide  in  solution,  the 
liquid  is  rendered  acid  with  sulphuric  acid,  some  ether  and  a 
few  drops  of  potassium  chromate  are  added,  and  the  solution 
well  shaken.  If  hydrogen  dioxide  be  present,  the  solution 
assumes  a  beautiful  blue  colour,  and  on  allowing  it  to  stand 
the  colour  is  taken  up  by  the  ether  and  a  deep  blue  layer 
separates  out.  This  blue  compound  is  probably  perchromic  acid, 
and  the  reaction  may,  in  a  similar  way,  be  employed  for  the 
detection  of  chromium.1 

When  hydrogen  dioxide  is  added  to  a  solution  of  iodide 
of  potassium  and  ferrous  sulphate,  iodine  is  set  free,  as  may 
easily  be  proved  by  the  formation  of  the  blue  iodide  of  starch 
(Schonbein).  This  reaction  is  so  delicate  that  one  part  of  the 
dioxide  in  twenty-five  million  parts  may  thus  be  detected.  Other 
oxidizing  agents  have  the  power  of  liberating  iodine  from  iodide 
of  potassium,  but  not  in  presence  of  ferrous  sulphate. 

For  the  purpose  of  determining  the  quantity  of  hydrogen 
dioxide  present  in  a  solution,  the  liquid  is  acidified  with  sulphuric 
acid,  and  then  a  standard  solution  of  potassium  permanganate 
added,  until  the  purple  tint  no  longer  disappears.  The  reaction 
here  occurring  is  thus  represented  :  — 

2KMn04  +  3H2S04  +  5H2O2  =  K2S04  +  2MnS04  +  8H20  +  502. 

Hydrogen  dioxide  occurs  in  small  quantities  in  the  atmosphere 
and  has  been  found  in  rain  and  in  snow  to  the  amount  of  about 
one  mgrm.  per  litre  ;  but  the  above  method  cannot  be  used  for 
estimating  its  quantity  in  this  case,  as  other  substances  contained 
in  the  air,  such  as  the  nitrites  and  organic  matter,  act  upon  the 
permanganate  solution  In  this  case  a  colorimetric  method 
proposed  by  Schone  2  may  be  used.  It  depends  upon  the  fact  that 
a  neutral  solution  of  hydrogen  dioxide  gradually  liberates  iodine 
from  a  neutral  solution  of  potassium  iodide  ;  thus  :  — 


1  Moissan,  Compt.  Bend.  97,  96  ;  Berthelot,  Compt.  Rend.  108,  24. 
-   Her.  7,  1695  ;  Annalm,  195,  228. 


OXIDES  AND  OXYACIDS  OF   CHLORINE  313 

The  liberation  of  iodine  is  accompanied  by  an  evolution  of  oxy- 
gen, and  is  very  small  in  comparison  to  the  amount  of  hydrogen 
peroxide  decomposed.  Solutions  containing  small  known  but 
varying  quantities  of  hydrogen  dioxide  are  first  prepared,  and  to 
each  of  these  solutions  potassium  iodide  and  starch  paste  are 
added,  the  degree  of  tint  which  the  several  solutions  attain, 
owing  to  the  formation  of  the  blue  iodide  of  starch,  is  then  com- 
pared with  that  obtained  in  a  similar  way  when  rain-water,  &c., 
is  employed. 

An  aqueous  solution  of  hydrogen  dioxide  is  now  largely  used 
for  the  bleaching  of  silk  and  wool.  For  this  purpose  a  liquor  is 
prepared  by  dissolving  sodium  peroxide  in  water  and  acidulating 
with  sulphuric  acid.  The  sodium  peroxide  is  manufactured  by 
the  combustion  of  sodium  and  is  brought  into  the  market  under 
the  name  of  soda-bleach.  Peroxide  of  hydrogen  is  also  used  for 
the  purpose  of  cleaning  and  bleaching  old  and  stained  engrav- 
ings and  oil  paintings,  and  as  an  auricome  for  bleaching  dark- 
coloured  hair. 


OXYGEN  AND  CHLORINE. 

OXIDES  AND  OXY- ACIDS  OF  CHLORINE. 

157  Although  chlorine  and  oxygen  do  not  combine  directly, 
two  distinct  compounds  of  these  elements  may  be  obtained  by 
indirect  means.  A  third  oxide,  described  by  Millon  and  others  as 
chlorine  tri-oxide,  has  been  proved  to  be  a  mixture  of  free  chlorine 
and  chlorine  peroxide.1  We  are  acquainted  with  no  less  than 
four  compounds  of  chlorine  with  oxygen  and  hydrogen,  which 
are  known  as  the  oxy-acids  of  chlorine.  Of  these,  the  first 
corresponds  to  the  oxide  of  chlorine,  that  is,  it  is  formed  by  the 
action  of  water  upon  the  latter.  The  following  are  the  com- 
pounds of  chlorine,  oxygen,  and  hydrogen  as  yet  known  .• 

Oxides  Oxy-acids. 

Chlorine  monoxide,  C12O  Hypochlorous  acid,  HC10 

Chlorous  acid,  HC1O2 
Chlorine  peroxide,  C1O2 

Chloric  acid,  HC1O3 
Perchloric  acid,  HC1O4 
1  Garzarolli-Thurnlackh,  Annalen,  209,  184. 


314  THE  NON-METALLIC  ELEMENTS 


Chlorine  and  oxygen  can  only  be  made  to  combine  -»togetiiar?» 
in  the  presence  of  a  basic  oxide  ;  thus,  if  chlorine  gas  be  led 
over  dry  mercuric  oxide,  chlorine  monoxide  and  mercuric  oxy- 
chloride  are  formed  ;  thus  :  — 


2HgO  +  2Cl2  =  Hg3OCl2  +  C120. 

The  same  reaction  takes  place  in  presence  of  water,  and  in 
this  case  a  colourless  solution  of  the  corresponding  hypochlorous 
acid  is  formed. 

C120  +  H20  =  2C10H. 

When  chlorine  is  passed  into  a  cold  dilute  solution  of  an 
alkali  such  as  caustic  potash,  instead  of  the  free  hypochlorous 
acid  the  corresponding  salt,  termed  a  hypochlorite,  is  formed  ; 
thus  :  — 

2KOH  +  C12  =  KOC1  +  KC1  +  H20. 

If  the  solution  of  the  alkali  be  concentrated,  or  if  it  be  heated 
whilst  the  gas  is  passed  through  it,  a  different  reaction  takes 
place,  in  which  a  salt  called  a  chlorate  is  formed  ;  thus  :  — 

6KOH  +  3C12  =  KC1O3  +  5KC1  +  3H2O. 

From  the  potassium  chlorate  thus  formed,  chloric  "acia  itself  can 
be  obtained,  and  by  reduction  of  this  acid  the  oxide  C102  may 
be  prepared.  PercHioric'acid  is  prepared  by  the  further  oxidation 
of  chloric  acid.  The  oxides  corresponding  to  chlorous,  chloric 
and  perchloric  acids,  viz.,  C12O8,  C1205,  and  C12O7,  have  not  yet 
been  prepared.  The  oxides  and  oxy-acids  of  chlorine  are  un- 
stable compounds,  as  indeed  might  be  expected,  owing  to  the 
feeble  combining  power  which  chlorine  and  oxygen  exhibit  to- 
wards one  another  ;  in  consequence  of  this  they  act  as  powerful 
oxidizing  substances,  many  of  them  being  most  dangerously 
explosive  bodies,  which  suddenly  decompose  into  their  con- 
stituents on  rise  of  temperature,  or  even  on  concussion.  It  is, 
however,  remarkable  that  perchloric  acid,  which  contains  the 
most  oxygen,  is  the  one  which  is  the  most  stable. 


CHLORINE  MONOXIDE  315 

CHLORINE  MONOXIDE  OR  HYPOCHLOROUS  ANHYDRIDE.    C12O. 

158  So  long  ago  as  1785  Bertliollet  noticed  that  chlorine  could 
be  combined  with  an  alkali  and  yet  preserve  the  peculiar  bleach- 
ing power  which  had  been  previously  discovered  by  Scheele,  and 
it  is  to  Berthollet  that  we  owe  the  practical  application  of  this 
important  property.  In  his  first  experiments  on  this  substance 
he  employed  chlorine  water,  but  afterwards  he  absorbed  the  gas 
by  a  solution  of  caustic  potash ;  and  the  liquor  thus  obtained, 
called  Eau  de  Javelles  from  the  name  of  a  bleach-works  where 
it  'was  prepared,  was  employed  for  bleaching  purposes  on  the  large 
scale.  Berthollet  described  these  experiments  to  James  Watt, 
who  was  at  that  time  staying  in  Paris,  and  he  brought  the  news 
to  Glasgow,  where  Tennant,  in  1798,  patented  an  improved 
process  for  bleaching,  in  which  lime  was  employed  instead  of  the 
potash,  as  being  a  much  cheaper  substance.1 

Up  to  the  year  1809-10,  when  Gay-Lussac  and  The'nard 
propounded  the  view  that  chlorine  might  be  considered  an 
element,  and  when  Davy  proved  that  this  supposition  was  correct, 
the  bleaching  liquors  were  supposed  to  contain  oxygenated  oiuri- 
ates  of  the  base.  Indeed  their  constitution  remained  doubtful 
until  the  year  1834,  when  Balard2  showed  that  the  alkaline 
bleaching  compounds  may  be  considered  to  be  a  mixture  or 
combination  of  a  chloride  and  a  hypochlorite.  Eau  de  Javelles 
therefore  contains  potassium  chloride  and  hypochlorite,  and 
bleaching-powder  solution  the  corresponding  calcium  salts  ; 
solid  bleaching-powder  has,  however,  a  different  constitution 
(Vol.  II.,  Pt.  I,  p.  194). 

Preparation. — Chlorine  monoxide  is  obtained,  as  seen  in 
Fig.  95,  by  the  action  of  dry  chlorine  gas  upon  cold  dry  oxide 
of  mercury,  which  is  contained  in  a  tube  (a  b),  which  should  be 
cooled  by  means  of  ice  or  a  stream  of  cold  water.3  The  crystal- 
lized mercuric  oxide  can,  however,  not  be  used  for  this  purpose, 
as  it  is  not  acted  on  by  dry  chlorine,  and  hence  the  precipitated 
oxide  must  be  employed,  it  having  been  previously  carefully 
washed  and  dried  at  300-400°.  The  reaction  which  takes  place  in 
this  case  has  already  been  described.  Mercuric  chloride,  HgCl2, 
is  not  formed  in  this  reaction,  but  the  oxychloride,  HgO,HgCl2. 

1  Tennant's  first  patent  was  declared  invalid  three  years  after  it  had  been 
granted,  as  it  was  proved  that  bleachers  in  Lancashire  and  at  Nottingham  had 
employed  lime  instead  of  potash  before  the  year  1798. 

2  Ann.  Chim.  Phys.  57,  225. 

3  V.  Meyer.  Ber.  16,  2999  ;  Ladenburg,  Ber.  17,  157. 


316 


THE  NON-METALLIC  ELEMENTS 


159  Properties. — Chlorine  monoxide  is  a  brownish  yellow 
coloured  gas,  which  has  a  peculiar  penetrating  smell,  somewhat 
resembling,  though  distinct  from,  that  of  chlorine.  Its  density 
is  43'5.1  By  exposure  to  a  low  temperature  the  gas  can  be  con- 
densed, as  in  the  tube  (D),  Fig.  95,  to  an  orange-coloured  liquid, 
which  boils  at  about  4-5°.  If  an  attempt  is  made  to  seal  up 
this  liquid  in  the  tube  in  which  it  has  been  prepared,  or  even  if 
the  tube  in  which  it  is  contained  be  scratched  with  a  file,  it  de- 
composes suddenly  with  a  most  violent  explosion  (Roscoe) ;  and 
when  poured  out  from  one  vessel  to  another  a  similar  explosion 


FIG.   95. 

takes  place  (Balard).  It  likewise  explodes  on  heating,  but  not 
so  violently,  two  volumes  decomposing  into  one  volume  of  oxygen 
and  two  of  chlorine.  According  to  Garzarolli-Thurnlackh  and 
Schacherl  it  does  not,  contrary  to  previous  statements,  undergo 
decomposition  in  direct  sunlight,  and  the  liquid,  if  all  organic 
matter  be  carefully  excluded,  may  be  distilled  without  decom- 
position. Most  easily  oxidizable  substances  and  many  finely 
divided  metals  take  fire  in  the  gas  and  produce  an  explosion  ; 
the  gas  is  also  decomposed  in  presence  of  hydrochloric  acid  into 
free  chlorine  and  water. 


C12O  -  2C12 


H20. 


2HC1 

1  Thurnlackh  and  Schacherl,  Annalen,  230,  273. 


HYPOCHLOROUS  ACID  317 

Chlorine  monoxide  is  readily  soluble  in  water,  the  latter  dis- 
solving 200  vols.  or  0-78  of  its  weight  of  the  gas  at  0°.  The 
solution  has  an  orange-yellow  colour. 

HYPOCHLOROUS  ACID.  HC1O. 

1  60  The  aqueous  solution  of  chlorine  monoxide  must  be  con- 
sidered as  a  solution  of  hypochlorous  acid,  a  compound  which 
in  the  pure  state  is  unknown. 

Preparation.  —  (1)  The  solution  is  best  prepared  by  shaking 
chlorine-water  with  precipitated  mercuric  oxide,  when  the  oxide 
quickly  dissolves  and  the  colour  of  the  solution  disappears, 
thus  :  — 

HgO  +  2C12  +  H20  =  HgCl2  +  2C10H. 

The  liquid  is  now  distilled  in  order  to  remove  the  mercuric 
chloride,  and  a  distillate  is  thus  obtained,  which,  although  it 
contains  only  half  as  much  chlorine  as  the  original  chlorine 
water,  possesses  an  equal  bleaching  power.1 

This  fact  is  expressed  in  the  following  equations,  which  also 
indicate  that  the  bleaching  effect  produced  by  chlorine  is  in 
reality  due  to  a  decomposition  of  water,  the  chlorine  combining 
with  the  hydrogen  and  liberating  the  oxygen.  It  is,  therefore, 
this  latter  element  which  is  the  true  bleaching  agent,  inasmuch 
as  it  oxidizes  and  destroys  the  colouring  agent. 

(a)  Bleaching  action  of  chlorine  water, 


(b)  Bleaching  action  of  the  hypochlorous  acid  formed  from 
the  chlorine  water, 


(2)  An  aqueous  solution  of  hypochlorous  acid  is  also  easily 
obtained  by  adding  to  a  solution  of  a  bleaching  compound 
exactly  the  amount  of  a  dilute  mineral  acid  requisite  to  liberate 
the  hypochlorous  acid  (Gay-Lussac).  For  this  purpose  a  dilute 
nitric  acid  containing  about  5  per  cent,  of  the  pure  acid  is 
allowed  to  run  slowly  from  a  tap-burette  into  a  filtered  solution 
of  common  bleaching-powder,  whilst  the  liquid  is  kept  well 
stirred  in  order  to  prevent  a  local  super-saturation,  which  would 

1  Gay-Lussac,  Ann.  Chim.  Phys.  43,  161. 


318  THE  NON-METALLIC  ELEMENTS 

cause  a  liberation  of  the  hydrochloric  acid  of  the  chloride,  and 
thus  again  effect  a  decomposition  of  the  hypochlorous  acid  into 
chlorine  and  water.  If  this  operation  be  conducted  with  care, 
no  chlorine  is  evolved,  or  at  any  rate  only  a  trace  if  a  slight 
excess  of  nitric  acid  has  been  added,  and  the  distillate  is 
perfectly  colourless.  Boric  acid  may  also  be  employed  with 
advantage  for  liberating  hypochlorous  acid  from  its  salts.1 

(3)  Another  method  of  obtaining  the  aqueous  acid  is  to  saturate 
a  solution  of  bleaching-powder  with  chlorine,  then  to  drive  off  the 
excess  of  chlorine  bypassing  a  current  of  air  through  the  liquid, 
and   then   to    distil.     The    following   equation    represents   the 
reaction  which  here  occurs  : — 

Ca(OCl)2  +  2C12  +  2H20  =  CaCl2  +  4C1OH. 

In  place  of  bleaching-powder  baryta- water  may  be  employed 
when  barium  hypochlorite  is  at  first  formed,  and  this  afterwards 
decomposed  as  shown  above.2 

(4)  Hypochlorous  acid  is  so  weak  an  acid  that  its  salts  are 
decomposed  by  carbonic  acid,  so  that  if  chlorine  gas  is  led  into 
a  solution  of  a  carbonate,  or  passed  through  water  containing 
finely  divided  calcium  carbonate  in  suspension  (1  part  to  40  of 
water),  no  hypochlorite    is   formed,  but  only  hypochlorous  acid 
(Williamson)  ;  thus  :— 

CaCO3  +  2C12  +  H2O  =  2C10H  +  CaCl2  +  CO2. 

Other  salts  of  the  alkali-metals  act  in  a  similar  way  when  a 
stream  of  chlorine  is  passed  through  their  aqueous  solutions  ; 
this  is  the  case  with  sulphate  and  phosphate  of  sodium,  in  these 
cases  an  acid  salt  is  formed  ;  thus : — 

Na2S04 + H20  +  C12  =  NaCl  +  NaHSO4 + C10H. 

Concentrated  aqueous  solutions  of  hypochlorous  acid  have  an 
orange  yellow  or  golden  yellow  colour,  and  an  odour  somewhat 
resembling  that  of  chloride  of  lime.  Only  dilute  solutions  of 
hypochlorous  acid  can  be  distilled  without  decomposition ;  con- 
centrated solutions  are  readily  decomposed  either  on  heating 
or  on  exposure  to  sunlight,  part  splitting  up  into  chlorine  and 
oxygen,  whilst  another  part  undergoes  oxidation,  yielding  chloric 
acid. 

1  Lauch,  Ber.  18.  2287.  2  Williamson,  Chem.  Soc.  Mem.  2,  234. 


THE  HYPOCHLORITES  319 

The  hypochlorites,  like  the  acid,  are  unstable  compounds, 
which  in  the  pure  state  are  almost  unknown.  Of  these  the  most 
important  is  formed  when  bleaching-powder  is  dissolved  in  water 
as  calcium  hypochlorite,  Ca(OCl)2,  although  it  is  probably  not 
present  as  such  in  the  solid  substance,  and  it  is  to  the  presence 
of  this  compound  that  the  bleaching  properties  of  the  solution 
are  due,  inasmuch  as  when  either  hydrochloric  or  sulphuric  acid 
is  added,  a  quantity  of  chlorine  eo]tial  to  that  contained  in  the 
compound  is  evolved.  In  the  first  case  half  the  chlorine  is  derived 
from  the  hypochlorite,  the  other  half  from  the  hydrochloric  acid, 
which  first  liberates  hypochlorous  acid,  and  then  decomposes  it 
into  chlorine  and  water  ;  thus  :  — 


(1)  2HCl  +  Ca(OCl)2  = 

(2) 


Tf  sulphuric  acid   is  used,  the  result  is  the  same,  as  this  acid 
decomposes  the  calcium  chloride  ;  thus  :  — 

CaCl2  +  Ca(OCl)2  +  2H2SO4  =  2CaSO4  +  2H2O  +  2C12. 

When  the  solutions  of  hypochlorites  are  heated,  they  undergo 
decomposition  into  chlorides  and  chlorates  in  the  following 
manner  : 

3KC10  =  2KC1  +  KC1O3. 

Hence  when  chlorine  is  passed  into  a  hot  solution  of  an  alkali 
the  product  is  a  mixture  of  chloride  and  chlorate. 


CHLOROUS  ACID  AND  THE  CHLORITES. 

161  This  acid,  like  chlorine  trioxide,  is  not  known  in  the 
free  state,  but  the  chlorites  can  be  prepared  by  adding  potassium 
hydroxide  to  an  aqueous  solution  of  chlorine  peroxide,  a  mixture 
of  potassium  chlorate,  and  potassium  chlorite,  KC1O2,  being 
obtained.1  The  chlorites  of  the  alkali  metals  are  soluble  in 
water,  and  from  their  solutions  the  insoluble,  or  difficultly  soluble 
chlorites  of  silver,  AgClO2,  and  of  lead,  Pb(ClO2)2,  may  be 
prepared  by  double  decomposition,  as  yellow  crystalline  powders. 
All  the  chlorites  are  very  easily  decomposed.  Thus  if  the  lead- 

1  Garzarolli-Thurnlackh  and  v.  Hayn,  Annalen,  209,  203. 


320  THE  NON-MET ALLIG  ELEMENTS 

salt  is  heated  for  a  short  time  to  100°,  it  decomposes  with 
detonation  ;  and  if  it  is  rubbed  in  a  mortar  with  sulphur  or 
certain  metallic  sulphides,  ignition  occurs.  The  soluble  chlorites 
possess  a  caustic  taste,  and  bleach  vegetable  colouring  matters, 
even  after  addition  of  arsenious  acid.  This  latter  reaction 
serves  to  distinguish  them  from  the  hypochlorites. 


CHLORINE  PEROXIDE.    C1O2. 

162  This  gas  was  first  prepared  and  examined  by  Davy  in 
1815  ;  it  was  obtained  by  him  by  the  action  of  strong  sulphuric 
acid  on  potassium  chlorate.  In  preparing  this  substance  special 
precautions  must  be  taken,  as  it  is  a  highly  explosive  and 
dangerous  body. 

Preparation. — Pure  powdered  potassium  chlorate  is  for  this 
purpose  thrown  little  by  little  into  concentrated  sulphuric  acid 
contained  in  a  small  retort.  After  the  salt  has  dissolved,  the  retort 
is  gently  warmed  in  warm  water.  In  this  reaction  chloric  acid 
is  in  the  first  instance  liberated,  and  then  decomposes  as  follows 
into  perchloric  acid,  chlorine  peroxide  and  water ;  thus  : — 

3HC103  =  HC104  +  2C102 + H2O. 

Properties. — The  heavy  dark  yellow  gas  thus  given  off  must 
be  collected  by  displacement,  as  it  decomposes  in  contact  with 
mercury  and  is  soluble  in  water ;  it  possesses  a  peculiar  smell, 
resembling  that  of  chlorine  and  burnt  sugar.  When  exposed  to 
cold  the  gas  condenses  to  a  dark -red  liquid,  which  boils1  at +  9°, 
and  at  —  79°  freezes  to  an  orange-coloured  crystalline  mass.  Its 
vapour  density  corresponds  to  the  formula  C1O2,2  and  the  double 
formula,  C12O4,  previously  employed  is  therefore  incorrect.  The 
gaseous,  and  especially  the  liquid  and  solid  peroxide  undergo 
sudden  decomposition,  frequently  exploding  most  violently, 
hence  their  preparation  requires  extreme  care.  According  to 
Schacherl,  liquid  chlorine  peroxide  may  be  distilled  without 
decomposition  if  every  trace  of  organic  matter  be  excluded.3 

In  order  to  obtain  an  aqueous  solution  of  the  gas,  a  mixture  of 
potassium  chlorate  and  oxalic  acid  may  be  heated  in  a  water 
bath  to  70°,  the  mixture  of  chlorine  peroxide  and  carbon  dioxide 

1  Pebal,  AnnaUn,  177,  1. 

2  Pebal  and  Schacherl,  Annalen,  213,  H3. 

3  Annalen,  206,  68. 


CHLORIC  ACID  AND  THE  CHLORATES  321 

then  evolved  being  passed  into  water.1     The  following  equation 
represents  the  action  here  occurring  :  — 


2KC10 


Chlorine  peroxide  can  be  preserved  without  change  in  the  dark  ; 
it  is,  however,  slowly  decomposed  into  its  elementary  con- 
stituents when  exposed  to  light,  and  this  decomposition  takes 
place  quickly  and  with  explosion  when  an  electric  spark  is 
passed  through  the  gas,  two  volumes  of  the  gas  yielding  one 
volume  of  chlorine  and  two  of  oxygen. 

When  phosphorus,  ether,  sugar,  or  other  easily  combustible 
substances  are  thrown  into  the  gas  they  take  fire  spontaneously. 
This  oxidizing  action  of  chlorine  peroxide  is  well  illustrated  by 
the  following  experiments.  About  equal  parts  of  powdered 
white  sugar  and  chlorate  of  potash  in  powder  are  carefully 
mixed  together  with  a  feather  on  a  sheet  of  writing  paper, 
the  mixture  then  brought  on  a  plate  or  stone  placed  in  a  draught 
chamber,  and  a  single  drop  of  strong  sulphuric  acid  allowed 
to  fall  upon  the  mixture,  when  a  sudden  ignition  of  the  whole 
mass  occurs.  This  is  caused  by  the  liberation  of  chlorine 
peroxide,  which  sets  fire  to  a  particle  of  sugar,  and  the  ignition 
thus  commenced  quickly  spreads  throughout  the  mass,  and 
the  sugar  is  all  burnt  at  the  expense  of  the  oxygen  of  the 
chlorate.  The  combustion  of  phosphorus  can  be  brought  about 
under  water  by  a  similar  reaction  :  for  this  purpose  some  crystals 
of  chlorate  of  potassium  and  a  few  small  lumps  of  yellow 
phosphorus  are  thrown  into  a  test  glass  half  filled  with  water, 
and  a  small  quantity  of  strong  sulphuric  acid  allowed  to  flow 
through  a  tube  funnel  to  the  lower  part  of  the  glass  where  the 
solids  lie.  As  soon  as  the  acid  touches  the  chlorate,  chlorine 
peroxide  is  evolved,  and  this  gas  on  coming  in  contact  with 
the  phosphorus  oxidizes  it,  and  bright  flashes  of  light  are 
given  off. 

Water  at  4°  dissolves  about  twenty  times  its  volume  of 
chlorine  peroxide  gas,  forming  a  bright  yellow  solution,  whilst  at 
lower  temperatures  a  crystalline  hydrate  is  formed.  If  this 
aqueous  solution  is  saturated  with  an  alkali,  a  mixture  of  chlorite 
and  chlorate  is  formed  ;  thus  :  — 

2KOH  +  2C1O2  =  KC1O2  +  KC103  +  H20. 

1  Calvert  and  Davies,  Journ.  Chem.  Soc.  2,  193. 
22 


322  THE  NON-METALLIC  ELEMENTS 

When  potassium  chlorate  is  treated  with  hydrochloric  acid  a 
yellow  gas  is  evolved,  first  prepared  by  Davy,  considered  by  him 
to  be  a  distinct  oxide  of  chlorine,  and  termed  Euchlorinc,  It 
has,  however,  been  shown  by  Pebal1  that  this  body  is  a  mixture 
of  free  chlorine  and  chlorine  peroxide  in  varying  proportions. 
This  mixture  possesses  even  more  powerful  oxidizing  properties 
than  chlorine  itself,  and  is  therefore  largely  used  as  a  disinfect- 
ant, being  not  only  very  effective  in  removing  by  oxidation 
putrescent  matter  in  the  air,  but  being  at  the  same  time  very 
easily  prepared. 

CHLORIC  ACID.    HC1O3. 

163  Chloric  acid  is  the  most  important  member  of  the  series 
of  chlorine  oxy-acids.     It  was  discovered  by  Berthollet  in  1786, 
and  it  is  obtained  when  the  lower  acids  or  aqueous  solutions  of 
the  oxides  of  chlorine  are  exposed  to  light. 

Preparation. — Chloric  acid  is  best  prepared  by  decomposing 
barium  chlorate  with  an  equivalent  quantity  of  pure  dilute 
sulphuric  acid  (Gay-Lussac,  1814) ;  thus: — 

Ba(C103)2  +  H2SO4  =  BaS04  +  2HC103. 

The  clear  solution  of  chloric  acid  must  be  poured  off  from  the 
deposited  precipitate  of  barium  sulphate,  and  carefully  evaporated 
in  vacuo  over  strong  sulphuric  acid.  The  residue  thus  prepared 
contains  forty  per  cent,  of  pure  chloric  acid  corresponding  to  the 
formula  HC1O3  -f  7H2O.  When  attempts  are  made  to  concen- 
trate the  acid  beyond  this  point,  the  chloric  acid  undergoes 
spontaneous  decomposition  with  rapid  evolution  of  chlorine  and 
oxygen  gases,  and  formation  of  perchloric  acid.  Chloric  acid 
can  also  be  prepared  by  decomposing  potassium  chlorate  with 
hydrofluosilicic  acid,  H2SiF6,  when  insoluble  potassium  fluosi- 
licate,  K2SiF6,  is  precipitated,  and  the  chloric  acid  remains  in 
solution  together  with  an  excess  of  hydrofluosilicic  acid.  This 
can  be  removed  by  the  addition  of  a  little  silica  and  by  subse- 
quent evaporation  when  the  fluorine  passes  away  as  gaseous 
tetrafluoride  of  silicon,  SiF4,  and  the  pure  chloric  acid  can  be 
poured  off  from  the  silica,  which  settles  as  a  powder  to  the 
bottom  of  the  vessel. 

164  Properties. — The  acid  obtained  in  this  way  in  the  greatest 
state  of  concentration  does  not  rapidly  undergo  change  at  the 

i  Annalen,  177,  1. 


CHLORIC  ACID  323 


ordinary  temperature,  but  it  forms  perchloric  acid  on  standing  for 
some  time  exposed  to  light.  Organic  bodies  such  as  wood  or  paper 
decompose  the  acid  at  once,  and  are  usually  so  rapidly  oxidized 
as  to  take  fire.  Aqueous  chloric  acid  is  colourless,  possesses 
a  powerful  acid  reaction  and  a  pungent  smell,  and  bleaches 
vegetable  colours  quickly.  It  is  a  monobasic  acid,  that  is,  it 
contains  only  one  atom  of  hydrogen  capable  of  replacement  by 
a  metal,  with  the  formation  of  salts. 

The  Chlorates. — Of  these  salts  potassium  chlorate  (or  chlorate 
of  potash),  KC1O3,  is  the  most  important.  It  is  easily  formed  by 
passing  chlorine  in  excess  into  a  hot  solution  of  caustic  potash ; 
thus : — 

3C12  +  6KOH  =.  5KC1  +  KC1O3+3H2O. 

The  chlorate  is  much  less  soluble  in  water  than  the  chloride 
formed  at  the  same  time,  so  that  by  concentrating  the  solution 
the  chlorate  is  deposited  in  tabular  crystals,  which  may  be 
purified  from  adhering  chloride  by  a  second  crystallization. 
Other  chlorates  can  be  prepared  in  a  similar  way  ;  thus,  for 
instance,  calcium  chlorate  is  obtained  by  passing  a  current  of 
chlorine  into  hot  milk  of  lime  when  the  following  reaction 
occurs : — 

6C12  +  6  Ca(OH)2=Ca(ClOs)2  +  5 CaCl2  +  6H20. 

All  the  chlorates  are  soluble  in  water,  and  many  deliquesce 
on  exposure  to  the  air.  The  potassium  salt  is  one  of  the  least 
soluble  of  these  salts,  100  parts  by  weight  of  water  at  0°  dis- 
solving about  3'3  parts  of  this  salt,  whilst  water  at  15°  dissolves 
twice  this  amount.  By  the  action  of  reducing  agents  such  as 
nascent  hydrogen  or  sulphur  dioxide,  chlorates  lose  the  whole  of 
their  oxygen  and  are  converted  into  chlorides.  A  chlorate  is 
recognized  by  the  following  tests  : — 

(1)  Its  solution  yields  no  precipitate  with  silver  nitrate,  but 
on  ignition  the  salt  gives  off  oxygen  gas,  and  a  solution  of 
the   residual   salt   (a   chloride)    gives  a   white  precipitate   on 
addition  of  silver  nitrate  and  nitric  acid. 

(2)  To  the  solution  of  the  chlorate  a  few  drops  of  indigo 
solution  are  added,  the  liquid  acidulated  with  sulphuric  acid, 
and  sulphurous  acid   (or  sodium  sulphite  dissolved  in  water) 
added  drop  by  drop.     If  a  chlorate  be  present  the  blue  colour  is 
discharged,  because  the  chloric  acid    is   reduced   to   a   lower 
oxide. 


324  THE  NON-METALLIC  ELEMENTS 

(3)  Dry  chlorates  treated  with  strong  sulphuric  acid  yield  a 
yellow  explosive  gas  (C1O2). 

The  composition  of  the  chlorates  has  been  very  carefully 
determined  by  Stas l  and  Marignac.2  The  following  numbers 
give  the  percentage  composition  of  silver  chlorate  according  to 
the  analyses  of  Stas : — 

Chlorine 18-5257 

Oxygen 25'0795 

Silver  .  56'3948 


100-000 


PERCHLORIC  ACID.    HC1O4. 

165  This  acid  was  discovered  by  Stadion  in  1816  ;  it  is  formed 
by  the  decomposition  of  chloric  acid  on  exposure  to  heat  or 
light  ;  thus  :  — 

3HC103  =  HC104  +  C12  +  202  +  H20. 

It  is  best  prepared  from  potassium  perchlorate,  which  can  be 
obtained  in  any  quantity  from  the  chlorate.  We  have  already 
remarked  under  oxygen,  that  when  potassium  chlorate  is 
heated  the  fused  mass  slowly  gives  off  oxygen,  and  a  point  is 
reached  at  which  the  whole  mass  becomes  nearly  solid,  owing 
to  the  formation  of  perchlorate,  together  with  potassium 
chloride. 

10KC1O  = 


The  mass  is  then  allowed  to  cool,  powdered,  and  well  washed 
with  water,  to  remove  the  greater  part  of  the  chloride  formed. 
In  order  to  get  rid  of  the  unaltered  chlorate,  the  crystalline 
powder  is  gently  heated  with  hydrochloric  acid  so  long  as 
chlorine  and  chlorine  peroxide  gases  are  evolved  :  a  subsequent 
washing  with  water  removes  the  remainder  of  the  chloride,  and 
the  pure,  sparingly-soluble  perchlorate  is  left. 

Preparation.  —  In  order  to  prepare  perchloric  acid,  the  pure  dry 
potassium  salt  is  distilled  in  a  small  retort  with  four  times  its 
weight  of  concentrated  (previously  boiled)  sulphuric  acid.  At  a 
temperature  of  110°  dense  white  fumes  begin  to  be  evolved,  whilst 
a  colourless  or  slightly  yellow  liquid,  consisting  of  pure  perchloric 

1  Nouvelles  Recherches  Chiiniques  sur  les  Lois  des  Proportions,  208. 

2  Bibl.  Univ.  45,  347. 


ANALYSIS  OF  PERCHLORIC  ACID  325 

acid,HC!O4,  distils  over(Roscoe).1  If  the  distillation  be  continued, 
this  liquid  gradually  changes  into  a  white  crystalline  mass, 
having  the  composition  HClO4-fH2O.  The  formation  of  this 
latter  body  can  be  readily  explained  ;  a  portion  of  the  pure  per- 
chloric acid  splits  up  during  the  distillation  into  the  lower  oxides 
of  chlorine,  oxygen  and  water,  which  latter  combines  with  the 
pure  acid  already  formed.  When  the  crystalline  hydrate  is  again 
heated  it  decomposes  into  the  pure  acid,  which  distils  over,  and 
into  an  aqueous  acid  which  boils  at  203°,  and  therefore  remains 
behind  in  the  retort.  This  reaction  is  employed  in  the  prepara- 
tion of  the  pure  acid,  HC1O4,  as  that  obtained  by  the  first  pre- 
paration is  generally  rendered  impure  by  sulphuric  acid  carried 
over  mechanically. 

An  aqueous  solution  of  the  acid  may  be  readily  prepared 
by  the  addition  of  hydrofluosilicic  acid  to  a  solution  of  the 
potassium  salt,  and  is  sometimes  used  for  the  estimation  of 
potassium.2 

Properties. — Pure  perchloric  acid  is  a  crystalline  substance 
which  melts  at  15°,3  but  is  usually  obtained  as  a  volatile  colour- 
less or  slightly  yellow  mobile  liquid,  having  at  15°'5  a  specific 
gravity  of  T782.  It  is  strongly  hygroscopic,  quickly  absorbing 
moisture  from  the  air,  and  emitting  dense  white  fumes  of 
the  hydrated  acid.  When  poured  or  dropped  into  water  it 
dissolves,  combining  with  the  water  so  vigorously  as  to  cause 
a  loud  hissing  sound  and  a  considerable  evolution  of  heat.  A 
few  drops  thrown  upon  paper  and  wood  cause  an  instantaneous 
and  almost  explosive  inflammation  of  these  bodies ;  and  if  the 
same  quantity  be  allowed  to  fall  upon  dry  charcoal,  the  drops 
decompose  with  an  explosive  violence  which  is  almost  equal 
to  that  observed  in  the  case  of  chloride  of  nitrogen.  If  the 
pure  acid,  even  in  very  small  quantity,  come  in  contact  with 
the  skin  it  produces  a  serious  wound,  which  does  not  heal  for 
months.  Perchloric  acid  undergoes  decomposition  on  distilla- 
tion ;  the  originally  nearly  colourless  acid  becomes  gradually 
darker,  until  it  attains  the  tint  of  bromine,  and  at  last  suddenly 
decomposes  with  a  loud  explosion.  The  composition  of  the 
substance  which  is  here  formed  is  unknown.  The  pure  acid  also 
undergoes  spontaneous  and  explosive  decomposition  when  pre- 
served for  some  days  even  in  the  dark. 

The  methods  employed  in  fixing  the  composition  of  this  acid 

1  Journ.  Chem.  Soc.  1863,  82.  2  Zeit.  angew.  Chem.  1893,  68. 

3  Berthelot,  Compt.  Mend.  93,  240. 


326  THE  NON-METALLIC  ELEMENTS 


may  here  be  referred  to  as  illustrating  the  mode  by  which  the 
quantitative  analysis  of  similar  bodies  is  carried  out. 

A  quantity  of  the  pure  acid  (HC1O4)  is  sealed  up  in  a 
small  glass  bulb  (Fig.  96),  whose  weight  has  been  previously 
ascertained,  and  the  bulb  and  acid  carefully  weighed.  The 
sealed  points  of  the  tube  are  then  broken,  the  acid  diluted 
with  water,  and  the  aqueous  solution  saturated  with  a  slight 
excess  of  solution  of  potassium  carbonate.  Acetic  acid  is 
next  added  in  slight  excess,  and  the  whole  evaporated  to 
dryness  on  a  water  bath.  The  potassium  acetate  being 
soluble,  and  the  potassium  perchlorate  being  insoluble  in 
absolute  alcohol,  the  whole  of  the  latter  salt  formed  by  the 
neutralisation  of  the  acid  is  obtained  in  the  pure  state  by 
washing  the  dry  mass  with  absolute  alchohol  and  drying  the 
residue  at  100°.  Thus  it  was  found  that  0'7840  gram  of 
the  acid  thus  treated  yielded  1*0800  gram  of  potassium  salt 
corresponding  to  0'7837  gram  of  pure  perchloric  acid,  or  the 


FIG.  96. 

acid  under  analysis  contained  99'85  per  cent,  of  HC104,  provided, 
of  course,  that  the  salt  obtained  really  had  the  composition 
indicated  by  the  formula,  KC1O4.  In  order  to  obtain  evidence 
on  this  point,  the  quantities  of  oxygen,  chlorine,  and  potassium 
contained  in  the  salt  were  determined  as  follows  : — 

(1)  0*9915  gram  of  the  dry  salt  was  mixed  with  pure  dry 
oxide  of  iron,  and  the  mixture  heated  in  a  long  tube  of  hard 
glass,  the  weight  of  which,  when  thus  filled,  was  determined. 
The  presence  of  the  oxide  of  iron  enabled  the  perchlorate  to 
yield  up    its  oxygen  at  a  lower    temperature    than   it   would 
have  done  if  heated  alone.     The  loss  of  weight  which  the  tube 
experiences  on  heating  represents  the  total  weight  of  oxygen 
contained  in  the  salt ;  in  this  case  it  amounted  to  0*4570  gram. 

(2)  The    residue  is  next  completely  exhausted  with  warm 
water,  and  the  chlorine  precipitated  in  the  solution  as  silver 
chloride ;  in  the   above    analysis   0'7683  gram    of  pure   silver 
was  needed  for  complete  precipitation,  and  this  corresponds  to 
40 '25 2  gram  of  chlorine. 

(3)  0*3165  gram  of  the  salt  was  next  carefully  heated  with 
an  excess  of   pure  sulphuric    acid,    and   the   residue    strongly 


PERCHLORATES.  327 


ignited.     In  this  way  the  potassium  perchlorate  is   converted 
into  sulphate,  which  was  found  to  weigh  0'2010  gram. 

From  these  numbers  the  percentage  composition  of  the  salt 
can  be  easily  obtained,  and  the  results,  as  shown  below,  are 
found  to  correspond,  within  the  unavoidable  errors  of  experiment, 
with  the  numbers  calculated  from  the  formula  : — 

Analysis  of  Potassium  Perchlorate. 

Calculated.  Found. 

Chlorine  ...  Cl  35'19  25'58  25'46 
Oxygen  ...  O4  63'52  46'18  46'09 
Potassium .  K  38'84  28'24  28-58 


137-55       100-00       100-13 


166  Hydrates  of  Perchloric  Acid. — The  monohydrate,  HC104-f 
H2O,  whose  mode  of  formation  has  been  mentioned,  is  obtained 
in  the  pure  state  by  the  careful  addition  of  water  to  the  pure 
acid,  HC104,  until  the  crystals  make  their  appearance.  This 
substance,  discovered  by  Serullas,  was  formerly  supposed  to  be  the 
pure  acid  ;  it  melts  at  50°  and  solidifies  at  this  temperature  again 
in  colourless  needle-shaped  crystals,  often  several  inches  in  length. 
The  liquid  emits  dense  white  fumes  on  exposure  to  the  air,  and 
oxidizes  paper,  wood,  and  other  organic  bodies  with  rapidity. 

As  has  been  stated,  the  monohydrate  decomposes  at  a  higher 
temperature  into  the  pure  acid,  and  a  thick  oily  liquid,  which 
possesses  a  striking  resemblance  to  sulphuric  acid,  boils  at  203°, 
and  has  a  specific  gravity  of  1'82.  This  liquid  contains  71 '6 
per  cent,  of  HC104,  and  does  not  correspond  to  any  definite 
hydrate.  An  acid  of  the  same  composition,  and  possessing  the 
same  constant  boiling  point,  is  obtained  when  a  weaker  acid  is 
distilled,  the  residue  then  -becomes  more  and  more  concentrated, 
until  the  above  composition  and  boiling  point  is  reached. 
Aqueous  perchloric  acid,  therefore,  exhibits  the  same  relations 
in  this  respect  as  the  other  aqueous  acids. 

Perchlorates. — Perchloric  acid  is  a  powerful  monobasic 
acid,  forming  a  series  of  salts,  termed  the  perchlorates,  which 
are  all  soluble  in  water,  and  a  few  of  which  are  deliquescent. 
Potassium  perchlorate,  KC1O4,  and  rubidium  perchlorate, 
RbC104,  are  the  least  soluble  of  the  salts,  one  part  of  the  former 
dissolving  in  58,  and  the  latter  requiring  92  parts  of  water  at 
21°  for  solution.  Both  these  salts  are  almost  insoluble  in 


328  THE  NON-METALLIC  ELEMENTS 

absolute  alcohol,  and  they  may  be,  therefore,  employed  for  the 
quantitative  estimation  of  the  metals. 

The  perchlorates  are  distinguished  from  the  chlorates  by  the 
following  reactions : — 

(1)  They  undergo  decomposition  at  a  higher  temperature  than 
the  chlorates. 

(2)  They  are  not  acted  upon  by  hydrochloric  acid. 

(3)  They  do  not  yield  an  explosive  gas,  C1O2,  when  heated 
with  strong  sulphuric  acid. 

(4)  They  are  not  reduced  to  chlorides  by  sulphur  dioxide. 

167  Constitution  of  the  Oxy -acids  of  Chlorine. — The  con- 
stitution of  these  acids  has  frequently  been  the  subject  of  discus- 
sion, and  cannot  as  yet  be  regarded  as  definitely  settled.  On 
the  assumption  that  chlorine  always  acts  as  a  monovalent,  and 
oxygen  as  a  divalent  element,  the  following  formulae  are  the 
only  possible  ones  for  these  acids  : — 

Hypochlorous  acid,  H — 0 — Cl. 
Chlorous  acid,  H— O— 0— Cl. 
Chloric  acid,  H— 0— O— O— Cl. 
Perchloric  acid,  H— 0— 0— 0— 0— Cl. 

It  has  however  been  found,  especially  in  the  case  of  the  carbon 
compounds,  that  substances  containing  oxygen  atoms  united 
together  in  this  manner  become  more  unstable  as  the  number 
of  oxygen  atoms  increases,  whilst  with  the  oxy-acids  of  chlorine 
the  contrary  is  the  case,  hypochlorous  acid  being  the  most  and 
perchloric  acid  the  least  unstable. 

Another  theory  is  that  chlorine  behaves  as  a  monad  in  hypo- 
chlorous  acid,  a  triad  in  chlorous  acid,  a  pentad  in  chloric  acid, 
and  a  heptad  in  perchloric  acid,  the  constitutional  formulae 
being  as  follows  : — 

/"^l  f~\     TT 

0=C1-0— H 

O 

\C1— 0— H 

0 

II 

0=C1— 0— H 

.5 


OXY-ACIDS  OF  BROMINE.  329 

This  formula  for  perchloric  acid  readily  explains  the  existence 
of  the  hydrate  of  perchloric  acid,  HC1O4,  H2O,  the  constitution 
of  which  would  then  be  represented  by  the  formula — 

O 

HO,    || 

\C1— 0— H 
HO/  || 
O 

A  substance  possessing  this  formula  should  from  analogy  be- 
have as  a  polybasic  acid  (compare  the  phosphoric  acids),  but 
hitherto  no  chemical  or  physical  evidence  of  the  polybasic 
nature  of  perchloric  acid  has  been  obtained,  all  the  salts  cor- 
responding to  the  formula,  HC1O4,  and  the  same  holds  true 
for  chloric  acid.  On  the  whole,  however,  the  balance  of  the 
evidence  is  in  favour  of  the  second  view,  which  is  also  in  agree- 
ment with  the  results  obtained  for  periodic  acid  (p.  334). 

Chlorine  peroxide  has  been  definitely  proved  to  have  the 
molecular  formula  C102,  and  in  this  compound  chlorine  must  be 
regarded  as  a  tetrad ;  this  fact  is  also  in  favour  of  the  view 
that  the  valency  of  chlorine  varies  in  its  different  compounds 
with  oxygen. 


OXYGEN  AND  BROMINE. 

OXY-ACIDS  OF  BROMINE. 

1 68  No  compound  of  bromine  and  oxygen  has  as  yet  been 
obtained,  but  oxy-acids  corresponding  to  those  of  chlorine  are 
known;  viz: — 

Hypobromous  acid,  HBrO. 

Bromic  acid,  HBr03. 


HYPOBROMOUS  ACID,  HBrO. 

This  acid  together  with  its  salts,  termed  the  hypobromites,  are 
formed,  in  a  similar  manner  to  hypochlorous  acid,  by  the  action 
of  bromine  on  certain  metallic  oxides  (Balard).  Thus  if  bromine 
water  be  shaken  up  with  mercuric  oxide,  and  if  the  yellow  liquid 


330  THE  NON-METALLIC  ELEMENTS 

thus  formed  be  treated  successively  with  bromine  and  the  oxide, 
a  solution  is  obtained  which  contains  in  every  100  cc.  6'2  per 
cent,  of  bromine  combined  as  hypobromous  acid,  the  reaction 
being  as  follows  :  — 

HgO  +  2Br2  +  H2O  =  2HOBr  +  HgBr2. 

The  greater  part  of  the  hypobromous  acid  contained  in  this 
strong  solution  is  decomposed  on  distillation  into  bromine  and 
oxygen.  It  can,  however,  be  distilled  in  vacua  at  a  temperature 
of  40°  without  undergoing  this  change.1 

Aqueous  hypobromous  acid  is  a  light  straw-yellow  coloured 
liquid,  closely  resembling  in  its  properties  hypochlorous  acid, 
acting  ,as  a  powerful  oxidizing  agent  and  bleaching  organic 
colouring  matters. 

By  the  action  of  bromine  on  lime,  a  substance  similar  to 
bleaching  powder  is  formed  and  this  salt  is  termed  bromide 
of  lime.2 

BROMIC  ACID,  HBr03. 

169  When  bromine  is  dissolved  in  hot  caustic  potash  or 
soda,  a  colourless  solution  is  produced  which  contains  a  mixture 
of  a  bromide  and  a  bromate  ;  thus  :  — 


The  difficultly  soluble  potassium  bromate  maybe  easily  separated 
by  crystallization  from  the  very  soluble  bromide.  Potassium 
bromate  is  also  formed  when  bromine  vapour  is  passed  into 
a  solution  of  potassium  carbonate  which  has  been  saturated  with 
chlorine  gas. 

Preparation  —  Free  bromic  acid  is  formed  when  chlorine'  is 
passed  into  bromine  water  ;  thus  :  — 

Br2  +  5C12  +  6H2O  =  2HBr03  +  1  OHC1. 

The  acid  is,  however,  best  obtained  by  the  decomposition  of 
the  slightly  soluble  silver  bromate.  This  salt  is  thrown  down 
on  the  addition  of  nitrate  of  silver  to  a  solution  of  a  soluble 
bromate  ;  the  precipitate  thus  prepared  is  well  washed  with 

1  Dancer,  Journ.  Chem.  Soc.  1862,  477.  2  Berzelius,  Jahresb.  10,  130. 


IODINE  PENTOXIDE  AND  IODIC  ACID  331 

water  and  then  treated  with  bromine ;   bromic  acid  remains  in 
solution  and  the  insoluble  silver  bromide  is  thrown  down;  thus  : — 

5  AgBrOs  +  3Br2  +  3H2O  =  5  AgBr  +  6HBr03. 

Properties — Obtained  according  to  the  foregoing  methods, 
bromic  acid  is  a  strongly  acid  liquid  reddening  and  ultimately 
bleaching  litmus  paper.  On  concentration  at  100°  the  aqueous 
acid  decomposes  into  bromine  and  oxygen,  and  it  is  at  once 
decomposed  by  reducing  agents  such  as  sulphur  dioxide  and 
sulphuretted  hydrogen,  as  also  by  hydrobromic  acid,  the  following 
reactions  taking  place  : — 

(1 )  2HBrO3  +  5SO9  +  4H2O  -  Br2  +  5H2S04. 

(2)  2HBrO3  +  5SH2  =  Br2  +  6H2O  +  5S. 

(3)  HBrO3  +  5HBr  =  3Br2  +  3H2O. 

Hydrochloric  and  hydriodic  acids  decompose  bromic  acid  in 
a  similar  manner  with  formation  of  the  chloride  or  iodide  of 
bromine. 

The  bromates  are  as  a  rule  difficultly  soluble  in  water,  and 
decompose  on  heating  into  oxygen  and  a  bromide,  but  unlike 
the  chlorates  no  perbromate  is  formed  in  the  process. 


PEKBROMIC  ACID, 

This  substance  is  stated  by  Kammerer1  to  be  formed  by  the 
action  of  bromine  on  dilute  perchloric  acid,  the  bromine 
liberating  chlorine.  Other  observers  have,  however,  failed  to 
obtain  the  substance  by  this  means,  and'  the  existence  of  the 
acid  and  of  its  salts  is,  therefore,  more  than  doubtful.2 


OXYGEN     AND     IODINE. 

OXIDE  AND  OXY-ACIDS  OF  IODINE. 

170  Only  one  oxide  of  iodine  is  known  with  certainty.  This 
is  the  pentoxide  I2O5,  which  unites  with  water  to  form  iodic 
acid,  HIO3.  Besides  these,  hyd rated  periodic  acid,  HIO4+  2H2O 
is  known  and  hypoiodous  acid  HIO  appears  to  exist  in  solution. 

1  J.  Pr.  Chem.  90,  190. 

2  Muir,    Journ.   Chem.    Soc.    1876,  ii.    469;   Wolfram,    Annalen,    198,    95; 
Mac  Ivor,  Chem.  News,  33,  35  ;  55,  203. 


332  THE  NON-METALLIC  ELEMENTS 


HYPOIODOTJS  ACID,  HIO. 

When  an  alcoholic  solution  of  iodine  is  treated  with  freshly 
precipitated  mercuric  oxide,  a  yellow  solution  is  formed  which 
does  not  turn  starch  blue  at  once,  but  only  after  a  time  ;  the 
solution  is  then  found  to  contains  mercuric  iodide  and  iodate. 
It  is  probable  that  the  solution  contains  hypoiodous  acid,  which 
soon  undergoes  decomposition  into  free  iodine  and  iodic  acid.1 
An  aqueous  solution  of  iodine  also  yields  with  alkalis  a  solution 
having  a  peculiar  odour,  which  possesses  bleaching  properties, 
and  probably  also  contains  a  hypoiodite.2  Lunge  and  Schoch, 
by  the  action  of  iodine  on  slaked  lime  and  water  at  the  ordinary 
temperature,  obtained  a  substance  having  a  peculiar  odour,  and 
resembling  chloride  of  lime  in  its  general  properties.  It  has 
probably  the  formula  CaOI2  or  Ca(IO)2  +  CaI2.3 


IODINE  PENTOXIDE,  I205  AND  IODIC  ACID,  HI03. 

171  This  acid  was  discovered  by  Davy  in  the  form  of  potas- 
sium iodate,  which  he  obtained  by  the  action  of  iodine  on 
caustic  potash ;  thus : — 

3I2  +  6KOH  =  5KI  +  KI03  +  3H2O. 

Preparation. — (1)  Free  iodic  acid  is  best  obtained  by  dissolv- 
ing iodine  in  pure  boiling  concentrated  nitric  acid,  which  oxidizes 
it  as  follows : — 

3I2+10HN03  =  6HI03+10NO  +  2H20. 

For  this  purpose  1  part  of  iodine  is  heated  in  a  retort  with  10 
parts  of  the  acid  until  the  whole  of  the  iodine  is  dissolved,  and 
no  further  evolution  of  red  fumes  takes  place.  The  solution  is 
then  evaporated  and  the  residue  heated  to  200°  until  every  trace 
of  nitric  acid  is  removed.  The  iodic  acid  thus  loses  water,  and 
a  white  powder  of  iodine  pentoxide  I2O5  is  obtained.  It  has  a 
specific  gravity  of  4'487,  and  when  heated  to  300°  decomposes 
into  iodine  and  oxygen.  This  substance  is  very  soluble  in 
water,  dissolving  with  evolution  of  heat  and  from  the  thick 

1  Kone,  Pogg.  Ann.  66,  802  ;  Compt.  Rend.  63,  968. 

2  J.  Pr.  Chem.  [I],  84,  385.  8  Ber.  15,  1883. 


CONSTITUTION  OF  IODIC  ACID  333 

syrupy   solution  thus  obtained  rhombic  crystals  of  iodic  acid, 
HIO3,  are  deposited. 

(2)  Iodic  acid  can  also  be  obtained  by  the  action  of  dilute  sul- 
phuric acid  on  barium  iodate,  which  is  prepared  as  follows  :  the 
requisite  quantity  of  iodine  is  dissolved  in  a  hot  concentrated 
solution  of  potassium  chlorate  and  a  few  drops  of  nitric  acid 
added;  immediately  a  violent  evolution  of  chlorine  gas  com- 
mences, and,  on  cooling,  the  potassium  iodate  crystallizes  out. 
This  salt  is  then  dissolved  in  water  and  barium  chloride  added 
to  the  solution,  when  barium  iodate  separates  out  as  a  white 
powder.  The  potassium  iodate  may  also  be  obtained  by  very 
carefully  heating  a  mixture  of  2  mols.  potassium  chlorate 
and  1  mol.  iodine,  a  simple  metathesis  taking  place.1 


(3)  Iodic  acid  is  likewise  formed  when  chlorine  is  passed  into 
water  in  which  iodine  in  powder  is  suspended  ;  thus  :  — 

I2  +  5C12  +  6H2O  =  2HIO3  +  10HC1. 

In  order  to  separate  the  hydrochloric  acid  which  is  formed 
at  the  same  time,  precipitated  oxide  of  silver  is  added  until 
the  acid  is  completely  precipitated  as  the  insoluble  silver 
chloride. 

Properties.  —  Crystallized  iodic  acid  has  a  specific  gravity  at  0° 
of  4'629  ;  it  is  insoluble  in  alcohol,  but  easily  soluble  in  water. 
The  concentrated  aqueous  solution  boils  at  104°,2and  first  reddens, 
and  then  bleaches  litmus  paper.  Phosphorus,  sulphur,  and  organic 
bodies  deflagrate  when  heat^  with  iodic  acid  or  with  the 
pentoxide.  Sulphur  dioxicj^Br  sulphuretted  hydrogen  as  well 
as  hydriodic  acid  reduces  iodia  acid  with  separation  of  iodine  ; 
thus  :  — 

(1)  2HI03  +  5S02  +  4H90  =  I,  +  5H9S04. 

(2)  2HIO3  +  5H9S  =  I2  +  5S  +  6H2O.~ 

(3)  HI03  +  5HI  =  3I2  +  3H2O. 

The  lodates.  —  Iodic  acid  is  a  monobasic  acid,  and  is  distin- 
guished from  chloric  acid  and  bromic  acid  by  the  fact  that  it 
forms  not  onty  the  normal  salts,  but  salts  which  are  termed 

1  Thorpe  and  Perry,  Journ.  Chem.  Soc.  1892,  i.  925. 

2  Ditte,  Ann.  Chem.  Phys.  [4],  21,  5. 


334  THE  NON-METALLIC  ELEMENTS 

acid-  or  hydrated-salts.  Thus  the  following  potassium  salts  are 
known : — 

Normal  potassium  iodate  KI03. 

Acid  potassium  iodate  KIO3,  HIO3. 

Di-acid  potassium  iodate  KIO3,  2HIO3. 

The  normal  iodates  are  chiefly  insoluble  or  difficultly  soluble  in 
water;  the  more  soluble  are  those  of  the  alkali  metals.  On 
heating  they  decompose  either  into  oxygen  and  an  iodide,  or 
oxygen  and  iodine  are  given  off,  leaving  a  residue  of  the  metal 
or  its  oxide ;  thus : — 

2Ba(I03)2  =  2BaO  +  2I2  +  5O2. 
2AgI03  =  2Ag  +  I2. 

In  order  to  detect  iodic  acid,  the  solution  after  acidifying  with 
hydrochloric  acid,  is  mixed  with  a  small  quantity  of  starch  paste 
and  then  an  alkaline  sulphite  or  a  solution  of  sulphurous  acid 
added  drop  by  drop,  thus  liberating  iodine  which  forms  with  the 
starch  the  blue  iodide. 

Sodium  iodate  occurs  in  nature  associated  with  sodium  nitrate 
in  Chili  saltpetre,  and  iodic  acid  is  not  unfrequently  met  with  in 
nitric  acid  prepared  from  this  source. 

The  constitution  of  iodic  acid  is  not  known  with  certainty. 
Unlike  chloric  acid  it  behaves  as  a  polybasic  acid,  and  the  formula 
H-O-O-O-I,  is  therefore  even  more  improbable  than  in  the 
latter  case.  If,  however,  we  assume  that  the  iodine  in  this  acid 
is  pentavalent  and  ascribe  to  it  the  constitutional  formula 

Q  "j;I-0-H,  we  are  still  unable  to   account   for  the  existence  of 

the  acid  and  diacid  salts  mentioned  above,  except  by  regarding 
them  as  molecular  compounds  of  the  anhydrous  salt  and  the 
acid.  Thomsen l  regards  the  molecular  formula  of  the  acid  as 
H2I2O6,  and  ascribes  to  it  a  constitutional  formula  in  which  the 
oxygen  atoms  are  regarded  as  tetravalent,  whilst  another  formula 
has  been  suggested  by  Blomstrand  ; 2  the  evidence  is  however 
as  yet  insufficient  to  decide  between  the  different  suggestions. 

1  Ber.  7,  112.  2  /.  Pr.  Chem.  [2],  40,  305. 


PERIODIC  ACID  335 


PERIODIC  ACID,  HIO4. 

172  This  substance  was  discovered  by  Magnus,1  and  subse- 
quently investigated  by  other  chemists,  especially  by  Ammer- 
miiller  and  Rammelsberg.  Normal  periodic  acid,  HIO4,  is  not 
known.  The  hydrate  H5IO6  or  HIO4  +  2H2O  is  formed  either 
by  the  action  of  iodine  on  aqueous  perchloric  acid,  thus : — 

HC104  + 1  +  2H20  =  H5I06  +  Cl, 

or  by  the  decomposition  of  silver  periodate  with  bromine. 

This  hydrate  is  a  colourless  transparent  crystalline  deliquescent 
solid  which  melts  at  133°  and  at  140°  is  completely  decomposed 
into  iodine  pentoxide,  water,  and  oxygen.  The  aqueous  solution 
has  a  strong  acid  reaction  and  it  acts  upon  reducing  agents  in 
a  similar  way  to  iodic  acid. 

Periodic  acid  forms  a  remarkable  series  of  salts,  the  com- 
position of  which  at  first  sight  seems  somewhat  complex.  Thus 
we  are  acquainted  with  salts  having  the  following  general 
formulas,  M  representing  a  monad  metal : 

MI04;  M4I209!  M^KV,  MJ2O!i;  M5IO6;  M12I2013- 

If,  however,  we  regard  iodine  as  a  heptad  in  these  compounds, 
the  hypothetical  periodic  anhydride,  I207,  would  have  the 
following  constitutional  formula  : 

O  O 

II  II 

0  =  1—0—1-0 

II        II 

O          O 

the  above  salts  may  then  be  looked  upon  as  derived  from  acids 
formed  from  this  anhydride  by  union  with  varying  numbers 
of  molecules  of  water,  in  the  manner  shown  below  : 

I2O7  +  H2O  =  2IO3.OH  =  2HIO4 

I9O7  +  2H2O  =  IO9(OH),— O— IO,(OH)0  =  H4I2O9 

I2O7  +  3H2O  =  2IO2(OH),  =  2HIO4.  2H9O 

I0O7  +  4H2O  =  IO(OH)4— O— IO(OH)4  =  H8I2On 

I.,O7  +  5H2O  =  2IO(OH)5  =  2HIO4,  4H2O 

1 A  +  6H20  =  I(OH)fl— O— I(OH)6  =  H19I9013 

I2O7  +  7H2O  =  2I(OH)7  =  2HIO4,  6H2O. 

1  Pogg.  Ann.  28,  514. 


THE  NON-METALLIC  ELEMENTS 


No  salts  are  known  corresponding  to  the  acid  I(OH)7,  but  it  will 
be  seen  that  the  composition  of  the  remaining  six  acids  corre- 
sponds exactly  with  one  or  other  of  the  series  of  salts  which  has 
already  been  prepared.  (Compare  the  constitution  of  the 
phosphoric  acids.) 

The  best  known  series  of  periodates  are  those  derived  from 
the  acids  HIO4,  H4I2O9,  H3IO5,  and  H5I00  which  are  termed 
metaperiodates,  diperiodates,  mesoperiodates,  and  paraperiodatcs 
respectively. 

The  periodates  can  be  obtained  in  several  ways ;  thus  if 
chlorine  be  allowed  to  act  on  a  mixture  of  sodium  iodate  and 
caustic  soda,  a  mixture  of  two  sodium  paraperiodates  (Na2H3 
IO6,  and  Na3H2IO6)  and  sodium  chloride  is  formed.  Another 
method  is  to  heat  barium  iodate,  which  is  thus  converted  into 
barium  periodate,  iodine  and  oxygen. 

5Ba(I03)2  =  Ba5(I06)2 + 4I2  +  8O2. 

The  barium  paraperiodate  may  be  heated  to  redness  without 
decomposition,  whereas  the  other  periodates  are  decomposed  at 
this  temperature  with  evolution  of  oxygen. 

The  periodates  are,  as  a  rule,  but  slightly  soluble  in  water ; 
their  solutions  give  with  silver  nitrate  and  nitric  acid  a  pre- 
cipitate of  silver  periodate,  a  different  silver  salt  being  obtained 
according  to  the  proportion  of  nitric  acid  present.1 


SULPHUR.     S  =  31-82. 

173  SULPHUR  has  been  known  from  the  earliest  times  as  it 
occurs  in  the  free  or  native  state,  in  the  neighbourhood  of  extinct 
as  well  as  of  active  volcanoes.  It  was  formerly  termed  Brim- 
stone or  Brennestone,  and  was  considered  by  the  alchemists  to 
be  the  principle  of  combustibility,  and  believed  by  them  to  re- 
present the  alterability  of  metals  by  fire.  The  compounds  of 
this  element  occur  in  nature  in  much  larger  quantities,  and  are 
much  more  widely  distributed  than  free  sulphur  itself.  The  com- 
pounds of  sulphur  with  the  metals,  termed  sulphides,  and  those 
with  the  metal  and  oxygen  termed  sulphates  are  found  in  large 
quantities  in  the  mineral  kingdom.  The  more  important  com- 
pounds of  sulphur  occurring  in  nature  are  the  following  : — 
1  Kimmins  Journ.  Chem.  Soc.  1887,  i.  356 ;  1889,  i.  148. 


OCCURRENCE  OF  SULPHUR 


337 


(1)  Sulphides.     Iron  pyrites  FeS2 ;    copper  pyrites  CuFeS2 . 
galena  PbS ;  cinnabar  HgS  ;  blende  ZnS ;  grey  antimony  Sb2S3  • 
realgar  As2S2 ;  orpiment  As2S3. 

(2)  Sulphates.     Gypsum  GaS04  -f-  2H2O  ;  gypsum  anhydrite 
€aSO4;  heavy  spar  BaSO4  ;  kieserite  MgSO4  +  H2O;  bitter  spar 
MgSO4  +  7H2O  ;  Glauber  salt  Na.2SO4  +  10H20  ;  green  vitriol 
FeS04  +  7H2O. 

Volcanic  gases  almost  always  contain  sulphur  dioxide  and 
sulphuretted  hydrogen,  and  when  these  two  moist  gases  come 
into  contact  they  mutually  decompose  with  the  deposition  of 
sulphur,  thus  : — 

S02  +  2H2S  =  3S  +  2H20, 


FIG.  97. 

It  is  very  probable  that  native  sulphur  is,  in  some  cases,  formed 
by  the  above  reaction.  The  apparatus  shown  in  Fig.  97  serves 
to  exhibit  this  change ;  the  sulphuretted  hydrogen  gas  evolved 
in  the  bottle  (C)  is  passed  into  the  large  flask  (A)  into  which 
is  led  at  the  same  time  sulphur  dioxide  from  the  small  flask 
(B).  The  walls  of  the  large  flask  are  soon  seen  to  become 
coated  with  a  yellow  deposit  of  sulphur. 

Sulphur  compounds  are  also  found  widely  distributed  in 
the  vegetable  and  animal  world,  in  certain  organic  compounds 
such  as  the  volatile  oils  of  mustard  and  of  garlic,  and  in  the 
acids  occurring  in  the  bile.  Sulphur  is  also  found  in  small 
quantities  in  hair,  and  wool,  whilst  it  is  contained  to  the 
amount  of  about  1  per  cent,  in  all  the  albuminous  sub- 

23 


338  THE  NON-METALLIC  ELEMENTS 


stances  which  form  so  important  a  constituent  of  the  animal 
body. 

174  Almost  all  the  native  sulphur  of  commerce  comes  from 
Italy,  where  it  is  found  in  the  Romagna  and  in  other  parts  of 
the  country,  but  especially  in  very  large  quantities  in  the 
volcanic  districts  of  the  island  of  Sicily,  where  it  occurs  in 
widespread  masses  found  chiefly  on  the  south  of  the  Madonia 
range  stretching  over  the  whole  of  the  provinces  of  Caltanissetta 
and  Girgenti,  and  over  a  portion  of  Catania.  No  fewer  than 
476  distinct  sulphur  workings  exist  in  Sicily,  from  which  the 
annual  production  in  the  year  1886  amounted  to  410,000  tons; 
of  this  quantity  only  30,226  tons  were  imported  into  the 
United  Kingdom,1  and  since  the  above  year  the  production 
of  the  Sicilian  mines  has  not  been  reduced,  although  the 
number  of  the  mines  worked  has  decreased.  The  deposits  of 
Sicilian  sulphur  occur  in  the  tertiary  formation  lying  imbedded 
in  a  matrix  of  marl,  limestone,  gypsum,  and  celestine.  The 
sulphur  occurs  partly  in  transparent  yellow  crystals  termed 
virgin  sulphur  and  partly  in  opaque  crystalline  masses,  to  which 
the  name  of  volcanic  sulphur  is  given.  Both  these  varieties  are 
separated  from  the  matrix  by  a  simple  process  of  fusion.  The 
method  often  described,  in  which  the  sulphur  ore  is  repre- 
sented as  being  placed  in  earthenware  pots  in  a  furnace,  the 
sulphur  distilling  out  into  other  pots  placed  outside  the 
furnace,  appears  to  be  unknown  in  Sicily.  In  the  Romagna 
an  apparatus  made  of  cast-iron  and  provided  with  a  receiver 
of  the  same  material  is  employed,  but  in  Sicily  a  very  simple 
method  of  melting  out  the  sulphur  has  long  been,  and  still 
continues  to  be,  in  vogue.  This  old  process  consists  in  placing 
a  heap  of  the  ore  in  a  round  hole  dug  in  the  ground  averag- 
ing from  2  to  3  metres  in  diameter  and  about  one-half  metre 
in  depth.  Fire  is  applied  to  the  heap  in  the  evening,  and 
in  the  morning  a  quantity  of  liquid  sulphur  is  found  to  have 
collected  in  the  bottom  of  the  hole  ;  this  is  then  ladled  out, 
the  combustion  being  allowed  to  proceed  further  until  the  whole 
mass  is  burnt  out.  By  this  process  only  about  one-third  of  the 
sulphur  contained  in  the  ore  is  obtained,  whilst  the  remaining 
two-thirds  burns  away  evolving  clouds  of  sulphurous  acid. 

This  rough  and  wasteful  process  has  been  greatly  improved 
by  increasing  the  quantity  of  ore  burnt  at  a  given  time, 
the  excavation  being  made  10  metres  in  diameter,  with 
1  J.  Soc.  Chem.  Ind.  1889,  313  ;  1890,  118. 


EXTKACTION  OF  SULPHUR 


339 


a  depth  of  2J  metres,  and  so  arranged  (on  the  side  of  a  hill, 
for  instance)  that  an  opening  can  be  made  from  the  lowest 
portion  of  the  hole  so  that  the  sulphur,  as  it  melts,  may  flow 
out.  These  holes  are  built  up  with  masses  of  gypsum  and  the 
inside  covered  with  a  coating  of  plaster  of  Paris  (see  Fig.  98). 
The  calcaroni,  as  these  kilns  are  termed,  are  then  filled  with  the 
sulphur  ore  which  is  built  up  on  the  top  into  the  form  of  a  cone, 
and  air  channels  (b  b  b)  are  left  in  the  mass  by  placing  large; 
lumps  of  the  ore  together.  The  whole  heap  is  then  coated  over 
with  powdered  ore  (c  c),  and  this  again  covered  with  a  layer  of 
burnt-out  ore,  after  which  the  sulphur  is  lighted  at  the  bottom. 
By  permitting  the  heat  to  penetrate  very  slowly  into  the  mass,  the 


FIG.  98. 

sulphur  is  gradually  melted,  and  running  away  by  the  opening  (a) 
at  the  bottom  of  the  heap,  is  cast  into  moulds.  By  this  process, 
which  takes  several  weeks  to  complete,  the  richest  ores,  con- 
taining from  30  to  40  per  cent.,  may  be  made  to  yield  from  20 
to  25  per  cent,  of  sulphur,  whilst  common  ores,  containing  from 
20  to  25  per  cent.,  yield  from  10  to  15  per  cent,  of  sulphur,  the 
remaining  portion  of  the  sulphur  being  used  up  for  combustion. 
Other  methods  of  extraction  by  means  of  solvents,  such 
as  bisulphide  of  carbon,  or  by  the  use  of  ordinary  fuel  instead 
of  sulphur  itself,  as  a  source  of  heat,  have  been  proposed,  but 
from  the  nature  of  the  country  and  its  inhabitants  these  have 
not  yet  proved  successful  in  Sicily,  and  the  raw  sulphur  still 
remains  the  cheapest  fuel  for  the  purpose.  Large  deposits  of 


340 


THE  NON-METALLIC  ELEMENTS 


sulphur  also  exist  in  Iceland,  Japan,  and  Mexico,  some  of  which 
are  worked  commercially. 

175  Refining  of  Sulphur. — Commercial  Sicilian  sulphur  con- 
tains about  3  per  cent,  of  earthy  impurities  which  can  be  removed 
by  distillation,  the  arrangement  being  shown  in  Fig.  99.  The 
sulphur  is  melted  in  an  iron  pot  (M)  and  runs  from  this  by 
means  of  a  tube  into  the  iron  retort  (G)  where  it  is  heated 
to  the  boiling  point ;  the  vapour  of  the  sulphur  then  passes  into 
the  large  chamber  (A)  which  has  a  capacity  of  200  cubic  metres. 


FIG.  100. 


FIG.  99. 


In  this  chamber  the  sulphur  is  condensed,  to  begin  with,  in  the 
form  of  a  light  yellow  powder  termed  flowers  of  sulphur,  just  as 
aqueous  vapour  falls  as  snow  when  the  temperature  suddenly 
sinks  below  0°.  After  a  time  the  chamber  becomes  heated 
above  the  melting  point  of  sulphur,  and  then  it  collects  as  a 
liquid  which  can  be  drawn  off  by  means  of  the  opening  (o). 
It  is  then  cast  in  slightly  conical  wooden  moulds,  seen  in  Fig.  100, 
and  is  known  as  roll  sulphur,  or  brimstone.  It  is  frequently 
also  allowed  to  cool  in  the  chamber  and  then  obtained  in  large 
crystalline  masses,  known  in  the  trade  as  block  sulphur. 


EXTRACTION  OF  SULPHUR  341 

In  France,  Germany,  and  Sweden,  sulphur  is  also  obtained  by 
the  distillation  of  iron  pyrites,  FeS2.  This  method,  which  was 
described  by  Agricola  in  his  work  De  Ee  Metallica,  depends  on 
the  following  decomposition  of  the  pyrites  : — 

3FeS2  =  Fe3S4+S2; 

and  the  change  which  occurs  is  exactly  similar  to  that  by 
means  of  which  oxygen  is  obtained  from  manganese  dioxide  ; 
thus : — 


This  decomposition  of  the  pyrites  is  sometimes  carried  on  in 
retorts,  but  more  generally  a  kiln  similar  to  a  lime-kiln  is 
employed  for  the  purpose,  having  a  hole  at  the  side  into  which 
a  wooden  trough  is  fastened.  A  small  quantity  of  fuel  is  lighted 
on  the  bars  of  the  furnace,  and  then  the  kiln  is  gradually  filled 
with  pyrites;  a  portion  of  the  sulphur  burns  away  whilst 
another  portion  is  volatilized ;  the  burnt  pyrites  is  from  time 
to  time  removed  from  below,  and  fresh  material  thrown  on  the 
top  so  that  the  operation  is  carried  on  uninterruptedly.  In  this 
way  about  half  the  sulphur  which  is  contained  in  the  pyrites 
can  be  obtained,  whilst  only  about  one-third  of  the  total  sulphur 
can  be  got  by  distilling  in  iron  cylinders. 

Sulphur  is  likewise  obtained  in  this  country,  though  in  smaller 
quantities,  as  a  by-product  in  the  manufacture  of  coal-gas. 
The  impure  gas  always  contains  sulphuretted  hydrogen,  which 
can  be  removed  by  passing  the  gas  over  oxide  of  iron,  when 
iron  sulphide  is  formed.  This  substance  on  exposure  to  air  is 
oxidised  with  separation  of  free  sulphur,  thus  : — 

Fe2S3  +  30  +  H2O  =  Fe2O3,H2O  +  3S. 

The  mass  can  then  be  again  employed  for  the  purification  of  the 
gas,  and  this  alternate  oxidization  and  sulphurization  can  be 
repeated  until  a  product  is  obtained  containing  50-60  per  cent, 
of  sulphur.  The  latter  may  be  separated  from  the  iron  oxide 
by  distillation  or  by  treatment  with  carbon  bisulphide  in  a 
suitable  apparatus.  In  most  cases,  however,  the  spent  oxide  is 
employed  for  the  manufacture  of  sulphuric  acid,  and  is  then 
burnt  in  kilns  similar  to  those  employed  for  burning  pyrites. 

Another  and  much  more  important  source  from  which  sulphur 
is  now  obtained  is  the  residue  or  waste  in  the  soda  manufac- 


342  THE  NON-METALLIC  ELEMENTS 

ture ;  this  consists  of  calcium  sulphide  mixed  with  chalk,  lime 
and  alkaline  sulphides.  The  sulphur  which  this  material  con- 
tains was  formerly  altogether  wasted ;  now,  however,  it  is 
economically  regained  in  the  alkali  works  on  a  very  large  scale. 
For  this  purpose  the  waste,  which  consists  mainly  of  calcium 
sulphide  CaS,  is  treated  in  the  presence  of  water  with  car- 
bonic acid  gas,  calcium  carbonate  being  thus  produced  whilst 
sulphuretted  hydrogen  is  evolved  : 

CaS  +  H2O+CO2  =  CaCO3  +  H2S. 

The  gas  obtained  is  then  burnt  with  an  insufficient  supply  of 
air  in  a  specially  devised  kiln  and  the  sulphur  thus  recovered 
in  the  free  and  pure  state.1 

176  Properties. — Sulphur  exists  in  several  allotropic  modifica- 
tions. Thus  it  can  be  obtained  either  crystalline  or  amorphous, 
and  at  least  two  varieties  of  the  amorphous  form  may  be  distin- 


FIG.  101. 

guished,  one  of  which  is  soluble  and  the  other  insoluble  in  carbon 
bisulphide.  A  form  is  also  known  which  is  soluble  in  water. 
Rhombic  or  a-Sulphur  occurs  in  nature  in  large  yellow  transparent 
octahedra.  Fig.  101  shows  the  form  of  the  natural  crystals  of 
sulphur  (a  being  the  simpler,  and  b  the  more  complicated  form) 
which  belong  to  the  rhombic  system,  and  have  the  following  rela- 
tion of  the  axes  :— a  :b:c  =  0'8106  : 1  :  T898.  In  addition  to  this 
primary  form,  no  less  than  thirty  different  crystallographic  modi- 
fications are  known  to  exist  in  the  case  of  sulphur.  Crystals  of 
rhombic  sulphur  have  also  been  found  in  the  sulphur  chambers, 
having  been  deposited  by  slow  sublimation.  The  specific  gravity 
of  this  form  of  sulphur  at  0°,  is  2*05  —  2'07  ;  it  is  insoluble  in 
water,  very  slightly  soluble  in  alcohol,  benzene,  and  ether,  but 
dissolves  readily  in  carbon  bisulphide,  chloride  of  sulphur,  petro- 
leum, and  turpentine.  Artificial  crystals  of  sulphur  are  best  ob- 
1  Chance,  Journ.  Soc.  Chem.  2nd.  (1888),  7,  162. 


ALLOTROPIC  FORMS  OF  SULPHUR  343 

tained  from  solution  in  carbon  bisulphide,  which  dissolves  at 
the  ordinary  temperature  about  one-third  of  its  weight  of 
sulphur ;  the  saturated  solution  on  being  allowed  to  evaporate 
slowly  deposits  large  transparent  octahedral  crystals.  Very  well 
developed  crystals  can  also  be  obtained  by  saturating  pyridine 
with  sulphuretted  hydrogen  gas  and  allowing  the  solution  to 
.stand  exposed  to  the  air  for  some  time.  The  sulphuretted 
hydrogen  is  partially  oxidised  by  the  air,  and  the  sulphur  which 
is  liberated  crystallises  out.1  a-Sulphur  melts  at  114*5° 
(Brodie),  forming  a  clear  yellow  liquid,  which  has  a 
specific  gravity  of  T803,  and  when  quickly  cooled,  solidifies 
again  at  the  same  temperature  ;  generally,  however,  it  remains 
liquid  at  a  temperature  below  its  melting  point,  and  then 
solidifies  at  a  slightly  lower  temperature,  which  varies  with  the 
point  to  which  the  liquid  has  been  heated. 

177  The  second  or  monosymmetric  modification,  known  as  /3- 
sulphur,  is  obtained  when  melted  sulphur  is  allowed  to  cool  at 


FIG.  102. 

the  ordinary  temperature  until  a  solid  crust  is  formed  on  the 
surface.  The  crust  is  then  broken  through,  and  the  portion  of 
sulphur  still  remaining  liquid  poured  out ;  the  sides  of  the 
vessel  will  then  be  found  to  be  covered  with  a  mass  of  long, 
very  thin  transparent  crystals  having  the  form  of  mono- 
symmetric  prisms  (Fig.  102),  the  ratio  of  the  axes  of  which  are 
expressed  by  the  following  numbers  :2  a:  b  :  c  =  T004  :  1  : 
1'004.  /3-Sulphur  appears  to  be  trimorphous,  since  no  less  than 
three  different  types  of  crystals,  all  belonging  to  the  mono- 
symmetric  system,  but  having  different  axial  ratios  and  optical 
properties,  have  been  observed.3 

Monosymmetric  sulphur  has  a  specific  gravity  of  1'96,  its 
melting-point  is  120°,  and  like  the  rhombic  modification,  it  is 
readily  soluble  in  carbon  bisulphide. 

178  According  to  the  conditions  under  which  the  passage  from 
the  fused  to  the  solid  state  takes  place,  sulphur  may  separate 

1  Ahrens,  Ber.  23,  2708.  2  Mitscherlich,  Pogg.  Ann.  24,  264. 

3  Muthmann,  Zeit.  Kryst.  17,  336. 


344  THE  NON-METALLIC  ELEMENTS 

either  in  the  form  of  rhombic  or  of  monosymmetric  crystals. 
The  rhombic  crystals  are  obtained  by  placing  about  200  grams 
of  sulphur  previously  crystallized  from  solution  in  carbon  bisul- 
phide in  a  flask  provided  with  a  long  neck,  which  is  afterwards 
bent  backwards  and  forwards  several  times  to  prevent  the  entry 
of  floating  dust.  The  sulphur  is  then  melted  by  placing  the 
flask  in  an  oil  bath  heated  to  120°,  and  when  the  contents  are 
liquid  the  flask  is  immersed  in  a  vessel  filled  with  water  at 
95°.  On  standing  for  some  time  at  a  temperature  of  about 
90°,  crystals  are  seen  to  form,  and  when  a  sufficient  quantity 
have  been  deposited  the  flask  is  quickly  inverted  ;  the  portion 
of  sulphur  still  liquid  then  flows  into  the  neck  and  there  at 
once  solidifies,  leaving  the  transparent  rhombic  crystals  in  the 
body  of  the  flask. 

When  a  large  mass  of  molten  sulphur  is  allowed  to  cool 
slowly,  rhombic  crystals  are  also  formed,  and  these  cannot  be 
distinguished  from  the  natural  crystals.  Thus  Silvestri  found 
such  rhombic  crystals  5  to  6  centimetres  in  length  in  a  mass 
of  sulphur  which  had  been  melted  during  a  fire  in  a  sulphur 
mine.  When  a  transparent  rhombic  crystal  of  sulphur  is 
heated  for  some  time  to  some  temperature  above  97'6°,  but 
below  its  melting-point,  it  becomes  opaque  when  touched 
with  a  prismatic  crystal,  owing  to  its  being  converted  into  a 
large  number  of  monosymmetric  crystals ; l  whereas,  on  the  other 
hand,  a  transparent  crystal  of  /3-sulphur  becomes  opaque  after 
standing  for  twenty-four  hours  at  the  ordinary  temperature, 
having  undergone  a  spontaneous  change  to  the  rhombic 
modification,  the  crystal  having  been  converted  into  a  large 
number  of  minute  rhombic  crystals.  This  conversion  is 
accelerated  by  vibration,  as,  for  instance,  when  the  crystals  are 
scratched,  and  also  when  they  are  exposed  to  sunlight ;  the 
change  is  always  accompanied  by  an  evolution  of  heat,  2'25 
cal.  being  liberated  by  the  conversion  of  31*82  parts  of  the 
monosymmetric  into  the  rhombic  variety.  A  saturated 
solution  of  sulphur  in  boiling  carbon  bisulphide  deposits 
rhombic  crystals  on  cooling,  whilst,  on  the  other  hand, 
solutions  in  alcohol,  ether  and  chloroform  give  rise  to  the 
monosymmetric  crystals.  A  saturated  solution  in  boiling 
benzene  deposits  the  /^-modification  at  temperatures  between 
75 — 80°,  and  the  a-variety  below  22°,  whilst  at  intermediate 
temperatures  mixtures  of  the  two  are  formed. 

1  Gernez.  Com.pt.  Rend.  98,  810  915,  100,  1343. 


ALLOTEOPIC  FOKMS  OF  SULPHUR  345 


179  Amorphous  sulphur  is  known  in  two  forms,  one  of  which 
is  soluble  in  carbon  bisulphide  and  the  other  insoluble.  The 
soluble  variety  is  produced  by  the  decomposition  of  sulphuretted 
hydrogen  water  in  the  air  and  by  the  action  of  acids  upon  the 
polysulphides  of  the  alkalis,  etc. 

Sulpkur  milk  (lac  sulphur  is),  &  body  known  to  Geber  and  now 
used  as  a  medicine,  is  sulphur  in  this  form.  It  is  deposited  as  a 
fine  white  powder  when  two  parts  of  flowers  of  sulphur  are  boiled 
with  thirteen  parts  of  water  and  one  part  of  lime  slaked  with 
three  parts  of  water,  until  the  whole  of  the  sulphur  is  dissolved. 
The  reddish-brown  solution  thus  prepared  contains  calcium 
pentasulphide,  which  is  decomposed  on  the  addition  of  hydro- 
chloric acid  with  evolution  of  sulphuretted  hydrogen  and 
deposition  of  milk  of  sulphur ;  thus : — 

CaS5  +  2HC1  =  CaCl2 + H2S  +  4S. 

The  insoluble  amorphous  modification  is  known  as  <y-sulphur, 
and  can  be  prepared  in  many  different  ways. 

When  melted  sulphur  is  further  heated,  the  pale  yellow 
liquid  gradually  changes  to  a  dark  red  colour  and  becomes 
more  and  more  viscid,  until  at  a  temperature  of  from  220°  to 
250°  it  becomes  almost  black  and  so  thick  that  it  can  only  with 
difficulty  be  poured  out  of  the  flask.  Observed  in  thin  films, 
this  change  of  colour  from  yellow  to  red  is  found  to  be  as- 
sociated with  a  distinct  change  in  the  absorption -spectrum, 
inasmuch  as  the  absorption  in  the  red  gradually  disappears, 
whilst  that  in  the  blue  is  gradually  increased  (Lockyer).  If  the 
temperature  be  raised  still  higher,  the  liquid  becomes  less  viscid, 
although  its  dark  colour  remains,  and  on  cooling  down  again 
the  above-described  appearances  are  repeated  in  inverse 
order.  If  the  viscid  sulphur  be  rapidly  cooled,  or  if  the  more 
mobile  liquid  obtained  at  a  higher  temperature  be  poured  in  a 
thin  stream  into  cold  water,  the  sulphur  assumes  the  form  of  a 
semi-solid  transparent  elastic  mass,  which  can  be  drawn  out 
into  long  threads.  This  is  known  as  plastic  sulphur ;  its  condi- 
tion is  an  unstable  one,  and  on  standing  it  gradually  becomes 
opaque  and  brittle.  The  plastic  variety  of  sulphur  can  be 
obtained  by  the  arrangement  shown  in  Fig.  103.  The  sulphur 
is  first  melted  and  then  heated  to  its  boiling-point  in  the  retort. 
The  sulphur  vapour  condenses  in  the  neck  of  the  retort,  and 
the  liquid  sulphur  runs  in  a  thin  stream  into  cold  water. 


346  THE  NON-METALLIC  ELEMENTS 

If  the  brittle  mass  be  treated  with  carbon  bisulphide,  a  small 
portion,  less  than  one  per  cent.,  dissolves,  the  remainder  being 
left  behind  in  the  form  of  a  dark  brown  powder,  the  colour  of 
which  is  due  to  small  traces  of  fatty  organic  matter.  In  the 
absence  of  this  the  colour  of  the  residue  is  lemon  yellow. 

Together  with  the  modification  soluble  in  carbon  bisulphide, 
"  flowers  of  sulphur  "  contains  a  light  yellow  insoluble  modifica- 
tion ;  and  if  a  solution  of  sulphur  in  the  above  menstruum  be 
exposed  to  the  sunlight,  a  portion  of  the  sulphur  separates  out 
in  the  insoluble  form.  The  insoluble  variety  is  also  produced, 
accompanied  by  the  soluble  form,  by  the  decomposition  of 
chloride  of  sulphur  with  water,  and  in  other  similar  reactions. 
On  preservation  it  gradually  changes  into  rhombic  sulphur, 


FIG.  103. 

and  this  transformation  can  also  be  produced  by  boiling  it  with 
alcohol  or  by  subjecting  it  to  a  pressure  of  8,000  atmospheres.1 

The  specific  gravity  of  this  variety,  obtained  from  flowers  of 
sulphur,  is  T9556  at  0°. 

Colloidal  or  8-sulphur. — According  to  Debus 2  an  amorphous 
variety  of  sulphur  which  is  soluble  in  water  is  contained  in 
Wackenroder's  solution  (p.  420).  It  forms  a  yellow,  semi- 
liquid  mass,  and  resembles  colloidal  silica  in  many  of  its  pro- 
perties. A  variety  which  is  also  soluble  in  water  is  formed3  when 
a  saturated  solution  of  sodium  thiosulphate  is  decomposed  by 
two  volumes  of  hydrochloric  acid  which  has  been  saturated  at 
25°  and  allowed  to  cool  to  10°. 

1  Spring,  Ber.  14,  2579.  2  Debus,  Journ  Chem.  Soc.  1888,  i.  282. 

3  Engel,  Compt.  Rend.  112.  866. 


VAPOUR  DENSITY  OF  SULPHUR  347 


Engel  has  also  found  that  if  the  yellow  solution  be  extracted 
with  chloroform  before  the  deposition  of  this  soluble  sulphur  and 
the  chloroform  allowed  to  evaporate,  rhombohedral  crystals  of 
sulphur  which  belong  to  the  hexagonal  system  are  deposited. 
These  are  denser  than  any  other  form  of  sulphur,  having  a 
specific  gravity  of  2  135,  and  on  preservation  become  opaque, 
the  insoluble  variety  of  sulphur  being  formed. 

In  addition  to  the  modifications  already  mentioned,  many 
others  have  been  described.  All  the  numerous  varieties  may 
however  be  referred  to  two  fundamental  forms,  (1)  the  rhombic 
form  soluble  in  carbon  bisulphide  and  (2)  the  insoluble  form, 
which  is  however  less  stable  than  the  rhombic  form,  into  which 
it  ultimately  passes. 

According  to  Berthelot,1  the  rhombic  form,  or  some  modifica- 
tion referable  to  it,  is  always  produced  when  the  sulphur  is  the 
electro-negative  element  of  the  molecule,  as  in  sodium  sulphide, 
sulphuretted  hydrogen,  &c.,  whilst  when  it  is  the  electro- 
positive element,  as  in  sulphur  dioxide,  sulphur  chloride,  &c., 
the  amorphous  form  is  produced  on  decomposition.  In  many 
cases  however  one  form  is  accompanied  by  more  or  less  of  the 
other  variety. 

1 80  Sulphur  boils,  according  to  Regnault,2  under  the  normal 
pressure  at  448°'4  or  at  450°  under  a  pressure  of  779'9mm-,  form- 
ing a  red  vapour.  The  density  of  the  vapour  was  found  by 
Dumas3  at  about  524°  to  be  6'56,  corresponding  to  a  molecular 
weight  of  189,  whilst  at  860°— 1040°  Deville  and  Troost4  found 
it  to  be  constant  at  a  value  of  2'23,  corresponding  to  the 
molecular  weight  of  64*2. 

The  conclusion  was  hence  drawn  that  the  molecules  of  sulphur 
at  temperatures  near  the  boiling  point  consist  of  six  atoms 
(Mol.  Wt.  =  190'9),  whilst  at  higher  temperatures  the  molecules 
are  made  up  of  only  two  atoms  and  have  a  weight  of  63'6.  The 
experiments  of  Biltz5  have  however  shown  that  the  vapour 
density  of  sulphur  varies  gradually  with  the  temperature,  being 
7'84  at  468°,  7'OJ)  at  524°,  and  473  at  606°,  but  finally  reaches 
the  constant  value  2'23.  Since  the  vapour-density  correspond- 
ing to  the  formula  S6  has  not  been  found  to  be  constant  through 
any  definite  range  of  temperature,  the  existence  of  molecules  of 
this  formula  cannot  be  considered  as  proved. 

1    Compt.  Rend.  44,  318.  378.  2  Relation  des  Experiences,  &c.,  torn.  ii. 

3  Ann.  Chim.  Phya.  50,  172.  4  Ann.  Chim.  Phys.  (3),  58,  257. 

5  Biltz,  Ber.  21,  2013. 


348  THE  NON-METALLIC  ELEMENTS 

Determinations  of  the  molecular  weight  of  sulphur  in  solution 
both  by  the  freezing-  and  boiling-point  methods  seem  to  lead  to 
the  formula  S8  or  Sg.1 

181  When  sulphur  burns  in  the  air  or  in  oxygen,  a  continuous 
spectrum  is  observed,  but  if  a  small  quantity  of  sulphur  vapour 
be  brought  into  a  hydrogen  flame,  a  series  of  bright  bands  is 
seen  when  the  blue  cone  in  the  interior  of  the  flame  is  examined 
or  when  the  sulphurized  flame  is  brought  on  any  cold  surface. 
This  blue  tint  is  almost  always  seen  when    a  pure  hydrogen 
flame  is  brought  for  an  instant  against  a  piece  of  porcelain,  the 
blue  colour  being  produced,  according  to  Barrett,2  by  the  sulphur 
contained  in  the  dust  in  the  air.     The  absorption  spectrum  of 
sulphur  has  been  obtained  by  Salet,3  and  the  emission  spectra, 
of  which  there  are  said  to  be  two,  a  channelled-space  and  a  line 
spectrum,  have  been  mapped  by  Pliicker  and  Hittorf,4  and  more 
recently  by  Salet.5     According  to  Mr.  Lockyer  two  other  spectra 
of  sulphur  occur,  viz.,  a  continuous  absorption  in  the  blue,  and 
a   continuous   absorption   in   the   red.     The   change  from  the 
channelled-space  spectrum  to  that  showing  absorption  in  the 
blue,  is  observed  when  the  vapour-density  changes,  the  first  of 
these  spectra  being  seen  when  the  vapour  possesses  a  normal 
density. 

182  Detection  and  Determination  of  Sulphur. — The  simplest 
mode   of  detecting    sulphur   in    a   compound   is   to   mix  the 
body  with   pure   carbonate   of  soda,  and   fuse    it   before   the 
blowpipe  on  charcoal  or,  to  avoid  the  introduction  of  sulphur 
from  the  gas  flame,  to   mix  the   substance  with  sodium  car- 
bonate   and    charcoal,  and   heat   in   a   small   closed    crucible. 
Sodium   sulphide  is  thus  formed,  and  this  may  then  be  re- 
cognized by  bringing  the  fused  mass  on  to  a  silver  coin  and 
adding  water.     The   smallest    quantity  of  sulphur    can    thus 
be   recognized   by  the  formation  of  a  brown  stain    of  silver 
sulphide.     Sulphur  is  almost  always  quantitatively  determined 
as  barium  sulphate.     If  the  body  is  a  sulphide,  as,  for  instance, 
pyrites,  it  is  finely  powdered,  and  either  fused  with  a  mixture 
of  carbonate  of  soda  and  nitre,  the  fused  mass  dissolved   in 
water,  and  the   filtrate,  after  acidifying  by  hydrochloric  acid 
precipitated  with  barium  chloride,  or  the  sulphide  is  oxidized 

1  Paterno  and  Nasini,  Ber.  21,  2153  ;  Beckmann,  Zeit.  phys.  Chem.  5.  76  -f 
Hertz,  Zeit.  phys.  Chem.  6,  358.  3  Phil.  Mag.  [4],  30,  321. 

s  Compt.  Rend.  74,  865.  4  Phil.  Trans.  1865,  1. 

5  Compt.  Rend.  73    559,  561,  742,  744. 


SULPHUKETTED  HYDROGEN  349 

with  a  mixture  of  nitric  and  hydrochloric  acids  or  fuming 
nitric  acid,  the  excess  of  acid  removed  by  evaporation  and 
barium  chloride  added,  whereby  the  insoluble  barium  sulphate 
is  formed,  and  this,  after  washing  and  drying,  is  ignited  and 
weighed.  A  further  method  is  to  fuse  the  mineral  with  2  parts 
of  caustic  soda  and  4  parts  of  sodium  peroxide,  the  melt  being 
then  acidified  and  precipitated  with  barium  chloride  as  above.1 

Atomic  Weight  of  Sulphur. — This  has  been  determined  by 
Berzelius,  Dumas,  Stas,  and  other  chemists  with  closely  con- 
cordant results.  At  a  mean  of  five  experiments  Stas  found  that 
100  parts  of  silver,  when  heated  in  sulphur  vapour  or  in 
sulphuretted  hydrogen,  yielded  114'854  parts  of  silver  sulphide  ; 
and  as  the  mean  of  six  others  that  100  parts  of  silver  sulphate, 
Ag2SO4,  when  heated  in  a  current  of  hydrogen,  left  a  residue  of 
69-203  silver.  Hence  when  O  =  15'88  and  Ag  =  107'13,  the 
atomic  weight  of  sulphur  is  31'82. 


SULPHUR  AND  HYDROGEN. 

183  These  elements  unite  to  form  at  least  two  distinct  com- 
pounds, viz.,  hydrogen  mono-sulphide  or  sulphuretted  hydrogen, 
H2S,  and  hydrogen  di-sulphide,  H2S2. 


SULPHURETTED  HYDROGEN,  OR  HYDROGEN  MONO-SULPHIDE. 

H2S.    33-82. 

The  preparation  of  a  solution  of  the  polysulphides  of  calcium 
by  boiling  lime  with  sulphur  is  described  in  the  Papyrus  of 
Leyden,  the  oldest  chemical  manuscript  known  (p.  4),  and 
Zosimus  frequently  alludes  to  the  unpleasant  smell  produced 
from  this  liquid,  which  was  known  to  him  as  the  "divine  water" 
(Greek  Oelov,  divine  or  sulphurous).  Geber  moreover  described 
the  preparation  of  milk  of  sulphur,  but  we  do  not  notice  either 
in  his  works  or  in  those  of  the  later  alchemists  that  any  further 
mention  is  made  of  the  fact  that  a  fetid  smell  is  given  off  in 
the  process.  Not  until  we  come  to  the  writers  of  the  sixteenth 
and  seventeenth  centuries  do  we  find  any  description  given 
of  sulphuretted  hydrogen,  and  then  it  is  described  under  the 
general  name  of  sulphurous  vapours.  Scheele  was  the  first  to 
1  Hempel,  Zeit.  Anorg.  Chem.  3,  193. 


350  THE  NON-METALLIC  ELEMENTS 

investigate  this  compound  with  care.  He  found  that  it  could 
be  formed  by  heating  sulphur  in  inflammable  air,  and  he 
considered  that  it  must  be  made  up  of  sulphur,  phlogiston 
and  heat. 

Sulphuretted  hydrogen  is  formed  when  hydrogen  gas  is  passed 
through  boiling  sulphur,  when  sulphur  vapour  is  burnt  in  an 
atmosphere  of  hydrogen,  or  when  dry  hydrogen  gas  is  passed 
over  certain  heated  sulphides.  Thus,  if  a  little  powdered 
antimony  trisulphide  be  placed  in  a  bulb  tube  and  heated 
by  a  flame,  and  if  a  slow  current  of  hydrogen  be  then  passed 
over  the  heated  sulphide,  the  escaping  gas  when  allowed 
to  bubble  through  a  solution  of  lead  acetate  will  produce  a  black 
precipitate  of  lead  sulphide,  thus  showing  the  formation  of 
sulphuretted  hydrogen.  The  reaction  is  thus  represented  :  — 


It  is  also  produced  in  the  putrefactive  decomposition  of 
various  organic  bodies  (such  as  albumin)  which  contain  sulphur, 
and  it  is  to  the  presence  of  this  substance  that  rotten  eggs 
owe  their  disagreeable  odour.  Sulphuretted  hydrogen  occurs, 
as  has  been  stated,  in  volcanic  gases,  whilst  certain  mineral 
waters,  such  as  those  of  Harrogate,  contain  dissolved  sul- 
phuretted hydrogen,  which  imparts  to  the  waters  their  peculiar 
medicinal  properties  as  well  as  their  offensive  smell. 

184  Preparation.  —  (1)  Sulphuretted  hydrogen  is  best  prepared 
by  acting  upon  certain  metallic  sulphides  with  dilute  acids  ;  in 
general,  ferrous  sulphide  (sulphide  of  iron,  obtained  by  melting 
together  iron  filings  and  sulphur)  is  employed  for  this  purpose. 
Ferrous  sulphide,  FeS,  dissolves  readily  in  hydrochloric  or  in 
dilute  sulphuric  acid,  sulphuretted  hydrogen  gas  being  liber- 
ated ;  thus  :  — 

FeS  +  H2S04  =  H2S  +  FeSO4. 
FeS  +  2HC1  =  H2S  +  FeCl2. 

The  apparatus  shown  in  Fig.  104  may  be  used;  the  materials 
are  placed  in  the  large  bottle  and  the  gas  which  is  evolved  is 
washed  by  passing  through  water  contained  in  the  smaller  one. 
When  a  regular  evolution  of  gas  for  a  long  period  is  needed,  the 
Kipp's  apparatus,  Fig.  105,  is  employed.  The  two  glass  globes  (a) 
and  (b)  are  connected  by  a  narrow  neck,  whilst  the  tubulus  of  the 


PREPAKATION  OF  SULPHUKETTED  HYDROGEN        351 


third  and  uppermost  globe  (c)  passes  air-tight  through  the  neck  of 
(6).  The  sulphide  of  iron  is  placed  in  globe  (b)  and  dilute  sulphuric 


FIG.  104. 


acid  poured  through  the  tube-funnel  until  the  lowest  globe  is 
filled  and  a  portion  of  the  acid  has  flowed  on  to  the  sulphide 


FIG.  105. 


of  iron.    When  it  is  desired  to  stop  the  current  of  gas,  the  stop- 
cock at  (e)  is  closed,  and  the  acid  is  forced  by  the  pressure 


352  THE  NON-METALLIC  ELEMENTS 

of  the  gas  accumulating  in  the  globe  (6)  up  the  tubulus  into 
the  uppermost  globe  (c). 

A  still  more  convenient  arrangement,  especially  for  use  in 
laboratories,  has  been  proposed  by  Ostwald1  in  which  only 
a  small  amount  of  acid  is  allowed  to  come  in  contact  with 
the  sulphide  of  iron  at  once,  and  is  thoroughly  used  up  in 
decomposing  the  sulphide,  any  excess  of  gas  evolved  being 
stored  in  a  specially  arranged  vessel. 

(2)  The  gas  thus  obtained  is,  however,  never  pure,  inasmuch  as 
the  artificial  ferrous  sulphide  always  contains  some  particles  of 
metallic  iron,  and  these  coming  into  contact  with  the  acid  evolve 
hydrogen  gas.     Hence  in  order  to  prepare  pure  sulphuretted 
hydrogen,  a  naturally  occurring  pure  sulphide,  viz.,  antimony 
trisulphide,  is  employed,  and  this  substance,  roughly  powdered, 
on   being  warmed  with  hydrochloric   acid,   evolves   a  regular 
current  of  the  pure  gas ;  thus  : — 

Sb2S3  +  6HC1  =  3H2S  +  2SbCl3. 

(3)  A  continuous  current  of  sulphuretted  hydrogen  may  like- 
wise be  obtained  by  heating  a  mixture  of  equal  parts  of  sulphur 
and   paraffin    (a   mixture    of   solid    hydrocarbons    having   the 
general  formula  CnH2ll+2).     By  regulating  the  temperature  to 
which  the  mixture  is  heated,  the  evolution  of  gas  may  be  easily 
controlled.     The  exact  nature  of  the  changes  which  here  occur 
remains  as  yet  undetermined.2 

(4)  In  order  to  obtain  a  perfectly  pure  gas  the  artificially  pre- 
pared sulphides  of  calcium  or  zinc  may  be  decomposed  by  dilute 
acids,  or  a  solution  of  magnesium  hydrosulphide,  Mg(SH)2,  may 
be  heated  to  60°,  at  which  temperature  it  is  decomposed. 

185  Properties. — Sulphuretted  hydrogen  obtained  by  any  of 
the  above  processes  is  a  colourless,  very  inflammable  gas,  pos- 
sessing a  sweetish  taste  and  a  powerful  and  very  unpleasant 
smell  resembling  that  of  rotten  eggs.  The  gas  may  be  collected 
over  hot  water  in  which  it  does  not  dissolve  so  readily  as  in 
cold.  When  a  light  is  brought  to  the  open  mouth  of  a  jar  filled 
with  the  gas,  it  burns  with  a  pale  blue  flame,  the  hydrogen 
uniting  with  the  oxygen  of  the  air  to  form  water,  and  the 
sulphur  partly  burning  to  sulphur  dioxide  (which  can  be  easily 
recognized  by  its  pungent  smell)  and  partly  being  deposited  as 
a  yellow  incrustation  on  the  inside  of  the  jar.  A  mixture  of 

1  Zcit.  Anal.  Chem.  31,  100.  2  Galletly,  Chem.  News,  24,  162. 


SULPHURETTED  HYDROGEN  353 

two  volumes  of  sulphuretted  hydrogen  and  three  volumes 
of  oxygen  explodes  violently  when  an  electric  spark  is  passed 
through  it,  complete  combustion  taking  place. 

Determination  of  Composition. — Sulphuretted  hydrogen  is 
decomposed  when  heated  by  itself,  this  decomposition  begin- 
ning at  as  low  temperature  as  400° l  and  being  complete  at  a 
white  heat.2  Upon  this  fact  a  method  is  based  for  the  deter- 
mination of  the  composition  of  sulphuretted  hydrogen.  This 
may  be  readily  accomplished  by  heating  some  metallic  tin  in  a 
given  volume  of  the  gas.  The  tin  decomposes  the  gas,  combin- 
ing with  the  sulphur  to  form  a  solid  sulphide,  and  setting  free 
the  hydrogen,  which  is  found  to  occupy  the  same  volume  as  the 
original  gas.  The  same  result  is  obtained  when  a  spiral  of 
platinum  wire  is  heated  to  bright  redness  in  the  gas  ;  sulphur 
is  deposited  in  the  solid  form  and  the  hydrogen  is  left,  whilst 
no  alteration  occurs  in  the  volume  of  the  gas.  Now,  as  the 
specific  gravity  of  the  gas  was  found  by  the  experiments  of  Gay- 
Lussac  and  Thenard  to  be  1'1912,  the  molecular  weight  is  34*8, 
or  correcting  this  number  by  the  more  accurate  results  deduced 
from  analytical  data,  we  have  33*82.  Deducting  from  this  the 
weight  of  one  molecule  of  hydrogen,  we  find  the  weight  of  the 
sulphur  contained  in  the  molecule  of  sulphuretted  hydrogen  to 
be  31 '8.2,  and  hence  the  formula  of  the  gas  is  SH2  and  the 
theoretical  density  1*175. 

When  inhaled,  even  when  mixed  with  a  considerable  volume 
of  air,  the  gas  acts  as  a  powerful  poison,  producing  insensibility 
and  asphyxia.  From  the  experiments  of  Thenard  it  appears  that 
respiration  in  an  atmosphere  containing  -$±^  part  of  its  volume 
of  this  gas  proves  fatal  to  a  dog,  and  smaller  animals  die  when 
only  half  the  quantity  is  present.  The  best  antidote  to  poison- 
ing by  sulphuretted  hydrogen  appears  to  be  the  inhalation  of 
very  dilute  chlorine  gas  as  obtained  by  wetting  a  towel  with 
dilute  acetic  acid  and  sprinkling  the  inside  with  a  few  grains  of 
bleaching  powder.  The  gas  is  dissolved  to  a  considerable  extent 
by  water,  one  volume  of  water  absorbing,  at  0°,  4 '37  volumes,  and 
at  15°,  3*23  volumes  of  the  gas.  The  general  expression  for  the 
solubility  of  sulphuretted  hydrogen  in  one  volume  of  water  at 
different  temperatures  between  2°  and  43°'3  is 

c  =  4-3706  -  0-083687  t  +  0'0005213  t-. 

1  Myers,  Annalen,  169,  124. 

2  Langer  and  Meyer,  Ber.  18,  135  c. 
24 


354  THE  NON-METALLIC  ELEMENTS 

The  solution  reddens  blue  litmus  paper  (whence  the  name  hydro- 
sulphuric  acid  has  sometimes  been  given  to  the  substance),  and 
possesses  the  peculiar  smell  and  taste  of  the  gas.  It  however 
soon  becomes  milky  on  exposure  to  air,  owing  to  the  hydrogen 
combining  with  the  oxygen  of  the  air,  whilst  the  sulphur  separ- 
ates out.  At  a  temperature  of— 18°  the  gas  combines  with 
water  to  form  a  hydrate  l  which  probably  has  the  formula 
H2S  +  7H2O.2  Under  a  pressure  of  about  seventeen  atmo- 
spheres, sulphuretted  hydrogen  gas  condenses  to  a  colourless 
mobile  liquid,  which  boils  at  —  61°'8  and  has  a  tension  of  14*6 
atmospheres  at  ll°'l  and  at  —  85°  freezes  to  an  ice-like  solid. 
Liquid  sulphuretted  hydrogen  was  first  prepared  in  1823  by 
Faraday  by  means  of  the  simple  bent  tube  apparatus  described 
on  page  165.  In  the  closed  limb  of  this  tube  Faraday  brought 
some  strong  sulphuric  acid,  and  above  it  he  placed  some  small 
lumps  of  ferrous  sulphide,  taking  care  to  separate  them  from 
the  acid  by  a  piece  of  platinum  foil  placed  in  the  tube.  The 
open  end  of  the  bent  tube  was  then  closed  hermetically  and 
the  sulphide  shaken  down  into  the  acid.  In  making  experi- 
ments of  this  kind  on  the  liquefiable  gases,  many  precautions 
must  be  taken  if  we  would  avoid  serious  accidents  from  ex- 
plosions. The  tube  must  be  chosen  of  thick  well-annealed 
glass  and  the  materials  used  must  be  pure  ;  thus  if  metallic 
iron  be  contained  mixed  with  the  sulphide,  hydrogen  gas  will 
be  given  off  and  the  tube  will  probably  burst ;  in  sealing,  the 
sides  of  the  glass  tube  must  be  allowed  to  fall  together  so  as 
to  form  a  strong  end,  otherwise  the  tube  will  give  way  at  the 
weakest  point.  Prepared  with  care  these  "  Faraday's  tubes " 
will  withstand  an  internal  pressure  amounting  to  many  tons 
per  square  inch  of  surface,  and  the  liquefied  gases  may  be  kept 
in  them  with  safety  for  years.  Liquefied  sulphuretted  hydrogen 
may  also  be  prepared  by  passing  the  gas  into  a  tube  cooled  to 
about  —  70°  in  a  bath  of  solid  carbonic  acid  and  ether. 

Another  mode  of  preparing  liquid  sulphuretted  hydrogen  is  to 
seal  up  a  quantity  of  liquid  hydrogen  persulphide  (p.  357)  placed 
in  one  limb  of  a  Faraday's  tube.  This  body  spontaneously  de- 
composes into  sulphuretted  hydrogen  and  free  sulphur,  and  by 
degrees  the  tension  of  the  gas  inside  the  tube  increases  so  much 
that  the  gas  becomes  a  liquid. 

1  Wohler,  Annalen,  33,  125. 

2  Forcrand,  Compt.  Rend.  106,  1357. 


THE  SULPHIDES  355 


Both  as  a  gas  and  in  solution  in  water  sulphuretted  hydrogen 
is  converted  into  water  and  sulphur  by  nearly  all  oxidising 
agents,  and  even  by  strong  sulphuric  acid,  so  that  this  acid 
cannot  be  used  for  drying  the  gas. 


H2S  +  H2S04  =  S  +  2H20  +  S0 


2. 


If  two  cylinders,  one  filled  with  chlorine  and  the  other  with 
sulphuretted  hydrogen  gas,  are  brought  mouth  to  mouth,  an 
immediate  formation  of  hydrochloric  acid  gas  and  deposition 
of  sulphur  occurs.  Fuming  nitric  acid  dropped  into  a  globe 
filled  with  sulphuretted  hydrogen  gas  causes  decomposition  with 
explosive  violence. 

1  86  Both  in  the  form  of  gas  and  as  a  solution  in  water,  sul- 
phuretted hydrogen  is  largely  used  in  analytical  operations  as 
the  best  means  of  separating  the  metals  into  various  groups, 
inasmuch  as  certain  of  these  metals  when  in  solution  as  salts, 
such  as  copper  sulphate,  antimony  trichloride,  &c.,  are  pre- 
cipitated in  combination  with  sulphur  as  insoluble  sulphides 
when  a  current  of  this  gas  is  passed  through  an  acid  solution 
of  the  salt  or  mixture  of  salts  ;  thus  :  — 

CuS04  +  H2S  =  CuS  +  H2SO4. 
2SbCl3  +  3H2S  =  Sb2S3  +  6HC1. 

Certain  other  metallic  salts  are  not  thus  precipitated  because 
the  sulphides  of  this  second  group  of  metals  are  soluble  in  dilute 
acids  ;  thus  sulphuretted  hydrogen  gas  does  not  cause  a  pre- 
cipitate in  an  acidified  solution  of  ferrous  sulphate,  but  if  the 
acid  be  neutralized  by  soda  or  ammonia,  a  black  precipitate  of 
iron  sulphide  is  at  once  thrown  down. 

FeS04  +  H2S  +  2NaHO  =  FeS  +  Na2SO4  +  2H2O. 

Again  a  third  group  of  metals  exists,  the  members  of  which  are 
under  no  circumstances  precipitated  by  the  gas,  their  sulphides 
being  soluble  in  both  acid  and  alkaline  solutions. 

Many  of  the  insoluble  sulphides  are  distinguished  by  their 
peculiar  colour  and  appearance,  so  that  sulphuretted  hydrogen 
is  used  as  a  qualitative  test  for  the  presence  of  certain  metals 
as  well  as  a  means  of  separating  them  into  groups. 

The  reaction  of  sulphuretted  hydrogen  on  several  metallic  salt 
solutions  may  be  exhibited  by  means  of  the  apparatus  seen  in 


356 


THE  NON-METALLIC  ELEMENTS 


Fig.  106.  The  gas  evolved  in  the  two-necked  bottle,  (A)  passes 
through  the  several  cylinders,  and  precipitates  the  sulphides  of 
the  metals  whose  salts  have  been  placed  in  these  cylinders ; 
thus,  B  may  contain  copper  sulphate,  C  antimony  chloride,  D 
a  solution  of  zinc  sulphate  to  which  acid  has  been  added,  and  E 
an  ammoniacal  solution  of  the  same  salt. 

Many  of  the  sulphides  can  also  be  obtained  by  the  direct 
combination  of  the  metal  with  sulphur.  A  class  of  bodies 
termed  hydrosulphides  is  also  known  in  which  only  half  of  the 
hydrogen  of  the  sulphuretted  hydrogen  has  been  replaced. 
Thus,  in  addition  to  potassium  sulphide,  K2S,  we  have  potassium 


FIG.  106. 

hydrosulphide,  KHS,  and  in  addition  to  calcium  sulphide  CaS, 
which  is  insoluble  in  water,  we  have  the  soluble  calcium  hydro- 
sulphide,  Ca(HS)2.  Sulphuretted  hydrogen  therefore  acts  as  a 
weak  dibasic  acid  (p.  123).  The  sulphides  of  the  metals  of  the 
alkalis  and  alkaline  earths  also  combine  with  sulphur  forming 
unstable  poly  sulphides  such  as  K2S5,  CaS5. 

Sulphuretted  hydrogen  immediately  tarnishes  silver  with 
formation  of  black  silver  sulphide  ;  hence  it  is  usual  to  gild 
silver  eggspoons  to  prevent  them  from  becoming  black  by  con- 
tact with  the  sulphuretted  hydrogen  given  off  from  the  albumin 
of  the  egg.  Hence,  too,  silver  coins  become  blackened  when 
carried  in  the  pocket  with  common  lucifer  matches. 


HYDROGEN  PERSULPHIDE  357 


HYDROGEN  BISULPHIDE  on  PERSULPHIDE.  H2S2  =  65'64. 

187  This  substance  was  discovered  by  Scheele,  and  afterwards 
more  completely  investigated  by  Berthollet.  It  was  first 
obtained  in  the  form  of  a  yellow  transparent  oily  liquid  by 
pouring  a  concentrated  aqueous  solution  of  pentasulphide 
of  potassium,  K2S5,  into  dilute  hydrochloric  acid,  when  the 
liquid,  on  standing,  deposits  the  substance  in  yellow  drops. 
Hence  Berthollet  believed  that  the  substance  possessed  an 
analogous  composition  to  the  body  from  which  it  was  formed, 
and  gave  it  the  formula  H2S5.  Thenard  next  examined  the 
compound,  and  came  to  the  conclusion  that,  as  in  many  of 
its  properties  it  resembled  the  then  newly-discovered  hydrogen 
dioxide,  the  composition  of  the  body  would  be,  most  probably, 
represented  by  the  formula  H2S2 ;  although  it  may  be  remarked, 
that  the  analyses  of  Thenard  l  showed  that  it  always  contained 
more  sulphur  than  the  above  formula  required.  Hofmann  has 
observed,2  that  when  yellow  ammonium  persulphide,  (NH4)2S3,  is 
mixed  with  strychnine,  a  crystalline  compound  is  formed  having 
the  composition  C21H22N2O2  +  H2S3,  and  this  when  treated  with 
hydrochloric  acid,  yields  oily  drops  of  the  persulphide  which, 
therefore,  probably  has  the  formula  H2S3.  Ramsay 3  finds  that  the 
compound  obtained  by  Berthollet's  process  contains  such  varying 
proportions  of  hydrogen  and  sulphur  as  are  represented  by  the 
formulae  H2S7  and  H2S10.  Still  more  recently  Schmidt*  has 
shown  that  strychnine  combines  with  sulphuretted  hydrogen  to 
form  a  beautiful  red  crystalline  body,  but  that  it  does  so  only  in 
presence  of  oxygen  ;  thus  : — 

2C21H22N202+  6H2S  +  30  =  (2C21H22N2O2  +  3H2S2)  +  3H2O. 

This  compound  yields  hydrogen  persulphide  on  treatment  with 
an  acid,  and  the  substance  thus  obtained  appears  to  possess 
identical  properties  with  that  prepared  according  to  other  methods. 
The  formula  H2S5  has  been  obtained  by  Rebs  for  the  persul- 
phide obtained  by  decomposing  the  various  poly  sulphides  of  the 
alkali  metals  by  acids.5  The  formula  of  the  compound  has  how- 

1  Ann.  Chim.  Phys.  48,  79.  2  Ber.  1,  81. 

3  Journ.  Chfim.  Soc.  1874,  857.  4  Ber.  8,  1267. 

5  Annalen,  246,  356, 


358  THE  NON-METALLIC  ELEMENTS 

ever  been  finally  found  to  be  that  suggested  by  Thenard,  for 
Sabatier1  has  succeeded  in  distilling  the  substance  under  a  pressure 
of  40  mm.  its  boiling  point  under  these  conditions  being  from 
68 — 85°,  and  thus  obtained  it  in  colourless  drops,  the  composition 
of  which  was  intermediate  between  those  required  by  the 
formulae  H2S2  and  H2S3.  Hence  Thenard's  view  would  be  con- 
firmed that  this  compound  has  a  composition  analogous  to  that 
of  hydrogen  dioxide,  but  it  seems  that  owing  to  the  ease  with 
which  it  decomposes  into  sulphuretted  hydrogen  and  sulphur, 
which  readily  dissolves  in  the  liquid,  it  usually  is  found  to 
contain  an  excess  of  the  latter  substance. 

Hydrogen  persulphide  is  usually  prepared  by  boiling  one  part 
by  weight  of  slaked  lime  with  sixteen  parts  of  water  and  two 
parts  of  flowers  of  sulphur,  the  clear  cold  solution  being  poured 
into  dilute  hydrochloric  acid.  A  heavy  yellowish  oil  separates 
out,  sinking  to  the  bottom  of  the  vessel.  It  possesses  a 
characteristic  pungent  odour,  gives  a  vapour  which  attacks  the 
eyes,  and  has  a  very  acrid  and  unpleasant  taste;  its  specific 
gravity  was  found  to  be  about  1*7.  At  the  ordinary  tempera- 
ture it  undergoes  a  slow  decomposition  yielding  sulphur  and 
evolving  sulphuretted  hydrogen  gas.  Hydrogen  persulphide 
is  however  more  stable  in  presence  of  an  acid  than  in  that  of  an 
alkali;  it  is  easily  soluble  in  carbon  bisulphide  and  benzene, 
and  scarcely  soluble  in  alcohol.  It  bleaches  organic  colouring 
matters,  and,  like  hydrogen  dioxide,  reduces  the  oxides  of  gold 
and  silver  so  rapidly  that  it  ignites  (Odling).  The  persulphide 
dissolves  phosphorus  and  iodine,  gradually  changing  these 
bodies  into  phosphorus  sulphide  and  hydriodic  acid.  On  the 
other  hand,  sulphur  dioxide  has  no  action  on  the  persulphide, 
which,  in  this  respect,  differs  essentially  from  sulphuretted 
hydrogen. 


SULPHUR   AND   CHLORINE. 

188  These  elements  combine  together  to  form  the  following 
compounds  : — 

Sulphur  monochloride,  S2C12,  Sulphur  dichloride,  SC12,  and 
Sulphur  tetrachloride,  SC14. 

1  Compt.  Rend.  100,  1346  and  1500. 


SULPHUR  MONOCHLORIDE 


359 


SULPHUR  MONOCHLORIDE.    S2C12  =•  134-02. 

This  compound,  the  most  stable  of  the  chlorides  of  sulphur,  is 
obtained  as  a  dark  yellow  oily  liquid  by  passing  a  current  of 
dry  chlorine  gas  over  heated  flowers  of  sulphur. 

Preparation. — An  apparatus  arranged  for  this  purpose  is 
shown  in  Fig.  107.  The  sulphur  is  placed  in  a  retort  and  the 
•chloride  which  distils  over  is  collected  in  the  cooled  receiver. 
By  rectification  it  can  be  obtained  as  a  clear  amber-coloured 
liquid  possessing  an  unpleasant  penetrating  odour,  having  a 


FIG.  107. 

specific  gravity  of  17055  (Kopp),  and  boiling  at  138°.  The 
density  of  its  vapour  is  4*70,  corresponding  to  a  molecular 
weight  of  135'  26,  so  that  the  compound  contains  two  atoms  of 
sulphur  in  the  molecule,  and  has  the  formula  S2C12,  its  exact 
molecular  weight  being  134'  02.  When  thrown  into  water 
sulphur  monochloride  gradually  decomposes  with  the  formation 
of  hydrochloric  acid,  thiosulphuric  acid  and  sulphur  ;  thus  :  — 


2S2C1 


3H2O  =  4HC1 


S2  +  H2S2O3. 


A  number  of  other  products,  among  which  pentathionic  acid 
may  be  named,  are  also  formed  by  the  further  reaction  of  these 
substances. 


360  THE  NON-METALLIC  ELEMENTS 

Metals  decompose  it  on  heating  with  liberation  of  sulphur 
and  formation  of  the  chloride  of  the  metal. 

Sulphur  dissolves  in  the  monochloride  so  readily  that  the  solu- 
tion forms,  at  the  ordinary  temperature,  a  thick  syrupy  liquid 
containing  6G  per  cent,  of  sulphur.  This  property  has  been  largely 
employed  in  the  arts  for  the  purpose  of  vulcanizing  caoutchouc. 


SULPHUR  BICHLORIDE.    SC12  =  102-2. 

189  Chlorine  gas  is  rapidly  absorbed  when  passed  into  sulphur 
monochloride  at  the  ordinary  temperature,  and  the  liquid  changes 
colour  until  it  finally  assumes  a  dark  reddish  brown  tint.  When 
this  liquid  is  heated  it  begins  to  boil  at  64°,  but  the  thermometer 
soon  rises,  as  the  compound  undergoes  decomposition  into  free 
chlorine  and  the  monochloride  which  remains  behind.  .  If,  how- 
ever, the  monochloride  be  placed  in  a  freezing  mixture  and  then 
saturated  with  dry  chlorine,  and  the  excess  of  chlorine  be  subse- 
quently removed  by  a  current  of  dry  carbonic  acid  gas,  a  liquid 
remains  which  analysis  shows  to  be  the  dichloride.1  The 
molecular  weight  of  the  dichloride  dissolved  in  acetic  acid 
determined  by  the  freezing  point  method  agrees  with  the 
formula  SC12.2  V  In  combination,  the  dichloride  appears  to  be 
much  more  stably  It  forms  definite  compounds  with  arsenic 
trichloride  ;  thus : — 

SCl2AsCl3  (H.  Rose), 

and  with  ethylene  and  amylene ;  thus : — 

C2H4SC12,  C5H10SC12  (F.  Guthrie). 

It  is  decomposed  by  water  in  a  similar  manner  to  the 
monochloride. 


SULPHUR  TETRACHLORIDE.    SC14  =  172-58. 

190  The  existence  of  this  compound  was  for  a  long  time  a 
matter  of  uncertainty,  but  Michaelis  3  has  shown  that  when  the 
dichloride  is  saturated  with  chlorine  at  —  22°  the  tetrachloride 
is  formed.  It  is  a  light  mobile  yellowish-brown  liquid,  which 

1  Hiibnerand  Gueront,  Zeitschrift fur  Chemie,  1870,  455;  Thorpe  and  Dalziel, 
Ghem.  News,  24,  159. 
3  Costa,  Gazz.  Chim.  20,  367.  3  Annalen,  170,  1. 


SULPHUR  TETRACHLORIDE  361 

at  once  begins  to  evolve  chlorine  when  taken  out  of  the 
freezing  mixture.  Tetrachloride  of  sulphur  forms  crystallized 
compounds  with  certain  metallic  chlorides,  thus : — 2A1C13  SC14 ; 
SnCl4  2SC14. 

The  decomposition  of  both  of  these  bodies  serves  as  an 
excellent  illustration  of  dissociation,  which  doubtless  all  com- 
pounds undergo  when  their  temperature  is  raised  sufficiently 
high,  although  we  are  as  yet  unable  to  obtain  temperatures 
elevated  enough  to  decompose  many  of  them.  The  follow- 
ing table  shows  the  composition  of  the  liquid  obtained  by 
saturating  (1)  the  dichloride,  and  (2)  the  monochloride  of 
sulphur  with  chlorine  at  the  given  temperatures : — 

Dissociation  of  Sulphur  Tetrachloride. 

Temp.                           SC14.  SC12. 

-  22°  ....  100-0  ....  O'OO 

-  15    ....       41-95  ....  58-05 

-  10    ....       27-62  ....  72-38 

7  ....  21-97  ....  78-03 

2  ....  11-93  ....  88-07 

+     0-7  ....  8-87  ....  91-13 

+     6-2  ....  2-43  ....  97-57 

Dissociation  of  Sulphur  Dichloride. 


Temp. 
20°     .... 

SC12. 
93-45  . 

S2C12. 
.     .     .       6-55 

30      .... 

87-22  . 

.     .     .     12-78 

50     .... 

75-41  . 

.     .     .     24-59 

65     .... 

66-78  . 

.     .     .     33-22 

85     .... 

54-06  . 

.     .     .     45-94 

90     .... 

26-48  . 

.     .     .     73-52 

100     .... 
110              .     . 

19-45  . 
12-35  . 

.     .     .     80-85 
.     .     .     87-65 

120     .... 

5-44  . 

.     .     .     94-56 

130 

o-oo  . 

.  100-00 

From  these  tables  it  is  seen  that  whilst  a  difference  of  7°, 
from  --  22°  to  -  -  15°,  reduces  the  percentage  in  the  case  of 
SC14,  from  100  to  41 '95,  a  difference  of  10°  in  the  case  of  SC12 
reduces  it  only  from  93*45  to  87*22,  and  an  elevation  of  100° 
does  not  completely  dissociate  the  dichloride. 


THE  NON-METALLIC  ELEMENTS 


SULPHUR  AND  BROMINE. 

SULPHUR  MONOBROMIDE.    S2Br2  =  222-36. 

191  Bromine  and  sulphur  form  compounds  which  are  much 
less  stable  than  the  corresponding  chlorine  compounds.  The 
monobromide  is  prepared  by  dissolving  sulphur  in  a  slight 
excess  of  bromine,  and  volatilizing  the  excess  by  means  of  a 
current  of  dry  carbon  dioxide.1  It  is  a  ruby  red  liquid,  which 
boils  at  about  200°,  and  which  by  repeated  distillation  can  be 
decomposed  completely  into  sulphur  and  bromine. 

Sulphur  tetrabromide,  SBr4,  is  formed,  according  to  Michaelis, 
by  the  action  of  phosphorus  chlorobromide  upon  sulphur 
dioxide ; 2 

S02  +  2PBr2Cl3  =  2POC13  +  SBr4. 

It  decomposes  at  once  into  bromine  and  the  monobromide. 


SULPHUR  AND  IODINE. 

SULPHUR  MONIODIDE.     S2I2- 315-46. 

192  When  sulphur  and  iodine  are  heated  together,  even  under 
water,  they  combine  to  form  a  blackish  grey  crystalline  solid, 
resembling  in  its  appearance  the  native  sulphide  of  antimony, 
and  melting  below  60°.  This  substance  is  sulphur  moniodide, 
S2I2.  The  same  body  can  be  obtained,  according  to  Guthrie,3 
in  fine  tabular  crystals  by  acting  upon  ethyl  iodide  with  sulphur 
monochloride,  when  ethyl  chloride  and  sulphur  moniodide  are 
formed,  thus : — 

2C2H5I  +  S2C12  =  2C2HfiCl  +  S2I2. 

1  M.  M.  Pattison  Muir,  Journ.  Chem.  Soc.  1875,  845. 

2  Zeit.  Chem.  (2),  7,  185. 

3  Journ.  Chem.  Soc.  1862,  57. 


FLUORIDE  OF  SULPHUR  363 


SULPHUR  HEXIODIDE.    SI6. 

A  compound  of  sulphur  arid  iodine  having  the  above  formula 
is  stated  to  be  deposited  in  crystals  which  are  isomorphous  with 
iodine,  when  a  solution  of  iodine  and  sulphur  in  carbon  bi- 
sulphide is  evaporated  (Landolt  and  vom  Rath).  This  substance 
however  appears  to  be  simply  a  mixture  of  the  two  elements, 
(McLeod).1 


SULPHUR  SUBIODIDE.    S3I2  =  347*28. 

This  substance  is  formed  by  the  action  of  H2S  upon  an 
aqueous  solution  of  the  double  compound  of  iodine  trichloride 
with  potassium  chloride. 

It  forms  a  red  precipitate  which  loses  iodine  in  the  air,  and 
melts  above  60°. 


SULPHUR  AND  FLUORINE. 

193  According  to  Davy  and  Dumas,2  a  compound  of  sulphur 
and  fluorine  is  obtained  by  distilling  lead  fluoride  with  sulphur ; 
the  composition  of  the  body  has,  however,not  yet  been  ascertained. 
If  sulphur  is  brought  into  contact  with  fused  fluoride  of  silver, 
silver  sulphide  is  formed  and  a  heavy  colourless  gas  is  given  off, 
which  does  not  condense  at  0°,  fumes  in  contact  with  air,  smells 
like  the  chlorides  of  sulphur  and  sulphur  dioxide,  and  etches 
glass  (Gore). 

When  sulphur  is  exposed  to  the  action  of  gaseous  fluorine,3 
it  becomes  incandescent,  and  gives  off  a  gas  which  has  a  smell 
resembling  that  of  chloride  of  sulphur.  This  gas  is  incom- 
bustible, and  does  not  take  fire  even  when  mixed  with  oxygen 
or  brought  into  contact  with  a  flame.  When  the  gas  is  heated 
in  a  glass  vessel  the  glass  becomes  etched,  silicon  fluoride  being 
formed. 

1  Brit.  Ass.  Report,  1892,  690. 

2  Ann.  Ghim.  Phys.  31,  437. 

3  Moissan,  Ann.  Chim.  Phys.  [6],  24,  239. 


364  THE  NON-METALLIC  ELEMENTS 

SULPHUR  AND  OXYGEN. 

OXIDES  AND  OXYACIDS  OF  SULPHUR. 

194  Sulphur  forms  with  oxygen  two  compounds,  which 
belong  to  the  class  of  acid-forming  oxides,  and  which,  therefore, 
when  brought  into  contact  with  water,  both  yield  acids  ;  thus  : — 

Sulphur  dioxide,  SO9,  yields  Sulphurous  acid,  H2SO3. 
Sulphur  trioxide,  S03,  yields  Sulphuric  acid,  H2S04. 

In  addition  to  these  we  are  acquainted  with  the  oxides  S203, 
and  S207,  as  well  as  with  the  following  oxyacids  of  sulphur. 

Persulphuric  acid HSO4. 

Hyposulphurous  acid     ....  HSO2. 

Thiosulphuric  acid H2S9O3. 

Dithionic  acid H2S2O6. 

Trithionic  acid H2S3O6. 

Tetrathionic  acid H2S4O6. 

Pentathionic  acid H2S5O6. 

Hexathionic  acid H2S6O6. 

The  names  given  to  the  six  last  acids  are  derived  from  Qelov 
sulphur. 

SULPHUR  DIOXIDE.    SO2  =  63'58. 

The  ancients  were  aware  that  when  sulphur  is  burnt  pungent 
acid  smelling  vapours  are  evolved.  Homer  mentions  that  the 
fumes  from  burning  sulphur  were  employed  as  a  means  of 
fumigation,  and  Pliny  states  that  they  were  employed  for 
purifying  cloth.  For  a  long  time  it  was  thought  that  sulphuric 
acid  was  produced  when  sulphur  was  burnt,  and  it  is  to  Stahl 
that  we  are  indebted  for  first  showing  that  the  fumes  of 
burning  sulphur  are  altogether  different  from  sulphuric  acid, 
standing  in  fact  half  way  between  sulphur  and  sulphuric  acid, 
and,  therefore,  termed,  according  to  the  views  of  the  time,  phlogis- 
ticated  vitriolic  acid.  Priestley  in  1775  first  prepared  the  pure 
substance  in  the  gaseous  state,  to  which  the  name  of  sulphurous 
acid  was  afterwards  given. 


SULPHUR  DIOXIDE  365 


Sulphur  is  an  easily  combustible  body  ;  according  to  Dalton 
it  ignites  at  a  temperature  of  260°,  burning  in  the  air  with  a  pale 
blue  flame,  and  in  oxygen  with  much  greater  brilliancy.  In  this 
act  of  combustion  sulphur  dioxide  is  formed,  and  each  atom  of 
sulphur  evolves,  according  to  the  experiments  of  Favre  and 
Silbermann,  70,539  thermal  units.  The  volume  of  the  sulphur 
dioxide  formed  is  equal  to  that  of  the  oxygen,  as  may  be  shown 
by  the  following  experiment. 

The  apparatus  employed,  shown  in  Fig.  108,  is  similar  in  its 


FIG.  108. 

arrangement  to  the  syphon  eudiometer  previously  described 
(p.  176),  except  that  on  one  of  the  limbs  a  globe-shaped  bulb  has 
been  blown,  and  this  can  be  closed  by  a  ground-glass  stopper. 
This  stopper  is  hollow,  and  through  it  are  cemented  two  stout 
copper  wires  ;  one  of  these  ends  in  a  small  platinum  spoon, 
whilst  to  the  other  a  small  piece  of  thin  platinum  wire  is 
attached,  and  this  lies  on  the  platinum  spoon.  A  fragment  of 
sulphur  is  then  placed  over  the  thin  wire  in  the  spoon,  and  the 
tube  having  been  filled  with  oxygen  gas,  and  the  stopper  placed 
in  position,  the  sulphur  is  ignited  by  heating  the  wire  with  a 
current,  care  being  taken  to  reduce  the  pressure  on  the  gas  by 


366  THE  NON-METALLIC  ELEMENTS 


allowing  mercury  to  run  out  by  the  stop-tap,  so  as  to  avoid  danger 
of  cracking  the  globe.  The  eudiometer  is  then  allowed  to  cool 
again,  when  it  will  be  found  that  the  level  of  the  mercury  rises 
to  the  same  point  at  which  it  stood  before  the  experiment. 
Hence  one  molecule  of  sulphur  dioxide  contains  one  molecule, 
or  31  '76  parts  by  weight  of  oxygen  and  therefore  the  molecule 
of  the  dioxide,  which  weighs  63*58,  contains  31*82  parts,  or  one 
atom  of  sulphur,  and  its  molecular  formula  is  SO2. 

195  Preparation.  —  (1)  Sulphur  dioxide  is  formed  not  only 
by  the  combustion  of  sulphur,  but  also  by  the  action  of  certain 
metals,  such  as  copper,  mercury,  or  silver,  on  concentrated 
sulphuric  acid  ;  thus  :  — 


Cu  +  2H2S04  =  CuS04  +  2H20  +  SO 


2. 


Sulphur  dioxide  is  easily  prepared  for  laboratory  use  by 
the  above  reaction.  For  this  purpose  a  flask  is  half  filled  with 
copper  turnings  or  fine  copper  foil,  and  so  much  strong  sulphuric 
acid  poured  in  that  the  copper  is  not  quite  covered.  The  mixture 
is  next  heated  until  the  evolution  of  gas  commences  ;  the  lamp 
must  then  be  removed,  as  otherwise  the  reaction  may  easily 
become  too  violent,  and  the  liquid  froth  over. 

(2)  Pure  sulphur  dioxide  is  also  produced  when  sulphur  and 
sulphuric  acid  are  heated  together  ;  thus  :  —  , 


(3)  It  is  also  formed  by  the  decomposition  of  a  sulphite,  such 
as  commercial  sodium  sulphite,  which,  when  treated  with  warm 
dilute  sulphuric  acid,  easily  evolves  the  gas  ;  thus  :  — 


Na2S03  +  H2S04  =  Na2S04  +  H2O  +  SO 


2. 


A  convenient  method  of  employing  this  decomposition  is  to 
allow  concentrated  sulphuric  acid  to  drop  into  a  saturated  solu- 
tion of  sodium  bisulphite,  which  is  now  an  article  of  commerce. 

(4)  Sulphur  dioxide  is  made  on  the  large  scale  for  the  prepara- 
tion of  the  sulphites,  especially  of  sodium  sulphite  and  calcium 
sulphite,  which  are  obtained  by  passing  the  gas  either  into  a 
solution  of  caustic  soda  or  into  milk  of  lime.  For  this  purpose 
charcoal  is  heated  together  with  sulphuric  acid,  when  carbon 
dioxide  is  evolved,  together  with  sulphur  dioxide  ;  but  the 


SULPHUR  DIOXIDE  367 


presence  of  the  former  compound  for  the  purpose  above  men- 
tioned is  not  detrimental ;  thus : — 

C  +  2H2S04  =  2H20  +  C02  +  2SO2. 

The  sulphur  dioxide  evolved  in  the  roasting  of  certain 
metallic  ores,  which  was  formerly  allowed  to  pass  off  into  the 
atmosphere,  is  now  frequently  utilized  for  the  preparation  of 
the  sulphites. 

(5)  Sulphur  dioxide  is  used  in  enormous  quantities  for  the 
manufacture  of  sulphuric  acid.  For  this  purpose  it  is  chiefly 
obtained  by  roasting  pyrites.  When,  however,  perfectly  pure 
sulphuric  acid  is  needed,  the  dioxide  is  prepared  by  burning 
pure  sulphur. 

196  Properties. — Sulphur  dioxide  is  a  colourless  gas,  which 
occurs  in  nature  in  certain  volcanic  emanations,  as  well  as  in  solu- 
tion in  volcanic  springs.  It  possesses  the  well-known  suffocating 
smell  of  burning  sulphur.  Its  specific  gravity  is  2*2639  (Leduc),1 
and  it  can,  therefore,  be  collected  by  downward  displacement 
like  chlorine.  If,  however,  the  gas  is  required  to  be  perfectly  free 
from  air,  it  must  be  collected  over  mercury.  Sulphur  dioxide 
does  not  support  the  combustion  of  carbon-containing  material, 
and  a  burning  candle  is  extinguished  when  plunged  into  the 
gas.  Some  of  the  metals  however  take  fire  when  they  are 
heated  in  the  gas  ;  thus,  potassium  forms  the  thiosulphate  and 
sulphite,  and  tin  and  finely  divided  metallic  iron  are  changed 
partly  into  sulphide  and  partly  into  oxide  ;  lead  dioxide,  PbO2, 
ignites  when  plunged  into  the  gas  and  loses  its  brown  colour, 
with  formation  of  white  lead  sulphate,  PbSO4. 

Sulphur  dioxide  is  easily  soluble  in  water,  as  is  seen  from  the 
following  table  : 2 — 

T  1  vol.  of  water  1  vol.  of  the  solution 

dissolves  S02.  contains  S02. 

0°    ....     79789  vols.  .     .     .     68'861  vols. 
20°     ....     39-374     ....     36-206 
40°    ....     18-766      ....     17-013 

A  solution  of  the  gas  saturated  at  0°  deposits  a  crystalline 
hydrate  which  melts  at  a  temperature  of  about  4C  and  which 
probably  possesses  the  formula  H2SO3  +  14H20.  The  solution 
of  the  dioxide  has  a  strongly  acid  reaction,  and  therefore  reddens 
1  Compt.  Rend.  117,  219.  2  Bunsen  und  Schonfeld,  Annalen,  95,  2. 


368 


THE  NON-METALLIC  ELEMENTS 


blue  litmus  paper,  which  the  perfectly  dry  gas  does  not,  as  the 
dioxide  only  forms  sulphurous  acid,  H2S03  by  union  with 
water. 

Sulphur  dioxide  condenses  to  a  mobile  liquid  when  exposed 
to  pressure  or  cold.  This  liquid  boils  at  —  80,1  its  vapour  at  0° 
having  a  tension  of  1*16506  meters  of  mercury.  The  critical 
temperature  is  +  155°'4  and  the  corresponding  pressure  78'9 
atmospheres.  The  condensation  of  this  gas  by  pressure  can 
easily  be  shown.  For  this  purpose  an  ordinary  but  strong  glass 


FIG.  109. 

tube  20  mm.  in  diameter  may  be  used  ;  this  is  drawn  out  to  a 
point  at  one  end,  whilst  into  the  other  end  fits  a  greased 

±  '  O 

caoutchouc  plug,  fastened  on  to  an  iron  rod.  The  tube  having 
been  filled  with  the  dry  gas  by  displacement,  the  plunger  is 
inserted,  and  the  gas  forcibly  compressed  ;  when  the  plunger 
has  been  driven  down  so  that  the  gas  occupies  about  one -fifth 
of  its  original  bulk,  drops  of  the  liquid  are  seen  to  form  and  to 
collect  in  the  drawn-out  point.  At  a  temperature  above  its 
boiling  point,  liquid  sulphur  dioxide  evaporates  quickly,  absorb- 
ing much  heat,  the  temperature  sinking  to— 50°  if  a  quick 

1  Pierre,  Compt.  Rend.  70,  92. 


LIQUID  SULPHUR  DIOXIDE  369 

stream  of  air  be  driven  through  the  liquid.  If  the  liquid  be 
placed  under  the  receiver  of  an  air-pump  and  the  air  rapidly 
withdrawn,  evaporation  takes  place  so  quickly  and  so  much  heat 
is  absorbed  that  a  portion  of  the  liquid  freezes  to  a  white  snow- 
like  mass.  •  The  formation  of  the  solid  may  also  be  observed  in 
the  condensing  tube  when  the  plunger  is  quickly  drawn  out 
again.  According  to  Pierre  its  specific  gravity  at -20-5°  is 
T4911,  and  it  dissolves  iodine,  sulphur,  phosphorus,  resins,  and 
many  other  substances  which  are  insoluble  in  water. 

In  order  to  prepare  liquid  sulphur  dioxide  in  larger  quantity, 
the  apparatus  Fig.  109  is  used.  The  gas  evolved  by  the  action  of 
sulphuric  acid  on  copper  is  purified  by  passing  through  the  wash 
bottle,  and  afterwards  passes  through  the  spiral  glass  tube, 
surrounded  by  a  freezing  mixture  of  ice  and  salt.  The  liquid 
which  condenses  and  falls  into  the  flask  placed  beneath  may 
be  preserved  by  sealing  the  flask  hermetically  where  the  neck 


FIG.  110. 

has  been  drawn  out.  It  may  also  be  preserved  in  glass  tubes 
provided  with  well-closing  glass  taps,  the  construction  of  one 
of  which  is  seen  in  Fig.  110. 

Sulphur  dioxide,  both  in  the  gaseous  state  and  in  aqueous 
solution,  exerts  a  bleaching  action  on  vegetable  colouring 
matters.  This  fact  was  known  to  Paracelsus,  and  it  is  still 
made  use  of  in  the  arts  for  bleaching  silk,  wool,  and  straw 
materials,  which  are  destroyed  by  chlorine.  The  decolorising 
action  of  sulphur  dioxide  depends  upon  its  oxidation  in  presence 
of  water  with  formation  of  sulphuric  acid,  the  hydrogen  which 
is  liberated  uniting  with  the  colouring  matter  to  form  a  colour- 
less body  ;  thus  : — 

SO,  +  2H20  =  H2S04  +  2H. 

Thus  the  bleaching  action  of  this  substance  is  a  reducing  one, 

25 


370  THE  NON-METALLIC  ELEMENTS 

whilst  that  of  chlorine  is  an  oxidising  one.  The  colouring  matter 
thus  destroyed  by  bleaching  with  sulphur  dioxide  may  often  be 
restored  when  the  cloth  is  exposed  to  the  air,  as  in  the  case  of 
linen  marked  with  fruit  stains,  or  when  brought  in  contact  with 
an  alkali,  as  when  bleached  flannel  is  first  washed  with  soap. 
The  reducing  action  of  sulphur  dioxide  is  also  made  use  of  in  the 
paper  manufacture,  in  order  to  get  rid  of  the  excess  of  chlorine 
left  in  the  pulp  after  bleaching,  when  the  following  decomposition 
takes  place : — 

SQ2  +  C12  +  2H20  =  H2S04  +  2HC1. 

Sulphur  dioxide  is  also  a  powerful  antiseptic,  and  has  been 
successfully  employed  for  preventing  the  putrefaction  of  meat, 
as  well  as  to  stop  fermentation.  It  is  used  in  the  sulphuring 
of  wine,  and  also  as  a  disinfecting  agent.1 

Sulphur  dioxide  has  been  shown  by  Tyndall  to  undergo  a 
remarkable  decomposition  when  exposed  to  light.  If  a  beam  of 
sunlight  be  passed  through  a  long  tube  filled  with  the  colour- 
less gas  a  white  cloud  is  seen  to  make  its  appearance,  and  this 
consists  of  finely  divided  particles  of  sulphur  and  sulphur  tri- 
oxide,  which  are  separated  by  the  chemical  action  of  the  light. 
The  gas  is  also  slowly  decomposed,  when  a  series  of  electric 
sparks  is  passed  through  it,  into  sulphur  and  sulphur  trioxide, 
but  this  decomposition  ceases  when  a  certain  quantity  of  the 
latter  compound  is  formed,  and  can  only  be  fully  carried  out 
when  the  trioxide  is  removed  by  allowing  it  to  dissolve  in  strong 
sulphuric  acid. 

197  In  order  to  detect  sulphur  dioxide  some  paper  steeped  in 
a  solution  of  potassium  iodate  and  starch  is  brought  in  the  gas  ; 
this  will  at  once  be  turned  blue  by  the  formation  of  the  iodide 
of  starch,  if  even  only  traces  of  the  gas  be  present,  iodine  being 
liberated,  as  is  shown  by  the  following  reaction : — 

2KI03  +  5S02  +  4H20  =  I2  +  2KHSO4  +  3H2S04. 

An  excess  of  sulphurous  acid  however  will  bleach  the  blue  paper 
again  with  formation  of  hydriodic  acid ;  thus  : — 

I2  +  S02  +  2H20  =  2HI  +  H2S04. 

1  Liquid  sulphur  dioxide  is  now  manufactured  by  Messrs.   Boake  &  Co.  of 
Stratford,  London,  and  stored  in  glass  syphons  or  steel  cylinders. 


SULPHUROUS   ACID  371 


This  last  reaction  serves  as  an  excellent  means  of  determining 
the  quantity  of  sulphur  dioxide  present  in  solution.  For  this 
purpose  a  small  quantity  of  starch  paste  is  added  to  the  solution, 
and  then  a  standard  solution  of  iodine  is  added  by  means  of  a 
burette  to  the  solution  until  a  permanent  blue  colour  from  the 
formation  of  iodide  of  starch  is  observed.  It  is,  however,  to  be 
borne  in  mind  that  the  above  reaction  does  not  take  place  unless 
the  solutions  are  sufficiently  dilute,  for  in  concentrated  solution 
sulphuric  acid  and  hydriodic  acid  mutually  decompose,  forming 
free  iodine,  sulphurous  acid  arid  water  ;  thus  : — 

2HI  +  H2S04  =  I2  +  H2S03  +  H20. 

Bunsen,  who  has  investigated  this  subject  thoroughly,  finds 
that  aqueous  sulphurous  acid  can  only  be  completely  oxidised  to 
sulphuric  acid  by  means  of  iodine,  when  the  proportion  of  sulphur 
dioxide  does  not  exceed  0*04  to  0*05  per  cent,  of  the  solution. 
A  standard  solution  of  sulphurous  acid  may,  of  course,  also  be 
used  for  the  quantitative  determination  of  iodine,  and  Bunsen 
has  made  use  of  this  reaction  for  the  foundation  of  a  general 
volumetric  method.  The  principle  of  this  method  depends  on 
the  fact  that  a  quantity  of  iodine,  equivalent  to  that  of  the 
substance  under  examination,  is  liberated,  and  the  quantity  of 
this  iodine  is  determined  volumetrically  by  a  dilute  solution  of 
sulphurous  acid.1 


SULPHUROUS  ACID.    H2S03. 

198  This  substance,  like  many  other  acids  whose  corresponding 
anhydrides  are  gaseous,  is  only  known  in  aqueous  solution.  This 
solution  smells  and  tastes  like  the  gas  and  has  a  strongly  acid 
reaction.  Exposed  to  the  light  it  is  decomposed  with  formation 
of  pentathionic  acid.  When  an  aqueous  solution  saturated  at  3° 
is  allowed  to  stand,  crystals  of  a  hydrate,  H2SO3  +  6H2O,  are 
obtained.2  Sulphurous  acid  differs  from  the  acids  which  have 
hitherto  been  described,  inasmuch  as  it  contains  two  atoms  of 
hydrogen,  both  of  which  may  be  replaced  by  metals.  It  is 
therefore  termed  a  dibasic  acid  ;  it  forms  two  series  of  salts 
termed  sulphites,  in  one  of  which  only  half  of  the  hydrogen  is 

1  Journ.  Chem.  Soc.  1856,  219. 

2  Geuther,  Annalen,  224,  219. 


372  THE  NON-METALLIC  ELEMENTS 

replaced  by  a  metal,  and  which  may  therefore  be  considered  as 
being  at  once  a  salt  and  a  monobasic  acid,  another  in  which  the 
whole  of  the  hydrogen  of  the  acid  has  been  replaced  by  a  metal. 
The  salts  of  the  first  series,  are  termed  acid  sulphites,  and  of  the 
latter  normal  sulphites. 

The  following  serve  as  types  of  these  different  salts  : — 

Acid  Sulphites,  or  Hydrogen  Sulphites.  Normal  Sulphites. 

HNaSO3  Na2SO8 

HKSO3  KNaSO3 

HAgS03  CaS03. 

The  acid  sulphites  of  potassium  and  sodium  are  obtained  by 
passing  sulphur  dioxide  gas  into  caustic  soda  or  caustic  potash 
as  long  as  it  is  absorbed.  If,  then,  exactly  the  same  quantity  of 
alkali  is  added  to  this  solution  as  was  originally  taken  for  the 
preparation,  the  normal  salts  are  obtained.  All  the  sulphites  of 
the  alkali  metals  are  easily  soluble  in  water,  the  normal  sulphites 
of  the  other  metals  being  either  difficultly  soluble  or  insoluble 
in  water.  They  dissolve  however  in  aqueous  sulphurous  acid 
and  exist  in  such  a  solution  as  acid  salts,  but  on  evaporation  they 
decompose  with  formation  of  the  normal  salt  and  sulphurous 
acid.  Salts  of  sulphurous  acid  are  also  known  in  which  the 
two  hydrogen  atoms  of  the  acid  have  been  replaced  by  differ- 
ent metals.  Thus  when  caustic  soda  is  added  to  a  solution  of 
acid  potassium  sulphite,  or  caustic  potash  to  a  solution  of  acid 
sodium  sulphite,  a  salt  of  the  formula  NaKSO3,  sodium 
potassium  sulphite  is  formed.  The  salts  produced  by  these 
two  methods  are  not  however  identical,  but  are  found  to 
possess  different  properties.  They  crystallise,  for  example,  with 
different  amounts  of  water  of  crystallisation,  and  yield  different 
products  when  acted  upon  by  ammonium  sulphide  or 
ethyl  iodide.  As  these  salts,  although  possessing  the  same 
composition,  have  different  chemical  properties,  the  atoms  in 
their  molecules  must  be  arranged  differently,  in  other  words 
they  have  a  different  chemical  constitution.  This  difference  is 
expressed  in  the  following  formulae,  according  to  which  one 
atom  of  metal  is  combined  directly  with  oxygen  and  the 
other  with  sulphur. 

Sulphurous  Acid.  Normal  Sodium  Sulphite. 

H.SO2.OH.  Na.SO2.ONa, 


CONSTITUTION  OF  THE  SULPHITES  373 

Sodium  Potassium  Sulphite. 
I.  II. 

Na.SO2.OK.  K.SO2.ONa. 

Substances  such  as  these,  which  possess  the  same  molecular 
weight  and  composition  but  differ  in  their  chemical  and 
physical  properties,  are  said  to  be  isomeric.  Comparatively 
few  instances  of  this  kind  occur  among  the  inorganic  compounds 
but  they  are  very  frequent  among  the  compounds  of  carbon. 
Sulphurous  acid  also  forms  salts  such  as  Na2S205,  which  are 
known  as  metabisulphites,  and  other  more  complicated  series, 
for  an  account  of  which  the  original  memoirs  must  be  con- 
sulted.1 

The  normal  sulphites  have  no  odour,  and  those  which  are 
soluble  in  water  possess  a  sharp  taste.  They  are  readily  detected 
by  the  fact  that  when  they  are  mixed  with  dilute  sulphuric  acid 
they  give  off  sulphur  dioxide  and  also  that  their  neutral  solu- 
tions give  a  precipitate  with  barium  chloride  which  is  soluble  in 
dilute  hydrochloric  acid,  whereas  if  nitric  acid  be  added  to 
this  solution  and  the  mixture  warmed,  a  precipitate  of  barium 
sulphate,  formed  by  oxidation  from  the  sulphite,  is  thrown  down. 


THIONYL  CHLORIDE.    SOC12,  =  118-08. 

199  All  oxy-acids  and  many  other  compounds  contain  the 
group  OH,  which  is  a  monad  radical  known  by  the  name  of 
hydroxyl.  It  is  thus  termed  because  it  is  capable  of  taking  the 
place  of  monad  elements  such  as  chlorine  and  bromine,  and  in 
its  compounds  may  be  replaced  by  other  monad  elements.  If 
the  hydroxyl  of  an  acid  be  replaced  by  chlorine  an  acid  chloride 
is  obtained.  The  chloride  of  sulphurous  acid,  or  thionyl  chloride, 
has  been  obtained  by  the  action  of  phosphorus  pentachloride  on 
sodium  sulphite  (Carius) ;  thus  : — 

S03Na2  +  2PC15  -  SOC12  +  2POC13  +  2NaCl. 

Thiony]  chloride  is  also  obtained  by  passing  sulphur  dioxide  over 
pentachloride  of  phosphorus  (Schiff) ;  thus  : — 

S02  +  PC15  =  SOC12  +  POC13. 

1  Schwicker,  Ber.  22,  1728  ;  Rorig,  J.  pr.  Chen.  37?  250  ;  Barth,  Zeit.  phys. 
Chem.  9,  176  ;  Divers,  Journ.  Chem.  Soc.  1886,  i.  533  ;  Hartog,  Compt.  Rend. 
109,  436. 


374  THE  NON-METALLIC  ELEMENTS 


It  may  likewise  be  prepared  by  the  direct  union  of  sulphur  and 
chlorine  monoxide  (Wurtz). 

It  is  a  colourless  highly  refractive  pungent  liquid  which  fumes 
on  exposure  to  air.  It  boils  at  78°  and  has  a  specific  gravity  at 
0°  of  T675.  Like  all  other  acid  chlorides,  when  brought  in 
contact  with  water  it  decomposes  into  its  corresponding  acid 
and  hydrochloric  acid  ;  thus  : — 

SOC12  +  2H20  =  S03H2  +  2HC1. 


THIONYL  BROMIDE,  SOBr,,  =  206'42. 

200  This  substance  cannot  be  prepared  in  the  same  way  as 
the  chloride,  but  is  formed  by  the  action  of  bromine  on 
thionyl-aniline  x  and  by  that  of  thionyl  chloride  on  potassium 
bromide.2  It  forms  a  brown  liquid  and  boils  at  136°  but  de- 
composes largely  on  heating  into  sulphur  dioxide  and  bromide 
of  sulphur. 


SULPHUR  TRIOXIDE.    S03,  =  79*46. 

20 1  This  body,  which  is  called  also  sulphuric  anhydride,  and 
formerly  was  termed  anhydrous  sulphuric  acid,  is  formed  when 
a  mixture  of  sulphur  dioxide  and  oxygen  is  passed  over  heated 
platinum- sponge.  In  place  of  pure  platinum-sponge,  platinised 
asbestos  may  be  employed ;  this  is  obtained  by  dipping  some 
ignited  asbestos  into  a  tolerably  concentrated  solution  of  platinum 
chloride  and  then  bringing  it  into  a  solution  of  sal-ammoniac. 
The  insoluble  double  chloride  of  platinum  and  ammonium 
(NH4)9PtCl6  is  deposited  on  the  threads  of  the  asbestos,  and 
when  it  has  been  dried  and  ignited  this  compound  is  converted 
into  finely  divided  platinum. 

In  order  to  show  the  oxidation  of  sulphur  dioxide  to  the 
trioxide  the  apparatus  Fig.  Ill  may  be  employed.  Sulphur 
dioxide  is  evolved  in  the  flask  (a)  and  is  mixed  in  the  wash- 
bottle,  which  contains  strong  sulphuric  acid,  with  the  oxygen  from 
a  gas-holder  coming  in  through  the  tube  (b).  The  mixture  next 
passes  through  the  cylinder  (e)  containing  pumice-stone  soaked 
in  strong  sulphuric  acid  in  order  to  remove  every  trace  of 

1  Michaelis,  Ber.  24,  747. 

2  Hartog  and  Sims,  Chem.  News.,  67,  82. 


SULPHUR  TRIOXIDE 


375 


moisture,  and  then  passes  at  (c)  over  the  platinised  asbestos. 
As  long  as  this  is  not  heated  no  change  is  observed ;  so  soon, 
however,  as  it  is  gently  ignited  dense  white  fumes  of  the  tri- 
oxide  are  formed  which  condense  in  a  receiver  (d)  cooled  by  a 
freezing  mixture  in  the  form  of  long  white  needles.  In  order 
to  obtain  these  crystals,  every  portion  of  the  apparatus  must  be 
absolutely  dry ;  if  even  a  trace  of  moisture  be  present  the  needles 
disappear  at  once,  liquid  sulphuric  acid  being  formed.  Wohler 
has  shown  that  instead  of  platinum,  certain  metallic  oxides,  such 
as  copper  oxide,  ferric  oxide,  and  chromic  oxide,  may  be  used. 


FIG.  ill. 

A  much  more  convenient  process  for  preparing  sulphur  trioxide 
than  the  above  is  by  the  distillation  of  fuming  oil  of  vitriol. 
This  substance,  sometimes  called  Nordhausen  sulphuric  acid, 
consists  of  a  solution  of  the  trioxide  in  sulphuric  acid,  and  is 
obtained  by  the  distillation  of  heated  ferrous  sulphate.  The 
writer  known  as  Basil  Valentine  mentions  that  by  this  process 
a  "  philosophical  salt "  can  be  obtained,  but  the  preparation 
of  the  "  sal  volatile  olei  mtrioli  "  from  fuming  acid  was  first 
described  by  Bernhardt  in  the  year  1775. 

In  order  to  prepare  the  trioxide,  the  fuming  acid  must 
be  gently  heated  in  a  retort,  and  the  trioxide  collected  in  a 


376  THE  NON-METALLIC  ELEMENTS 

well-cooled  and  perfectly  dry  receiver,  where  it  is  obtained  in 
the  form  of  long  transparent  needles.  Sulphur  trioxide  is  also 
obtained  by  heating  concentrated  sulphuric  acid  with  an  excess  of 
phosphorus  pentoxide  ;  thus  : — 

H2S04  +  P2O5  =  SO3  +  2HPO3. 

The  same  substance  can  be  obtained  in  several  other  ways,  for 
instance  by  heating  dry  antimony  sulphate.  Sulphur  trioxide 
is  now  manufactured  on  the  large  scale  by  Messrs.  Chapman 
and  Messel  at  Silvertown.1 

Properties. — Sulphur  trioxide  forms  transparent  prisms  which 
melt  at  14°' 8  and  solidify  at  the  same  temperature.  The  melted 
trioxide  often  remains  for  a  considerable  length  of  time  in  the 
liquid  state  at  a  temperature  below  its  ordinary  point  of  solidi- 
fication, but  on  agitation  it  at  once  solidifies,  the  temperature 
rising  to  14°'8.  The  liquid  trioxide  has  a  specific  gravity  of  1*97 
at  20°,  and  boils  at  46° ;  its  coefficient  of  expansion  between 
25°  and  40°  is  0*0027,  a  number  which  is  much  higher  than  that 
ordinarily  observed  for  liquid  bodies  and  amounts  to  two-thirds 
of  the  coefficient  of  the  gases. 

A  second  modification  of  the  trioxide  is  said  to  be  obtained 
when  the  melted  mass  is  allowed  to  stand  at  a  temperature  below 
25°,  being  then  transformed  into  a  mass  of  silky  needles  which 
do  not  melt  below  50°.  When  these  silky  needles  are  melted 
they  undergo  a  change  into  the  first  modification.2  The  exist- 
ence of  two  modifications  is  denied  by  Weber,3  the  difference 
observed  being  due  to  impurities. 

Sulphur  trioxide  absorbs  moisture  readily  from  the  atmosphere 
and  evolves  dense  white  fumes  in  the  air.  Thrown  into  water 
it  dissolves  with  a  hissing  sound,  forming  sulphuric  acid  and 
evolving  a  large  amount  of  heat.  When  brought  into  contact 
with  anhydrous  baryta,  BaO,  a  combination  with  formation  of 
barium  sulphate,  BaSO4,  occurs  with  such  energy  that  the  mass 
becomes  red  hot. 

If  the  vapour  of  sulphur  trioxide  is  led  through  a  red-hot 
porcelain  tube  it  is  decomposed  into  two  volumes  of  sulphur 
dioxide  and  one  volume  of  oxygen.  This  fact  indicates  that  the 
formula  of  the  substance  is  SO3,  and  this  conclusion  is  borne  out 
by  the  vapour  density  which  is  2'75. 

1  Journ.  Soc.  Chem.  Ind.  4,  520. 

2  Schultz-Sellack,  Ber.  3,  216. 

3  Pogg.  Ann.  159,  313. 


SULPHUEIC  ACID  377 


SULPHURIC  ACID.    H2SO4. 

202  Sulphuric  acid  is,  without  doubt,  the  most  important  and 
useful  acid  known,  as  by  its  means  nearly  all  the  other  acids  are 
prepared,  whilst  its  manufacture  constitutes  one  of  the  most 
important  branches  of  modern  industry  owing  to  the  great 
variety  of  purposes  for  which  it  is  needed,  as  there  is  scarcely 
an  art  or  a  trade  in  which  in  some  form  or  other  it  is  not 
employed.  It  is  manufactured  on  an  enormous  scale,  no  less 
than  850,000  tons  being  at  present  annually  produced  in  Great 
Britain,  and  this  production  is  undergoing  constant  increase.1 

It  appears  probable  that  Geber  was  acquainted  with  sulphuric,, 
or,  as  it  was  formerly  called,  vitriolic  acid,  in  an  impure  state  'r 
but  the  writer  known  as  Basil  Valentine  was  the  first  fully  to 
describe  the  preparation  of  this  acid  from  green  vitriol  or 
ferrous  sulphate,  and  to  explain  that  when  sulphur  is  burnt 
with  saltpetre  a  peculiar  acid  is  formed. 

Originally,  sulphuric  acid  was  obtained  exclusively  by 
heating  green  vitriol  according  to  a  decomposition  which  we 
shall  study  hereafter.  The  present  method  of  preparing  the 
acid  is  said  to  have  been  introduced  into  England  from  the 
Continent  by  Cornelius  Drebbel ;  but  the  first  positive  infor- 
mation which  we  possess  on  the  subject  is  that  a  patent  for 
the  manufacture  of  sulphuric  acid  was  granted  to  a  quack  doctor 
of  the  name  of  Ward.2  For  this  manufacture  he  employed  glass 
globes  of  about  40  t<>  50  gallons  in  capacity ;  a  small  quantity 
of  water  having  been  poured  into  the  globe,  a  stoneware  pot  was 
introduced,  and  on  to  this  a  red-hot  iron  ladle  was  placed.  A 
mixture  of  sulphur  and  saltpetre  was  then  thrown  in  to  this 
ladle,  and  the  vessel  closed  in  order  to  prevent  the  escape  of  the 
vapours  which  were  evolved.  These  vapours  were  absorbed  by 
the  water,  and  thus  sulphuric  acid  was  formed.  This  product,  from 
the  mode  of  its  manufacture,  was  termed  oil  of  vitriol  made  by 
the  bell,  as  contradistinguished  from  that  made  from  green  vitriol, 
and  it  cost  from  Is.  6d.  to  2s.  6d.  per  Ib. 

Dr.  Roebuck  of  Birmingham  was  the  first  to  suggest  a  great 
improvement,  in  the  use,  instead  of  glass  globes,  of  leaden 
chambers,  which  could  be  constructed  of  any  wished-for  size. 

1  For  a  complete  account  of  the  manufacture  of  sulphuric  acid  see  Lunge's 
excellent  treatise  on  the  subject.     Gurney  and  Jackson,  London,  1891. 

2  See  Dossie's  Elaboratory  Laid  Open,  1758,  Intro,  p.  44. 


378  THE  NON-METALLIC  ELEMENTS 


Such  leaden  chambers  were  first  erected  in  Birmingham  in  1746, 
and  in  the  year  1749  at  Prestonpans  in  Scotland.  The  mode  of 
working  this  chamber  was  similar  to  that  adopted  with  the  glass 
globes ;  the  charge  of  sulphur  and  nitre  was  placed  within  the 
chamber,  ignited,  and  the  door  closed.  After  the  lapse  of  a 
certain  time,  when  the  greater  portion  of  the  gases  had  been 
absorbed  by  the  water  in  the  chamber,  the  door  was  opened,  the 
remaining  gases  allowed  to  escape,  and  the  chamber  charged 
again. 

The  leaden  chambers  first  set  up  were  only  six  feet  square, 
and  for  many  years  they  did  not  exceed  ten  feet  square,  but  in  these 
all  the  acid  employed  in  the  country  was  manufactured,  whilst 
much  was  exported  to  the  Continent,  where  the  chamber  acid  still 
goes  by  the  name  of  English  sulphuric  acid.  The  first  vitriol 
works  in  the  neighbourhood  of  London  were  erected  at  Battersea 
in  the  year  1772,  by  Messrs.  Kingscote  and  Walker,  and  in  1783 
a  connection  of  the  above  firm  established  works  at  Eccles,  near 
Manchester.  This  manufactory,  the  first  erected  in  Lancashire, 
contained  four  chambers,  each  twelve  feet  square,  and  four  others, 
each  of  which  was  forty-five  feet  long  and  ten  feet  wide. 

In  the  year  1 788  a  great  stimulus  was  given  to  the  manufacture 
of  sulphuric  acid  by  Berthollet's  application  of  chlorine,  dis- 
covered by  Scheele  in  1774,  to  the  bleaching  of  cotton  goods,  and, 
from  that  time  to  the  present,  the  demand  has  gradually  extended 
until  it  has  become  enormous  and  almost  unlimited  in  extent. 

The  next  improvement  in  the  manufacture  consisted  in  making 
the  process  continuous.  The  foundations  of  this  mode  of 
manufacture  appear  to  have  been  laid  by  Chaptal,  and  the 
principle  employed  by  him  is  that  which  is  at  the  present  day  in 
use.  The  improvements  thus  proposed  were  (1)  the  introduction 
of  steam  into  the  chamber  instead  of  water,  (2)  the  continuous 
combustion  of  the  sulphur  in  a  burner  built  outside  the  chamber, 
(3)  sending  the  nitrous  fumes  from  the  decomposition  of  nitre 
placed  in  a  separate  vessel,  along  with  the  sulphur  dioxide  gas 
and  air  into  the  chamber. 

203  The  theory  of  the  formation  of  sulphuric  acid l  in  the 
leaden  chamber  has  been  the  subject  of  much  discussion,  and 
even  at  the  present  day  the  views  of  chemists  differ.  The  view 
originally  proposed  by  Berzelius  may  be  simply  expressed  by 
saying  that  although  sulphur  dioxide  in  presence  of  water  or 

1  Peligot,  1844,  Ann.  Chim.  Phys.  [3]  2,  263  ;  R.  Weber,  1866,  Pogg.  Ann. 
127,  543  ;  Raschig.  Annalen,  241,  242  ;  Lunge,  loc.  cit. 


THEORY  OF  SULPHURIC  ACID  MANUFACTURE          379 

steam  is  unable  rapidly  to  absorb  atmospheric  oxygen,  it  is  able 
to  take  up  oxygen  from  such  oxides  of  nitrogen,  as  N203  or  NO2. 
If,  therefore,  these  oxides  are  present  in  the  chamber  they  give 
up  part  of  their  oxygen  to  the  sulphur  dioxide,  and  are  reduced 
to  nitric  oxide,  NO.  This  is,  however,  able  to  absorb  free 
oxygen,  and  is  at  once  reconverted  into  N2O3  or  NO2.  This 
continuous  reaction  may  be  represented  as  follows  :— 

(1)  N02  +  S02  +  H20  =  H2S04  +  NO. 

(2)  NO  +  0  =  N02. 

Another  view,  founded  upon  that  of  Davy,  is  due  to  Lunge. 
According  to  him,  nitrogen  peroxide,  NO2,  is  not  formed  in  the 
chambers,  and  is  therefore  not  the  essential  agent  which  brings 
about  the  oxidation  of  the  sulphur  dioxide,  nor  does  reduction 
to  nitric  oxide,  NO,  usually  occur,  the  active  substance  being 
nitrogen  trioxide,  N203,  (p.  508).  In  the  first  instance,  the 
sulphur  dioxide  combines  with  nitrogen  trioxide,  oxygen  and 
water  to  form  nitrosyl-sulphonic  acid,  S02(OH)NO0,  according 
to  the  equation  (1) : — 

(1)  2S02  +  N9O3  +  0.2  +  H2O  =  2SO,(OH)N00. 

(2)  2S02(OH)N02  +  H20  =  2H2S04  +  N2O3. 

This  substance  on  meeting  with  an  excess  of  water  vapour  is 
decomposed  into  sulphuric  acid  which  falls  to  the  bottom  of  the 
chamber,  and  nitrogen  trioxide  which  is  ready  to  react  again 
(equation  2).  The  existence  of  nitrosyl-sulphonic  acid  is  well 
known  to  the  manufacturers  of  sulphuric  acid,  since  it  is  formed 
as  a  white  crystalline  substance  when  the  supply  of  steam  has 
been  insufficient,  and  is  termed  by  them  "  chamber  crystals." 

It  seems  probable  that  in  practice  both  these  reactions  occur, 
the  formation  of  sulphuric  acid  at  the  commencement  of  the 
chambers  being  accompanied  by  the  presence  of  nitric  oxide,  NO, 
whilst  in  the  subsequent  stages  the  active  substance  is  nitrogen 
trioxide,  NgOg.1 

It  is  thus  clear  that  according  to  either  theory  nitrous  fumes 
act  as  a  carrier  between  the  oxygen  of  the  air  and  the  sulphur 
dioxide,  so  that,  theoretically,  a  small  quantity  of  these  fumes  will 
suffice  to  cause  the  combination  of  an  infinitely  large  quantity 
of  sulphur  dioxide,  oxygen,  and  water  to  form  sulphuric  acid. 

1  A  complete  discussion  of  this  somewhat  intricate  subject  will  be  found  in 
vol.  i.  chap.  x.  of  Lunge's  Sulphuric  Acid  and  Alkali,  1891. 


380 


THE  NON-METALLIC  ELEMENTS 


Practically,  however,  this  is  not  the  case,  because  instead  of 
pure  oxygen,  air  must  be  used,  and  four- fifths  of  this  consists  of 
nitrogen,  which  so  dilutes  the  other  gases  that  in  order  to  obtain 
the  necessary  action  a  considerable  quantity  of  these  oxides  of 
nitrogen  must  be  added.  Besides  this,  nitrogen  has  to  be 
constantly  removed  from  the  chambers,  and  in  its  passage 


Fia.  112. 

carries  much  of  the  nitrous  fume  away  with  it,  although  most 
of  this  can,  as  we  shall  see,  be  recovered  and  used  over  again. 

The  above  reaction  can  be  illustrated  on  the  small  scale  by  the 
apparatus  shown  in  Fig.  112,  in  which  sulphur  contained  in  the 
bulb-tube  is  allowed  to  burn  in  a  stream  of  air,  supplied  from 
the  double  aspirator ;  the  sulphur  dioxide  and  air  pass  through 
the  wide  glass  tube  into  the  large  glass  globe,  but  carry  in  on 
their  way  the  nitrous  fumes  generated  in  the  small  flask  (a), 


SULPHURIC  ACID  MANUFACTURE 


381 


from  nitre  and  sulphuric  acid.  The  flask  (b)  contains  boiling 
water,  from  which  steam  passes  into  the  globe.  The  outlet  tube 
(c)  of  the  globe  communicates  with  a  draught.  By  alternately 
increasing  and  diminishing  the  supply  of  sulphur  dioxide,  the 
disappearance  and  reappearance  of  the  red  nitrous  fumes  can  be 
readily  shown.  If  the  flask  be  kept  dry  whilst  the  two  gases 
are  passed  in,  the  white  leaden-chamber  crystals  are  seen  to  be 
deposited  on  the  glass.  When  aqueous  vapour  is  admitted, 
the  crystals  dissolve  with  formation  of  sulphuric  acid  and 
ruddy  fumes. 


FIG.  113. 


204  The  leaden  chambers  for  the  manufacture  of  sulphuric 
acid  are  now  constructed  of  a  much  larger  size  than  was  formerly 
the  case,  but  vary  considerably  in  different  works  ;  they  are 
frequently  30  meters  in  length,  6  to  7  meters  in  breadth,  and 
about  5  meters  in  height,  and  have  therefore  a  capacity  of  from 
900  to  1,000  cubic  meters  (about  38,000  cubic  feet).  The 
chambers  are  made  of  sheet  lead  weighing  35  kilos  per  square 
meter  (or  7  Ibs.  to  the  square  foot),  and  soldered  together  by 
melting  the  edges  of  the  two  adjacent  sheets  by  means  of  the 
oxyhydrogen  blow-pipe.  The  leaden  chamber  is  supported  by  a 


382 


THE  NON-METALLIC  ELEMENTS 


wooden  framework  to  which  the  leaden  sheets  are  attached  by 
strips  of  the  same  metal,  and  the  wooden  framework  is  generally 


raised  from  the  ground  on  pillars  of  brick  or  iron  and  the  whole 
erection  protected  from  the  weather,  sometimes  by  a  roof,  but 


SULPHURIC  ACID  MANUFACTURE 


383 


at  any  rate  by  boarding  to  keep  off  most  of  the  rain.     The  space 
below  the  chamber  is  used  either  for  the  sulphur  burners  or  for 


11. 


Decim.  10 

I      , 


the  concentrating  pans. 

The  general  appearance  or  bird's-eye  view  of  a  sulphuric  acid 


THE  NON-METALLIC  ELEMENTS 


chamber  is  shown  in  Fig.  113,  whilst  the  arrangement  arid  con- 
struction of  one  of  the  most  complete  forms  of  sulphuric  acid 
plant  now  in  use  in  this  country  is  shown  in  Figs.  114,  115,  and 
116.  Three  chambers,  termed  respectively  Nos.  1,  2,  and  3 
(Fig.  114),  are  placed  side  by  side  supported  on  iron  pillars 
ten  feet  high.  Each  chamber  has  the  dimensions  already  given, 
and  each,  therefore,  has  a  capacity  of  38,500  cubic  feet.  A 
longitudinal  section  of  the  chamber  (No.  2)  in  the  direction  (DC) 
is  shown  in  Fig.  115,  and  a  sectional  elevation  in  the  direc- 
tion (AB)  is  shown  in  Fig.  116.  From  this  last  figure  it  is 
seen  that  the  roof  of  the  chamber  is  not  horizontal  but  slightly 


slanting  so  as  to  enable  the  rain  to  run  off  into  gutters  placed  to 
receive  it. 

205  Beginning  at  the  first  part  of  the  process  we  find  the 
pyrites-kilns,  or  burners  placed  across  the  ends  of  the  chamber 
as  seen  in  plan  at  A,  Fig.  114,  in  longitudinal  section  and  in 
elevation  at  A,  Fig.  116,  and  in  cross  section  at  A,  Fig.  115. 
The  broken  pyrites,  FeS2,  is  filled,  in  moderately  sized  lumps, 
into  the  burners,  which  have  previously  been  heated  to  red- 
ness, and  when  the  burning  is  once  started  the  fire  is 
kept  up  by  placing  a  new  charge  on  the  top  of  that  nearly 
burnt  out.  The  ordinary  charge  for  each  burner  of  pyrites, 
containing  about  48  per  cent,  of  sulphur,  is  6  to  8  cwt.,  which 


SULPHURIC  ACID  MANUFACTURE 


385 


is  burnt  out  in  twenty-four  hours,  and  the  kilns  are  charged  in 
regular  succession,  so  that  a  constant  supply  of  gas  is  evolved 
during  the  whole  time,  whilst  the  quantity  of  air  which  enters 
the  kiln  is  carefully  regulated  by  a  well-fitting  door  placed 
below. 

The  hot  sulphur  dioxide,  nitrogen,  and  oxygen  gases,  are 
drawn  from  the  pyrites  burners,  through  the  whole  system  of 
tubes,  towers,  and  chambers,  by  help  of  the  powerful  draught 
from  a  large  chimney  which  is  placed  in  connection  with 
the  apparatus.  These  gases  first  pass  from  each  kiln 'into 
a  central  flue,  built  in  the  middle  of  the  kiln,  and  thence  into 


lal : i 


15  Meter. 
I 


an  upright  brick  shaft  through  a  horizontal  earthenware  flue,  or 
cast-iron  pipe,  into  the  lower  part  of  the  square  denitrating  tower 
seen  in  section  at  G  in  Fig.  116.  This  tower,  from  a  to  I,  is 
about  45  feet,  or  14  meters,  in  height ;  it  is  built  up,  from  a  to  c, 
to  a  height  of  25  feet,  or  8  meters,  of  lead  lined  with  fire  brick, 
and  of  this  about  15  feet,  or  5  meters,  from  d  to  e,  are  filled  up 
with  pieces  of  flint. 

The  object  of  this  Glover's  tower,  or  denitrating  tower  as  it  is 

termed,  is  to  impregnate  the  sulphur  dioxide  as  it  comes  from 

the  burners  with  nitrous  fumes  derived  from  a  later  stage  of  the 

^operation.     This  is  effected  by  allowing  strong  nitrated  acid  to 

flow  down  the  tower  together  with  a  stream  of  chamber-acid. 

26 


386 


THE  NON-METALLIC  ELEMENTS 


Strong  sulphuric  acid,  as  we  shall  see,  has  the  power  of  absorb- 
ing nitrous  fumes,  with  formation  of  nitrosyl-sulphonic  acid, 
S0.2(OH)(NO.2),  and  these  are  given  off  again  when  the  acid 


comes   into   contact  with    the   hot   sulphur  dioxide  from 
kilns  :  — 


m  fe 


the 


2S02(OH)(N02)  +  S0 


3SO2(OH)2  +  2NO. 


SULPHURIC  ACID  MANUFACTURE  387 


Two  reservoirs  are  placed  at  the  top  of  the  Glover's  tower ;  one 
containing  the  strong  nitrated  acid,  the  other  containing  the 
chamber-acid.  Both  the  strong  nitrated  and  the  chamber-acid 
are  allowed  to  flow  down  together  over  the  column  of  flint  stones 
in  given  proportions,  and  when  the  mixture  comes  into  contact 
with  the  upward  current  of  hot  sulphur  dioxide,  the  nitrous 
fumes  dissolved  in  the  strong  acid  are  given  off  and  swept 
away,  together  with  the  gases  from  the  burners,  direct  into  the 
chambers.  Although  the  nitrated  acid  loses  its  nitrous  fumes 
when  diluted  with  water,  it  does  not  do  so  in  presence  of 
chamber-acid  of  the  ordinary-strength,  so  that  the  denit rating 
effect  of  the  Glover's  tower  depends  on  the  reducing  action  of 
the  sulphur  dioxide,  rather  than  on  any  dilution  by  the  chamber- 
acid.  This  addition  is  mainly  made  for  the  purpose  of  cheaply 
concentrating  the  chamber-acid,  for  not  only  does  the  strong 
acid  lose  its  dissolved  nitrous  fumes,  with  the  production  of  a 
considerable  amount  of  acid,  but  the  weak  chamber-acid  coming 
in  contact  with  the  hot  dry  gases  which  enter  the  tower  at  a 
temperature  of  340°,  parts  with  a  large  quantity  of  its  water, 
which  goes  into  the  chamber  as  steam,  whilst  the  concentrated 
acid,  falling  to  the  bottom  of  the  tower,  flows  into  a  reservoir,  z, 
Fig.  114,  placed  to  receive  it. 

On  issuing  from  the  tower,  the  gas,  having  now  been  cooled 
by  contact  with  the  stream  of  acid  to  a  temperature  of  about  60°, 
passes  into  the  cast-iron  pipe,  x,  Fig.  114  (4  feet  6  inches  in 
diameter),  whence  it  is  delivered  at  the  further  end  of  chamber 
No.  1  at  a  height  of  8  to  9  feet  above  the  floor. 

206  The  supply  of  nitrous  fumes,  which  is  needed  to  act  as 
carrier  of  the  atmospheric  oxygen  to  the  sulphur  dioxide,  is 
furnished  by  the  three  nitre  pots  seen  in  plan  and  in  section 
in  Fig.  117.  The  charges  of  30  Ibs.,  or  13'5  kilos,  of  nitrate 
of  soda,  and  33  Ibs.,  or  15  kilos,  of  sulphuric  acid,  of  sp. 
gr.  r?5,  are  run  into  the  pots  from  the  outside,  and  after  the 
lapse  of  two  hours,  when  each  charge  is  exhausted,  the  fused 
bisulphate  of  soda  (technically  termed  sale  nixum)  is  run  off  into 
a  pan  placed  on  a  platform  outside  the  ove-n,  and  a  new  charge 
introduced.  The  decomposition  of  the  nitre  is  accelerated  by 
heat  from  the  pyrites  burners,  placed  below  the  brick  arch 
which  separates  them  from  the  pots,  and  the  nitric  fumes  are 
gathered  into  a  cast-iron  pipe,  z,  Fig.  117,  which  discharges  its 
contents  into  the  long  horizontal  main  carrying  the  products 
from  the  pyrites  burners  into  the  chamber.  Here  also  the 


388 


THE  NON-METALLIC  ELEMENTS 


nitric  acid  vapour  parts  with  some  of  its  oxygen  and  is  reduced 
by  the  sulphur  dioxide  to  the  lower  oxide  which  acts  as  a 
carrier  between  the  oxygen  and  sulphur  dioxide. 

The  mixture  of  oxygen,  nitrogen,  sulphur  dioxide,  nitrous 
fumes  and  vapour  of  water  now  meets  with  steam  introduced 
into  the  chamber  by  the  tubes,  s  s,  Fig.  115,  and  the  reaction  as 
already  described  sets  in.  Having  travelled  through  the  length 
of  chamber  No.  1,  the  gases  pass  by  means  of  the  connecting 
shaft  (v)  shown  in  Fig.  116,  into  the  second  chamber,  where 
they  likewise  meet  with  steam  jets,  and  having  passed  through 
this  chamber,  and  having  deposited  a  further  amount  of  liquid 


FIG.  117. 

sulpnuric  acid,  which  falls  on  the  floor  of  the  chamber,  the  gases 
are  drawn  into  the  third  or  exhaust  chamber  by  the  flue  (w) 
shown  in  Fig.  116.  Here,  if  the  process  is  properly  worked,  all  the 
sulphur  dioxide  is  converted  into  sulphuric  acid,  and  red  nitrous 
fumes  must  always  be  visible.  For  the  purpose  of  determining 
the  proper  working  of  the  process  the  percentage  of  sulphur 
dioxide  contained  in  the  gases  entering  the  first  chamber,  and 
that  of  the  oxygen  in  the  gases  leaving  the  third  chamber,  is 
regularly  ascertained  in  carefully  managed  works. 

The  nitrous  fumes  having  been  added  in  excess  of  the  quantity 
required  to  convert  the  SO2  into  H2SO4  still  remain  in  chamber 


SULPHURIC  ACID  MANUFACTURE  389 

No.  3,  and,  in  order  to  absorb  these,  a  Gay-Lussac  tower  (G", 
Figs.  114  and  116)  is  employed,  the  capacity  of  which  ought  to 
be  at  least  one  hundredth  part  of  that  of  all  the  chambers. 
This  consists,  like  the  Glover's  tower,  of  a  square  tower  50  feet 
in  height,  made  of  strong  lead  (7—8  Ib.)  and  lined  for  35  feet 
with  2  inch  thick  glazed  fire  tiles,  and  filled  with  coke.  The 
exit  gases  from  chamber  No.  3  are  drawn  in  at  the  bottom  of 
this  coke  column,  and  escape  to  the  chimney  by  the  exit  tube 
(i,  Fig.  116)  at  the  top.  In  their  passage  they  come  in  contact 
with  a  finely  divided  shower  of  strong  cold  acid  (sp.  gr.  1*75) 
obtained  by  concentrating  the  chamber-acid.  This  strong  sul- 
phuric acid  absorbs  the  excess  of  nitrous  fumes  which  would 
otherwise  pass  away  up  the  chimney,  and  having  thus  become 
saturated  with  nitrous  fumes,  runs  away  through  the  spout  Jc 
into  reservoirs  for  the  so-called  nitrated  acid,  built  under 
chamber  No.  3,  the  position  of  which  (mm,  Fig.  114)  is  shown 
on  the  plan.  From  these  reservoirs  the  nitrated  acid  is  allowed 
to  run  into  one  of  the  cast-iron  air  boilers  (nnn)  shown  on  the 
plan,  whence,  by  air  pressure,  it  is  forced  up  to  the  cistern  on 
the  top  of  the  Glover's  tower  for  employment  in  the  first  part 
of  the  process  as  already  described. 

207  The  continuous  process  of  acid  making  in  the  chambers 
is  only  carried  on  until  the  acid  has  attained  a  specific  gravity  of 
1'53  to  1*62,  or  contains  62 — 70  per  cent,  of  the  pure  acid,  H2SO4, 
inasmuch  as  an  acid  stronger  than  this  begins  to  absorb  the 
nitrous  fumes.  In  order  to  obtain  a  stronger  acid,  either  the 
arrangement  of  the  Glover's  tower,  as  described,  is  employed 
or,  in  works  where  the  Glover  is  not  used,  the  chamber-acid 
is  run  into  the  leaden  concentrating  pans  (po)  placed  under 
chamber  No.  2,  shown  in  plan  in  Fig.  114,  and  in  section  in 
Fig.  115.  The  flame  and  heated  air  from  the  fires  (q)  play 
over  the  surface  of  the  acid  contained  in  these  pans,  the  water 
passes  away  in  the  form  of  steam,  and  the  strong  acid,  remains. 
By  this  means  the  acid  can  be  concentrated  until  it  attains  a 
specific  gravity  of  1*72,  or  contains  79  per  cent,  of  pure  acid ; 
beyond  this  degree  of  concentration  the  hot  acid  begins  rapidly 
to  attack  the  lead  of  the  pans,  and  it  therefore  cannot  be  fur- 
ther evaporated  in  them.  It  is  then  run  off  into  the  acid 
cooler  (p),  a  leaden  trough  surrounded  by  cold  water,  whence  it 
passes  into  the  strong-acid  cisterns  (r,  Fig.  115).  In  this  form 
the  acid  is  technically  known  as  B.  O.  V.,  brown  oil  of  vitriol, 
as  it  is  always  slightly  coloured  from  the  presence  of  traces  of 


390 


THE  NON-METALLIC  ELEMENTS 


SULPHURIC  ACID  MANUFACTURE  391 

organic  matter,  and  it  is  in  this  condition  that  it  is  very  largely 
sold  for  a  great  variety  of  purposes. 

208  In  order  to  drive  off  the  remaining  portions  of  water,  the 
acid  must  be  concentrated  or  rectified  in  platinum  or  glass  vessels. 
A  common  arrangement  for  concentrating  in  platinum  stills  is 
shown  in  Fig.  118,  as  manufactured  by  Messrs.  Johnson,  Matthey, 
and  Co.,  of  London.  By  means  of  this  apparatus  no  less  than 
200  cwt.  (10,000  kilos)  of  brown  oil  of  vitriol  can  be  daily 
concentrated,  yielding  a  product  containing  98  per  cent,  of 
real  acid. 

The  retort  or  still  (A,  Fig.  118)  consists  of  plates  of  platinum, 
the  joints  of  which  are  autogenously  soldered.  This  rests  on 
the  iron  ring  (c).  The  chamber-acid  runs  from  the  stopcock 
through  a  platinum  tube  on  to  the  heated  thick  bottom  of  the 
still,  where  it  is  quickly  concentrated,  whilst  the  aqueous  vapour 
escapes  by  the  head  of  the  still  (L).  As  soon  as  the  level  of  the 
concentrated  acid  reaches  the  top  of  the  platinum  funnel  (D),  it 
begins  to  flow  off  by  means  of  the  tube  (E)  into  the  platinum 
vessel  (F),  round  which  a  current  of  cold  water  circulates. 
Having  been  thus5  cooled,  the  acid  passes  into  the  stoneware  jar 
(H),  surrounded  by  /water,  and  thence,  by  means  of  the  lead  or 
stoneware  funnel  (i)  into  the  reservoir  (K). 

Messrs.  Johnson,  Matthey,  and  Co.  <  have  introduced  an  im- 
proved form  of  platinum  concentrating  apparatus,  by  means  of 
which  all  evaporation  in  leaden  pans  is  avoided,  and  thus  the 
operation  not  only  considerably  cheapened,  but  the  acid  ob- 
tained in  a  purer  condition.  This  new  arrangement  is  repre- 
sented in  Fig.  119.  A  A  are  pans  made  of  platinum  plates, 
which  are  corrugated  at  the  bottom,  and  heated  by  a  fire  placed 
below.  In  these  the  concentration  proceeds  until  the  acid 
attains  a  strength  of  from  78  to  80  per  cent,  of  H2S04.  It  then 
runs  into  the  retort  (B),  also  having  a  corrugated  surface,  and 
the  perfectly  concentrated  acid  which  is  thus  obtained  is  cooled 
by  passing  through  the  worm  (D),  made  of  platinum  tube. 

In  many  English  works  the  sulphuric  acid  is  rectified  in  glass 
and  not  in  platinum  vessels.  These  glass  vessels  are  large 
retorts  made  of  well-annealed  and  evenly-blown  glass  (Fig.  120 
a),  of  such  a  size  as  to  contain  twenty  gallons  of  the  acid.  Each 
retort  is  placed  on  an  iron  sand-bath  (&),  round  which  the 
flames  from  a  fire  are  allowed  to  play,  but  so  that  the  flame 
does  not  touch  the  retort.  A  glass  head  (c)  fits  loosely  into 
the  neck  of  the  retort,  and  through  this  the  aqueous  vapour 


392 


THE  NON-METALLIC  ELEMENTS 


1 


SULPHURIC  ACID  MANUFACTURE  39S 

carrying  with  it  a  little  acid  fume,  passes  into  a  condensing 
box.  The  plan  of  a  rectifying  house  containing  twenty-four 
retorts  is  shown  in  Fig.  121.  The  acid  having  been  concentrated 
in  the  leaden  pans  (A  A  A),  passes  along  the  leaden  tubes  (BB  B)^ 
from  which  the  retorts  are  filled  by  means  of  the  upright  leaden 
tubes  (d,  Fig.  120),  which  can  be  bent  so  as  to  discharge  the 
acid  into  the  neck  of  the  retort.  After  the  rectification  is  com- 
plete the  retorts  are  allowed  to  cool  for  twelve  hours,  and  the 
acid  is  then  drawn  out  by  means  of  leaden  syphons  into  the 
stoneware  coolers  (it  Fig.  120). 

In  order  to  effect  a  continuous  rectification  in  glass  vessels, 
the  following  arrangement  has  been  adopted  in  some  works. 
Three  of  the  retorts  are  placed  one  above  the  other,  as  is  shown 


FIG.  120. 


in  Fig.  122.  As  soon  as  the  acid  in  retort  (B)  has  attained  a 
specific  gravity  of  1-84,  the  retort  is  connected  with  a  system  of 
syphon  tubes  (///),  and  acid  of  the  specific  gravity  of  T74,  and 
having  a  temperature  of  150°,  is  allowed  to  run  into  the  upper- 
most retort  (D)  by  means  of  the  stopcock.  This  acid  gradually 
passes  through  the  three  retorts,  D,  c,  and  B,  and  when  it  has 
reached  the  last  one  it  has  attained  a  specific  gravity  of  1*84, 
and  is  allowed  to  run  off  through  a  cooling  chamber  Qi)  into 
the  carboy.  Another  recent  and  efficient  system  of  continuous 
concentration  in  glass  vessels  has  been  patented  by  Webb,1 
and  improved  by  Levinstein,2  by  means  of  which  acid  containing 
about  96  per  cent,  of  H2SO4  (sp.  gr.  1-84)  can  be  obtained  in  a 

1  Eng.  patent,  2,343  (1891)  ;  17,407  and  18,891. 

2  Eng.  patent,  19,213  (1892). 


394 


THE  NON-METALLIC  ELEMENTS 


single  operation  from  acid   of  sp.  gr.  T625,  containing  about 
71  per  cent,  of  H2SO4. 

209  According  to  theory,  100  parts  of  sulphur  burnt  should 
yield  306*25  parts  of  pure  sulphuric  acid.  In  practice,  however, 
this  theoretical  yield  is  never  attained,  and  for  several  reasons ; 
in  the  first  place  because  a  certain  amount  of  loss  must  neces- 
sarily take  place  in  working  with  such  enormous  volumes  of 
gas,  and  in  the  second  place  inasmuch  as  an  unavoidable  loss 
occurs  in  the  processes  of  concentration ;  and  thirdly,  owing  to 


FIG.  121. 


the  fact  that  an  amount  of  sulphur  varying  from  2  to  5  per  cent, 
remains  behind  in  the  burnt  ore,  and  this  amount  cannot  be 
accurately  allowed  for.  As  a  general  rule  a  yield  of  270  to  280 
parts  of  pure  acid  from  100  of  sulphur  is  practically  considered 
about  the  proper  production,  so  that  about  5  per  cent,  of  sulphur 
is  lost  on  the  average,  of  which,  however,  only  a  portion  passes 
out  in  the  gaseous  form  into  the  air.  In  cases  where  special 
precautions  are  taken  the  yield  sometimes  reaches  from  294  to 
297,  but  when  the  manufacture  is  not  carefully  conducted  much 
more  serious  losses  occur.  Thus  in  his  eighth  annual  report 


SULPHURIC  ACID  MANUFACTURE 


395 


{1871)  Dr.  R.  Angus  Smith  gives  (p.  17)  a  table,  showing  the 
total  escape  of  sulphur  acids  (calculated  as  sulphuric  acid)  from 
twenty-three  chemical  works.  From  this  it  appears  that  whilst 
from  some  of  the  works  no  escape  of  these  acids  occurs,  the  average 
loss  of  sulphuric  acid  in  the  twenty-three  works  in  question  is 
7*606  per  cent,  on  the  total  quantity  obtainable  from  the  sulphur 
burnt,  and  that  the  loss  in  the  case  of  four  works  actually  rises 
to  more  than  20  per  cent.,  in  one  case  amounting  to  an  escape  of 
159  Ibs.  of  sulphuric  acid  every  hour.  Facts  like  these,  says  the 


FIG.  122. 


inspector,  dispose  of  the  argument  often  used  by  the  manu- 
facturers, that  they  require  the  acid,  and  that  it  is  to  their 
interest  to  keep  it,  and  of  course  condense  it  to  the  best  of  their 
power.  Indeed,  certain  makers  are  fully  aware  that  they  are 
allowing  sulphuric  acid  to  escape  in  large  quantities,  but  their 
reply  is  that  it  is  cheaper  to  permit  a  large  escape  and  work 
rapidly  rather  than  have  large  chambers  and  condense  the  whole 
of  their  gases. 

By  the  Alkali  Act  of  1881  the  limit  of  4  grains  per  cubic  foot 
has  been  set  to  the  amount  of  sulphuric  anhydride  which  may 


396  THE  NON-METALLIC  ELEMENTS 

be  left  in  the  gases  escaping  from  the  chambers.  The  average 
actually  attained  is  1*284  grains  per  cubic  foot.1 

The  amount,  again,  of  nitrate  of  soda  or  Chili-saltpetre  used, 
varies  considerably  even  in  the  best  works,  according  to  the  rate 
at  which  the  reaction  is  permitted  to  proceed,  and  the  complete- 
ness and  rapidity  with  which  the  nitrous  fumes  can  be  recovered 
in  the  Gay-Lussac  tower  and  again  brought  into  the  chamber. 
Manufacturers  who  employ  Glover  and  Gay-Lussac  towers  use  on 
an  average  3'5  to  6'5  parts  of  nitrate  for  every  100  of  sulphur 
burnt,  whilst  at  works  where  these  appliances  are  not  in  use  the 
quantity  of  nitre  required  may  rise  to  from  12  to  13  parts.  The 
larger  the  quantity  of  nitrous  fumes  present  in  the  chamber,  the 
quicker  will  be  the  formation  of  sulphuric  acid,  and  the  proportion 
of  fumes  which  pays  best  is  a  question  for  the  manufacturer  in  each 
instance  to  decide.  A  certain  loss  of  oxides  of  nitrogen  cannot, 
of  course,  be  avoided ;  the  fumes  are  partly  not  completely  con- 
densed, and  pass  out  by  the  chimney,  and  partly,  in  all  prob- 
ability, reduced  by  the  sulphur  dioxide  to  nitrous  oxide  or  even 
to  nitrogen,  which,  as  they  cannot  combine  again  with  the 
atmospheric  oxygen,  must  escape  into  the  air. 

In  order  to  convert  100  parts  of  sulphur  into  sulphuric  acid, 
about  210  parts  of  water  in  the  form  of  steam  are  needed.  This 
steam  is  costly  in  its  production,  but  an  attempt  made  by 
Sprengel  to  reduce  this  item  of  expenditure  by  employing  a 
jet  of  water  in  the  form  of  spray  or  in  a  state  of  very  minute 
division  has  proved  unsuccessful. 

21  o  None  of  these  processes  yield,  it  must  be  remembered, 
chemically  pure  acid,  inasmuch  as,  in  the  first  place,  the  water 
cannot  thus  be  completely  removed,  and  secondly,  because  im- 
purities, such  as  sulphate  of  lead,  arising  from  the  action  of  the 
acid  on  the  leaden  concentrating  pans,  and  arsenic  derived  from  the 
pyrites,  are  not  got  rid  of  by  this  process  of  simple  concentration. 

In  order  to  prepare  pure  sulphuric  acid,  the  commercial 
product  must  be  distilled  in  a  glass  retort  until  one-third  has 
passed  over;  then  the  receiver  is  changed  and  the  acid  dis- 
tilled nearly  to  dryness.  It  not  unfrequently  happens  that  in 
this  process  the  acid  bumps  violently  on  ebullition,  owing 
to  a  small  quantity  of  solid  lead  sulphate  being  deposited 
on  the  bottom  of  the  retort ;  the  addition  of  small  pieces  of 
platinum  foil  or  wire  stops  this  to  a  certain  extent,  but  a  better 
preventive  is  either  to  heat  the  retort  at  the  sides  rather  than 
1  Report  of  Inspector  of  Alkali  Works,  1892,  p.  8. 


PROPERTIES  OF  SULPHURIC  ACID  397 

.at  the  bottom,  or,  when  the  ebullition  becomes  percussive,  to 
allow  the  liquid  to  cool,  then  to  pour  off  the  clear  acid,  leaving 
the  deposit  behind,  and  to  proceed  with  the  distillation  of  the 
•clarified  liquid.  A  slow  current  of  air  passed  through  the 
boiling  acid  has  also  been  found  efficacious. 

211  Properties.  —  The  acid  thus  purified  by  distillation  still  con- 
tains about  1*5  per  cent,  of  water  which  cannot  be  removed  by 
this  process.  If,  however,  the  distillate  be  cooled,  the  pure  acid 
containing  100  per  cent,  of  H2SO4,  separates  out  in  the  form  of 
crystals  which  melt  at  10  '5°.  These  crystals  when  once  melted 
generally  remain  liquid  for  a  considerable  time,  even  when 
cooled  below  their  freezing  point,  the  liquid  only  solidifying 
when  it  is  agitated  or  when  a  small  crystal  of  the  acid  is  added, 
the  temperature  then  rising  to  10'5°.  The  specific  gravity  of  the 
pure  liquid  acid  is  l'^3  7  at  15°  compared  with  water  at  4°  (or,  as 
it  is  usually  expressed,  15°/4°),1  whilst  according  to  Lunge  and 
Naef,2  it  is  I  '8384.  When  the  pure  acid  is  heated,  it  begins  to 
fume  at  30°  inasmuch  as  it  then  partially  decomposes  into 
water  and  sulphur  trioxide.  This  dissociation  increases  with 
increase  of  temperature  until  at  338°,  the  boiling-point  of  the 
liquid  (Marignac),  a  large  quantity  of  trioxide  is  volatilized,  so 
that  the  residue  contains  from  98'4  to  98'8  per  cent,  of  the 
real  acid,  and  then  this  liquid  may  be  distilled  without  altera- 
tion. The  vapour  of  sulphuric  acid  when  it  is  more  strongly 
heated  completely  decomposes  into  water  and  the  trioxide. 
According  to  Deville  and  Troost  the  vapour  density  at  440° 
is  25,  whilst  for  equal  volumes  of  aqueous  vapour  and  sulphur 
trioxide  the  calculated  vapour  density  is 

17-88  +  79-46 


When  heated  still  more  strongly,  the  trioxide  thus  formed  itself 
splits  up  into  oxygen  and  sulphur  dioxide.  This  decomposition 
may  be  readily  shown  by  allowing  sulphuric  acid  to  drop  slowly 
into  the  platinum  flask  (a,  Fig.  123),  which  is  filled  with  pumice- 
stone  and  heated  strongly  by  the  lamp  ;  the  mixture  of  gases 
which  escapes  consists  of  one  volume  of  oxygen  to  two  volumes 
of  sulphur  dioxide,  which  latter  gas  is  absorbed  by  passing 
through  water  containing  caustic  soda,  the  oxygen  escaping  in 
the  free  state,  whilst  any  undecomposed  sulphuric  acid  is  con- 
densed in  the  U-tube  and  collects  in  the  flask  (d).  It  has  been 

1  Marignac,  Ann.  Chim.  Phys.  [3],  192  ;  Mendelejeff,  Ber.  17,  2536. 

2  Chem.  Industrie,  1883,  37. 


398 


THE  NON-METALLIC  ELEMENTS 


proposed  to  use  this  process  for  the  preparation  of  oxygen  on 
the  large  scale,  as  the  material  is  cheap  and  the  sulphur 
dioxide  can  again  be  used  for  the  manufacture  of  sulphuric  acid. 
212  When  sulphuric  acid  is  mixed  with  water  a  considerable 
evolution  of  heat  takes  place  and  a  contraction  ensues.  The 
amount  of  heat  which  is  evolved  by  mixing  sulphuric  acid  and 
water  has  been  exactly  determined  by  Thomsen  l ;  his  results 
are  given  in  the  following  table  in  which  the  columns  marked  I. 


FIG.  123. 

give  the  number  of  molecules  of  water,  and  II.  the  heat  evolved 
in  calories : — 

I. 

49-3      , 
99-7 


H2SO 


19-1 


IT. 
i-  x  H,,04. 

6336 
9355 
11294 
13020 
14851 
16147 


200-4 

401-7 

804-4 

1609-7 


II. 

+  x  H20. 
16572 
16744 
16950 
17196 
17522 
17737 


From  the  above  numbers  it  is  seen  that  the  addition  of  the  first 
molecule  produces  an  amount  of  heat  represented  by  6,336 
thermal  units,  or  about  one-third  of  the  total,  whilst  the 
addition  of  two  molecules  of  water  gives  off  about  one-half 

1  Therm.  Unter.  III.  34. 


PROPERTIES  OF  SULPHURIC   ACID  399 

the  total  quantity.  The  heat  evolved  by  a  further  addition  of 
water  is  less  for  each  successive  molecule  of  water,  but  it  has 
been  found  impossible  to  determine  the  point  at  which  no  further 
evolution  of  heat  is  caused  by  further  dilution. 

Hydrates  of  Sulphuric  Acid. — When  a  mixture  of  equal  mole- 
cules of  acid  and  water  is  cooled  down,  the  mixture  solidifies  to 
a  mass  of  prismatic  crystals,  which  possess  the  composition 
H2SO4  +  H2O,  and  melt,  according  to  Pierre  and  Puchot,  at  7°'5. 

Another  hydrate  of  the  formula  H2S04  +  4H2O,  has  been 
isolated  by  Pickering1  from  a  solution  of  sulphuric  acid  in 
water.  It  forms  large,  well-defined  crystals  and  melts  at— 25°. 
This  substance  is  characterised  as  a  definite  chemical  compound 
by  the  facts  that  it  melts  and  freezes  at  a  definite  temperature, 
and  that  its  freezing  point  is  lowered  by  the  addition  of  either 
of  its  components.  The  existence  of  many  other  hydrates  of  the 
acid  has  been  surmised  from  the  properties  of  its  solution  in 
water,  but  no  others  have  as  yet  been  isolated. 

213  Sulphuric  acid  is  largely  used  in  the  laboratory  not  only 
for  the  preparation  of  most  of  the  other  acids,  but  also  in  con- 
sequence of  its  powerful  hygroscopic  properties  for  the  purpose 
of  drying  gases.  To  effect  this,  the  gas  is  best  led  through 
tubes  filled  with  fragments  of  pumice-stone  which  have  been 
boiled  in  strong  sulphuric  acid.  The  acid  is  also  employed  to 
dry  solid  bodies,  or  to  concentrate  liquids,  especially  in  cases 
where  the  application  of  a  high  temperature  is  likely  to  pro- 
duce a  decomposition  of  the  substance,  the  bodies  to  be  dried 
being  placed  over  a  vessel  containing  the  acid  in  a  closed  space 
or  in  a  vacuum. 

Sulphuric  acid  when  concentrated  does  not  act  in  the  cold 
upon  many  of  the  metals,  although  it  does  so  in  some  cases  when 
heated.  Thus  copper,  mercury,  antimony,  bismuth,  tin,  lead,  and 
silver  are  attacked  by  the  hot  acid,  with  evolution  of  sulphur 
dioxide,  but  are  not  acted  on  by  the  cold  dilute  acid ;  thus  :— 
2Ag  4  2H2S04  =  Ag2S04  +  SO2  +  2H2O. 

Gold,  platinum,  iridium,  and  rhodium  are  unacted  upon,  even  by 
boiling  sulphuric  acid,  and  this  acid  is,  therefore,  employed  in 
the  separation  of  silver  and  gold.  The  more  easily  oxidizable 
metals,  such  as  zinc,  iron,  cobalt,  manganese,  are  dissolved  by 
the  dilute  acid  with  evolution  of  hydrogen  and  formation  of  a 
sulphate,  but  also  act  upon  the  hot  concentrated  acid  with 
production  of  sulphur  dioxide. 

1  Journ.  Chem.  Soc.  1890,  i.,  1339. 


400  THE  NON-METALLIC  ELEMENTS 

Many  organic  bodies  are  decomposed  by  sulphuric  acid,  which 
abstracts  from  them  the  elements  of  water.  Thus,  for  instance, 
oxalic  acid,  C2H204,  by  heating  with  strong  sulphuric  acid  is 
decomposed  into  carbon  dioxide,  CO2,  carbon  monoxide,  CO, 
and  water,  H2O ;  and  alcohol,  C2H60,  is  transformed  by  means 
of  this  acid  into  ethylene  gas,  C2H4,  and  water,  H2O.  Wood, 
sugar,  and  other  substances  are  blackened  by  sulphuric  acid, 
this  body  withdrawing  from  them  the  hydrogen  and  the  oxygen 
which  they  contain  with  production  of  water. 

214  The  Sulphates. — The  salts  of  sulphuric  acid  are  termed 
sulphates,  and  as  this  acid  is  dibasic,  like  sulphurous  acid,  two 
series  of  sulphates  exist,  viz.,  the  normal  salts,  such  as  Na2SO4 
and  CaSO4,  and  the  acid  salts  such  as  NaHSO4. 

Many  sulphates  occur  native,  existing  as  well-known  and 
important  minerals ;  such  are  : — gypsum,  CaS04  -f  2H2O ;  heavy 
spar,BaSO4;  celestine,SrSO4;  Glauber's  salt,  Na2SO4  +  10H2O; 
and  Epsom  salts,  MgSO4  +  7H2O. 

Most  of  the  sulphates  are  soluble  in  water,  and  crystallize 
well,  and  these  can  be  readily  prepared  by  dissolving  the  metal 
in  dilute  sulphuric  acid,  or  the  oxide  or  carbonate  if  the  metal 
does  not  readily  dissolve.  Some  few  sulphates,  viz.,  calcium 
sulphate  and  the  sulphates  of  lead  and  strontium,  are  only 
very  slightly  soluble,  whilst  barium  sulphate  is  insoluble  in 
both  water  and  dilute  acids.  This  fact  is  made  use  of  for  the 
detection  of  sulphuric  acid.  A  soluble  barium  salt,  usually 
the  chloride,  is  added  to  the  solution  supposed  to  contain  a  sul- 
phate ;  if  sulphuric  acid  be  present,  a  heavy  white  precipitate 
of  barium  sulphate,  BaSO4,  falls  down,  which  is  insoluble  in 
dilute  hydrochloric  acid.  In  order  to  detect  free  sulphuric  acid, 
together  with  sulphates,  as  for  instance  in  vinegar,  which  is 
sometimes  adulterated  with  oil  of  vitriol,  the  liquid  must  be 
evaporated  on  a  water-bath  with  a  small  quantity  of  sugar.  If 
free  sulphuric  acid  is  present  a  black  residue  is  obtained. 

Free  sulphuric  acid  is  found  in  the  water  of  certain  volcanic 
districts.  It  has  already  been  mentioned  that  sulphur  dioxide 
occurs  in  volcanic  gases,  and  these  when  dissolved  in  water 
gradually  absorb  oxygen  from  the  air  and  pass  into  sulphuric 
acid.  The  Rio  Vinagre  in  South  America,  which  is  fed  from 
volcanic  springs  and  receives  its  name  on  the  account  of 
the  acid  taste  of  the  water,  contains  free  sulphuric  acid.  A 
singular  occurrence  of  free  sulphuric  acid  has  been  noticed  in 
the  salivary  glands  of  certain  mollusca;  thus,  according  to 


SULPHURIC  ACID 


401 


Bodeker    and   Troschel,   those  of  the  Dolium    galca    contain 
about  2*47  per  cent. 

215  The  following  table  by  Lunge,  Isler  and  Naef1  exhibits  the 
percentage  of  real  acid,  H2SO4,  contained  in  aqueous  sulphuric 
acid  of  varying  specific  gravities. 


Degree 
Twaddell 

Specific 
gravity 
15°/4°  in 
vacua 

Percentage 
of  H2S04 

Degree 
Twaddell 

Specific 
gravity 
15°/4°  in 
vacua 

Percentage 
of  H2S04 

0 

1.000 

0-09 

99 

1-495 

59-22 

3 

1-015 

2-30 

102 

1-510 

60-65 

6 

1-030 

4-49 

105 

1-525 

62-06 

9 

1-045 

(5-67 

108 

1-540 

63-43 

12 

1-060 

8-77 

111 

1*555 

64-67 

15 

1-075 

10-90 

114 

1-570 

65-90 

18 

1  090 

12-99 

117 

1-585 

67-13 

21 

1-105 

15-03 

120 

1-600 

68-51 

24 

1-120 

17-01 

123 

1-615 

69-89 

27 

1-135 

18-96 

126 

1-630 

71-16 

30 

1-150 

20-91 

129 

1-645 

72-40 

33 

1-165 

22-83 

132 

1-660 

73-64 

36 

1-180 

24-76 

135 

1-675 

74-97 

39 

1195 

26-68 

138 

1-690 

76-30 

42 

1-210 

28-58 

141 

1-705 

77-60 

45 

1-225 

30-48 

144 

1720 

78-92 

48 

1-240 

32-28 

147 

1-735 

80-24 

51 

1-255 

34-00 

150 

1-750 

81-56 

54 

1-270 

35-71 

153 

1-765 

82-88 

57 

1-285 

37-45 

156 

1-780 

84-50 

60 

1-300 

3919 

159 

1-795 

86-30 

63 

1-315 

40-93 

162 

1-810 

88-30 

66 

1-330 

42-66 

165 

1-825 

91-00 

69 

1-345 

4428 

166 

1-830 

92-10 

72 

1-360 

•    45-88 

167 

1-835 

93-43 

75 

1-375 

47-47 

168 

1-840 

95-60 

78 

1-390 

49-06 

1-8410 

97-00 

81 

1-405 

50-63 

1-8415 

97-70 

84 

1-420 

52-15 

1-8410 

98-20 

87 

1-435 

53-59 

1-8400 

99-20 

90 

1-450 

55-03 

1.8390 

99-70 

93 

1-465 

56-43    • 

1-8384 

100-00 

96 

1-480 

57-83 

1  Sulphuric  Acid  and  Alkali,  vol.  i.  p.  119  (1891). 
27 


402  THE  NON-METALLIC  ELEMENTS 


FUMING  SULPHURIC  ACID. 

216  This  substance,  which  is  a  solution  of  varying  quantities 
of  sulphur  trioxide  in  sulphuric  acid,  was  known  before  the 
sulphuric  acid  manufactured  from  sulphur,  being  termed  Nord- 
Tiausen  sulphuric  acid,  from  the  fact  that  it  was  prepared  at 
Nordhausen  in  the  Hartz,  by  heating  roasted  green  vitriol. 

Preparation. — (1)  When  green  vitriol  or  ferrous  sulphate, 
FeSO4  +  7H2O,  is  roasted  in  the  air  it  loses  water  and  becomes 
oxidized  to  a  basic  ferric  sulphate,  Fe2S2O9,  which  is  then 


FIG.  124. 

further  heated  in  clay  retorts,  as  shown  in  Fig.  124,  when  the 
following  decomposition  takes  place  : — 

Fe2S2O9  =  2S08  +  Fe203. 

The  sulphur  trioxide  thus  formed,  partly  combines  with  the 
water  which  is  still  present  to  form  sulphuric  acid,  whilst 
the  other  portion  of  the  trioxide  dissolves  in  the  sulphuric 
acid  thus  produced. 

Fuming  sulphuric  acid  is  now  almost  entirely  prepared  in 
Bohemia  in  the  works  of  J.  D.  Starck.  The  solution  of  green 
vitriol  obtained  by  the  oxidation  of  the  pyrites  is  evaporated 
down,  and  the  residue  ignited,  care  being  taken  that  the  "  vitriol- 
stone  "  thus  obtained  is  as  free  as  possible  from  ferrous  sulphate* 
inasmuch  as  if  this  body  be  present  sulphur  dioxide  is  formed 


FUMING  SULPHURIC  ACID  403 

in  the  subsequent  distillation,  and  this  carries  away  with  it  large 
quantities  of  the  easily  volatile  trioxide ;  thus : — 

2FeS04  =  Fe203  +  SO3  +  SO2. 

The  more  completely  the  vitriol-stone  is  oxidized,  the  larger 
is  the  yield  of  fuming  acid,  which  on  an  average  amounts  to 
from  34  to  50  per  cent.  In  some  works  the  green  vitriol  is 
allowed  to  crystallize  out,  and  the  oxidized  mother  liquors  alone 
used  for  the  production  of  the  fuming  acid.  Fuming  sulphuric 
acid  was  formerly  chiefly  used  in  the  arts  for  dissolving  indigo  ; 
at  present,  however,  it  is  largely  employed  in  the  preparation  of 
the  soluble  sulphonic  acids  of  colouring  matters,  and  in  the  manu- 
facture of  artificial  alizarin.  For  this  purpose  an  acid  is  needed 
which  contains  more  trioxide  than  the  commercial  substance,  and 
hence  it  is  prepared  specially  by  the  manufacturers  themselves 
by  heating  the  fuming  acid  in  cast-iron  retorts  and  absorbing 
the  trioxide,  which  is  given  off,  in  another  portion  of  the  acid 
in  well-closed  receivers. 

(2)  Sulphur  trioxide  is  formed,  as  has  been  already  stated 
(p.  374),  when  a  mixture  of  oxygen  and  sulphur  dioxide  is 
passed  over  heated  platinum  sponge.  It  has  been  proposed  to 
employ  this  reaction  for'  the  preparation  of  common  sulphuric 
acid  on  the  large  scale,  the  sulphur  dioxide  being  obtained  by 
the  combustion  of  sulphur,  or  the  roasting  of  pyrites,  and  this 
together  with  air  passed  over  heated  platinized  asbestos,  the 
fumes  of  the  trioxide  being  collected  in  water.  It  was  found 
that  this  process  cannot  be  practically  carried  out,  inasmuch  as 
the  platinum  soon  loses  this  peculiar  property,  probably  owing 
to  the  fact  that  dirt  and  particles  of  dust  collect  on  the  surface 
of  the  metal.  For  the  production  however  of  a  strongly  fuming 
acid,  the  following  process,  according  to  Winkler,1  answers  well. 
Common  sulphuric  acid,  as  has  been  shown,  decomposes  on 
heating,  into  aqueous  vapour,  sulphur  dioxide,  and  oxygen.  If 
the  mixture  of  gases  be  washed  by  sulphuric  acid  in  order  to 
remove  the  water  and  particles  of  dust,  and  then  the  mixture 
of  dioxide  and  oxygen  passed  over  heated  platinized  asbestos 
the  trioxide  is  formed  and  may  be  collected  in  sulphuric  acid. 

Properties. — Fuming  sulphuric  acid  is  a  colourless,  thick,  oily 

liquid  when  pure,  but  is  generally  coloured  slightly  brown  from 

the  presence  of  organic  matter.     It  has  a  specific  gravity  of  from 

1*86  to  T89,  and   evolves  on  exposure  to  the  air  dense  white 

1  DingL  Polyt.  Journ.  218,  128. 


404  THE  NON-METALLIC  ELEMENTS 

fumes,  inasmuch  as  the  volatile  trioxide  escapes  and  combines 
with  the  aqueous  vapour  of  the  air  to  form  sulphuric  acid. 

When  the  fuming  acid  is  cooled,  white  crystals  of  the  com- 
pound, H2SO4  +  SO3,  separate  out.  This  substance  melts,  ac- 
cording to  Marignac,  at  35°,  fumes  strongly  in  the  air,  and 
decomposes  easily  on  heating  into  its  constituents.  The  name 
disulphuric  acid  or  pyrosulpkuric  acid,  H2S2O7,  has  been  given 
to  this  substance  as  it  forms  a  series  of  very  stable  salts ;  thus 
sodium  disulphate,  Na2S^O7,  is  obtained  by  heating  the  acid 
sodium  suh>hate,  HNaSO4,  so  long  as  water  is  given  off; 
thus : — 

SQ     fONa 

S°2    \ONa 

When  still  more  strongly  heated  this  salt  decomposes  into  the 
normal  sulphate  and  sulphur  trioxide. 

Sulphur  trioxide  and  sulphuric  acid  also  unite  together  to 
form  tdra-sulphuric  acid,  3  SO3  +  H2SO4  =  H2S4013,  an  oily 
liquid,  and  another  compound,  having  the  composition 
S03  +  3H2S04,  a  transparent  crystalline  mass  melting  at  26°. 


CHLORIDES    AND    BROMIDES    OF    SULPHURIC 

ACID. 

CHLOKOSULPHONIC  ACID,  OR  SULPHTJRYLHYDROXYCHLORIDE. 

=  115-65. 


217  Williamson  first  obtained  this  substance  by  the  direct 
union  of  hydrochloric  acid  and  sulphur  trioxide.1 

It  is,  however,  best  prepared  by  the  distillation  of  a  mixture 
of  concentrated  sulphuric  acid  and  phosphorus  oxychloride, 
thus  :  — 


Hence  it  is  seen  that  chlorosulphonic  acid  may  be  considered  to 

be  sulphuric  acid,  in  which  the  group  hydroxyl,  OH,  is  replaced 

by  chlorine.     It  is  a  colourless  liquid,  fuming  strongly  in   the 

1  Proc.  Roy.  Soc.  7,  11. 


SULPHURYL  CHLORIDE  405 

air,  having  a  specific  gravity  of  1*766  at  18°,  and  boiling  at 
158°.  Its  vapour  decomposes  on  heating,  and  at  216°  its 
vapour  density  is  found  to  be  32'8,  or  the  dissociation  is  nearly 
perfect. 

This  decomposition  is  probably  represented  by  the  equation  :  l 

201.  S03H  =  S02  +  C12  +  H20  +  S03. 

When  thrown  into  water  it  decomposes  with  explosive 
violence,  forming  hydrochloric  and  sulphuric  acids,  and  when 
added  to  strong  sulphuric  acid,  disulphuric  acid  and  hydrochloric 
acid  are  formed,  thus  :  — 

OH 


r 
S02    glH  +  s02 


)OH 


r  r4! 

SULPHURYL  CHLORIDE.     SO0  \  ^i  =133*96. 

•   (  vl 

218  This  body  was  first  obtained,  mixed  with  ethylene  di- 
chloride  by  Regnault  in  the  year  1838  by  the  action  of  chlorine 
upon  a  mixture  of  olefiant  gas  and  sulphur  dioxide.2  Sulphuryl 
chloride  is  also  formed  when  a  solution  of  the  two  gases  in 
anhydrous  acetic  acid  is  allowed  to  stand.  It  is,  however, 
most  readily  obtained  by  heating  chlorosulphonic  acid  in  closed 
tubes  at  a  temperature  of  180°  for  12  hours;3  thus  :  — 

r  OH  ci          r  OH. 


or  by  saturating  camphor  with  sulphur  dioxide  and  then  passing 
in  chlorine,  the  camphor  remaining  unaltered.4 

Sulphuryl  chloride  is  a  colourless  liquid  boiling  at  70°,  possess- 
ing a  strongly  pungent  odour,  fuming  strongly  in  the  air  and 
having  a  specific  gravity  of  1'659  at  20°;  it  decomposes  in  presence 
of  a  small  quantity  of  water  into  chlorosulphonic  acid  and  hydro- 
chloric acid,  and  with  an  excess  of  water  into  sulphuric  acid 
and  hydrochloric  acid  ;  thus  :  — 


S02  f  2H20  =  S02 

1  Herrmann  and  Kochlin,  Bcr.  18,  604.     2  Ann.  Chim.  Phys.  (2)  69,  170. 
3  Behrend,  Bcr.  8,  1004.  4  Schulze,  J.  Pr.  Chem.  (2)  23,  351. 


406  THE  NON-METALLIC  ELEMENTS 

The  vapour  density  of  sulphuryl  chloride  is  normal  at  100°, 
but  falls  to  one  half  of  this  at  442°,  at  which  temperature  it 
is  completely  dissociated  into  sulphur  dioxide  and  chlorine.1 
The  affinity  between  the  sulphur  dioxide  and  chlorine  is  very 
weak,  and  the  latter  is  very  readily  withdrawn  by  any  com- 
pound having  an  attraction  for  chlorine  ;  sulphuryl  chloride  is, 
therefore,  frequently  used  as  a  chlorinating  agent  in  organic 
chemistry  2 

DISULPHURYL  CHLORIDE.     S2O5C12  =  213-42. 

219  The  chloride  of  disulphuric  acid  was  first  prepared  by 
Rose  3  by  the  action  of  chloride  of  sulphur  on  sulphur  trioxide, 
thus : — 

S2C12  +  5SO3  =  S205C12  +  5SO2. 

The  same  compound  has  also  been  obtained  by  Michaelis 4  by 
heating  sulphur  trioxide  with  phosphorus  oxychloride ;  thus  : — 

6S03  +  2POC13=3S.205C12  +  P205. 

It  is  likewise  formed  when  common  salt  is  heated  with 
sulphur  trioxide,  and  when  this  latter  substance  is  brought  in 
contact  with  sulphuryl  chloride  ;  thus  :— 

SOJC1' 


^*  \  01. 

It  is  colourless  fuming  liquid,  boiling  at  146°,  and  having  a 
specific  gravity  at  18°  of  1*819.  Water  decomposes  it  into 
sulphuric  acid  and  hydrochloric  acid. 

SULPHUR  OXYTETRACHLORIDE.     S2O3C14  =  252*04. 

220  Millon  first  obtained  this  substance  by  the  action  of  moist 
chlorine  upon  sulphur  or  chloride  of  sulphur.  It  is  best  pre- 
pared by  cooling  down  a  mixture  of  chloride  of  sulphur  and 
chlorosulphonic  acid  to  —  15°,  and  then  saturating  the  liquid 
with  chlorine,  when  the  following  decomposition  takes  place  : — 

S03HC1  +  SC14  =  S203C14  +  HC1. 

1  Herrmann  and  Kbchlin,  Ber.  16,  602. 

2  Armstrong,  Proc.  Chem.  Soc.,  1891,  60.  3  Pogg.  Ann.  44   291. 
4  Zeitsch.  Chcm,  (2)  7,  149. 


SULPHUR  SESQUIOXIDE  407 

Sulphur  oxytetrachloride  forms  a  white  crystalline  mass,  which 
has  a  very  pungent  smell  and  attacks  the  mucous  membrane 
violently.  It  dissolves  in  water  with  a  hissing  noise,  forming 
hydrochloric,  sulphuric,  and  sulphurous  acids.  Exposed  to 
moist  air,  it  deliquesces  with  evolution  of  chlorine,  hydrochloric 
acid,  and  sulphur  dioxide,  leaving  a  residue  of  thionyl  chloride 
and  disulphuryl  chloride.  When  the  compound  is  heated  it 
partially  sublimes  in  fine  white  needles,  whilst  another  portion 
decomposes  into  sulphur  dioxide,  chlorine,  thionyl  chloride,  and 
disulphuryl  chloride.  When  kept  in  closed  tubes  this  body 
liquefies  with  the  formation  of  thionyl  and  sulphuryl  chlorides ; 
thus : — 

S203C14  =  SOC12  +  S02C12. 

An  oxy-chloride,  S2OC14,  is  formed  as  a  red  liquid,  boiling  at 
60°,  by  the  action  of  chloride  of  sulphur  on  sulphuryl  chloride.1 

SULPHURYL  BROMIDE.    SO2  j  ^'  =  222-30. 

221  Bromine   combines   with    sulphur    dioxide,   forming    a 
volatile  white  solid  crystalline  mass,  which  when  acted  upon 
with   silver  oxide  forms  sulphur  trioxide;2  thus: — 

S02Br2  +  Ag2O  =  S03  +  2AgBr. 

Some  doubt  has  been  thrown  on  the  existence  of  this  com- 
pound by  later  observers.3 

SULPHUR  SESQUIOXIDE.    S203  =  111-28. 

222  So  long  ago  as  the  year  1804  Buchholz  found  that  when 
sulphur  is  heated  with  fuming  sulphuric  acid  an  intensely  blue- 
coloured  solution  is  formed,  and  in  the  year  1812  F.  C.  Vogel 
showed  that  this  blue  body  is  also  produced  by  the  action  of 
sulphur  on  sulphur  trioxide.     In  later  years  this  subject  has 
frequently  attracted  the  attention  of  chemists,  but  the  nature  of 
the  blue   substance   remained   unexplained   until   R.   Weber 4 
showed  that  it  consists  of  a  new  oxide  of  sulphur. 

In  order  to  prepare  this  substance,  carefully  dried  flowers  of 
sulphur  are  added,  in  small  quantities,  to  recently  prepared  and 
liquid  sulphur  trioxide,  a  fresh  quantity  of  sulphur  only  being 
added  when  that  already  present  has  entered  into  combination. 

1  Ogier,  Compt.  Rend.,  94,  446  ;  Bull.  Soc.Chem.  37,  293. 

2  Odling,  Journ.  Chcm.  Soc.  1855,  2. 

a  Michaelis,  Lehrbuch,  i.,  733  (5th  Edition).  4  Pogg.  Ann.  156,  531. 


408  THE  NON-METALLIC  ELEMENTS 

In  order  to  moderate  the  reaction,  the  test-tube  in  which  the 
solution  is  made  must  be  placed  in  water  at  a  temperature  of 
from  12°  to  15°.  The  sulphur  on  falling  into  the  trioxide  dis- 
solves in  the  form  of  blue  drops  which  sink  down  to  the  bottom 
of  the  test-tube  and  then  solidify.  As  soon  as  a  sufficient 
quantity  of  this  substance  has  been  formed,  the  supernatant 
sulphur  trioxide  is  poured  off  and  the  residue  removed  from  the 
test-tube  by  very  gently  warming  it. 

Sulphur  sesquioxide  forms  bluish-green  crystalline  crusts,  in 
colour  closely  resembling  malachite.  At  the  ordinary  temperature 
it  slowly  decomposes  into  sulphur  dioxide  and  free  sulphur,  and 
this  decomposition  takes  place  more  readily  when  the  substance 
is  warmed  ;  thus  : — 

2S203  =  3S02  +  S. 

This  compound  dissolves  in  fuming  sulphuric  acid,  giving  rise  to 
a  blue  solution  which  on  the  addition  of  common  sulphuric  acid 
gradually  changes  to  a  brown.  Water  decomposes  the  sesqui- 
oxide with  the  separation  of  sulphur  and  the  formation  of 
sulphuric  acid,  sulphurous  acid,  and  thiosulphuric  acid. 


HYPOSULPHUROUS  ACID.    H2S2O4. 

223  This  compound  was  discovered  by  Schiitzenberger 1  who 
gave  it  the  name  of  hydrosulphurous  acid  and  assigned  to  it  the 
formula  H2SO2,  its  sodium  salt  being  formulated  as  NaHSO2. 
Bernthsen,2  however,  subsequently  showed  that  the  simplest 
formula  of  the  sodium  salt  is  NaS09,  the  molecular  formula 
being  probably  double  this,  Na2S2O4,  and  the  free  acid  H2S204. 
The  zinc  salt  of  the  acid  is  obtained  by  the  action  of  metallic 
zinc  upon  an  aqueous  solution  of  sulphur  dioxide  contained  in 
a  closed  vessel ;  no  evolution  of  hydrogen  takes  place,  but  the 
zinc  dissolves  and  a  salt  of  hyposulphurous  acid  is  formed, 
the  reaction  being  the  following  (Bernthsen) : — 

Zn  +  2S02  =  ZnS2O4. 

The  liquid  thus  obtained,  which  was  looked  upon  by 
Schiitzenberger  as  the  free  acid,  possesses  powerful  reducing 
properties.  It  bleaches  organic  colouring  matters  more  quickly 
than  sulphurous  acid  and  precipitates  the  metals  silver  and 

1  Compt.  Rend.  69,  169.  2  Annalcn,  208,  142  ;  211,  285. 


HYPOSULPHUEOUS  ACID  409 

mercury  from  solutions  of  their  soluble  salts.  None  of  the  salts 
have  been  prepared  pure,  but  a  solution  of  the  sodium  salt  may 
be  obtained  by  the  action  of  zinc  on  a  solution  of  acid  sodium 
sulphite,  the  liquid,  which  is  contained  in  a  well  closed  bottle, 
being  kept  well  cooled  by  cold  water.  The  following  reaction 
takes  place  (Bernthsen)  : — 

4NaHSO3  +  Zn  =  ZnSO3  +  Na2SO3  +  H2O  +  Na2S2O4. 

A  portion  of  the  zinc  sulphite  is  converted  into  a  basic  salt  and 
sulphur  dioxide  thus  set  free,  which  is  then  reduced  in  its  turn 
by  the  excess  of  zinc  present.  The  greater  portion  of  the 
sodium  sulphite  crystallizes  out  along  with  the  sulphite  and 
basic  sulphite  of  zinc,  but  some  still  remains  in  solution.  In 
order  to  remove  this,  the  mother  liquor  containing  the  hypo- 
sulphite is  poured  off  into  a  flask  and  three  or  four  times  its 
bulk  of  strong  alcohol  added ;  the  alcoholic  solution,  on  standing 
well  corked,  deposits  a  second  crop  of  crystals  of  zinc-sodium 
sulphite,  and  the  supernatant  liquid  on  again  being  poured  off 
into  a  well-stoppered  flask,  crystallizes  into  a  mass  of  colourless 
crystals  of  the  hyposulphite  which  only  needs  to  be  pressed 
between  blotting  paper  or  between  a  cloth  and  dried  in  a 
vacuum.  These  crystals  however  only  contain  about  40  per  cent, 
of  sodium  hyposulphite,  much  of  the  sulphur  being  present  as 
sulphate,  sulphite  and  thiosulphate.  (Bernthsen.) 

Sodium  hyposulphite  is  also  formed  when  a  current  of 
electricity  is  passed  through  a  solution  of  acid  sodium  sulphite, 
the  h}^drogen,  which  is  evolved  at  the  negative  pole,  reducing 
the  salt.  Sodium  hyposulphite  is  employed  by  the  dyer  and 
calico-printer  for  the  reduction  of  indigo,  as  it  possesses  the 
same  reducing  properties  as  the  free  acid.  In  the  moist  state 
or  in  solution  when  exposed  to  the  air  it  absorbs  oxygen  quickly 
and  changes  at  once  into  sodium  metabisulphite. 

Na2S204  +  O  =  Na2S205. 

The  aqueous  solution  decomposes,  even  when  not  exposed  to 
the  air,  with  the  formation  of  sodium  thiosulphate.  In  order 
to  prepare  hyposulphurous  acid,  a  dilute  solution  of  oxalic  acid  is 
added  to  a  solution  of  a  hyposulphite ;  a  yellow  liquid  is  then 
obtained  which  soon  decomposes,  thiosulphuric  acid  being 
formed,  and  this  being  very  unstable  decomposes  into  sulphur 
and  sulphur  dioxide. 


410  THE  NON-METALLIC  ELEMENTS 

The  ease  with  which  hyposulphurous  acid  undergoes  decompo- 
sition accounts  for  the  fact  that  the  existence  of  this  compound 
was  so  long  overlooked.  Berthollet  showed  so  long  ago  as  1789 
that  iron  dissolves  in  aqueous  sulphurous  acid  without  any 
evolution  of  gas,  and  Fourcroy  and  Vauquelin  found  in  1798 
that  zinc  and  tin  also  act  in  a  similar  manner.  That  a  lower 
product  of  oxidation  of  sulphur  is  thus  formed  was  even  then 
known,  but  up  to  the  time  of  Schiitzenberger's  discovery  it  was 
supposed  that  the  product  of  the  reaction  was  thiosulphuric 
acid. 

224  According  to  Schtitzenberger's  formula  the  acid  may  be 
looked  upon  as  derived  from  the  unknown  anhydride  SO, 

H2SO2  =  H2O  +  SO ; 

whilst,  if  Bernthsen's  view  be  correct,  it  must  correspond  to  the 
anhydride  S2O3 ; 

H2S204  =  H20  +  S203. 

Bernthsen  has  succeeded  in  showing  that  for  every  two  atoms 
of  sulphur  in  the  form  of  hyposulphite*,  one  atom  of  oxygen  ig 
required  to  oxidise  the  substance  to  a  sulphite,  which  may  be 
effected  by  means  of  an  ammoniacal  solution  of  copper  sulphate, 
and  three  atoms  of  oxygen  to  oxidize  it  to  a  sulphate,  a  reaction 
which  may  be  realised  by  the  use  of  iodine  solution.  Both  of 
these  results  are  in  agreement  only  with  the  formula  H2S2O4,  as 
will  be  seen  from  a  comparison  of  the  following  equations : — 

(1)  S203  +  O  =  2S02 
2SO  +  2O  =  2SO2 

(2)  S2O3  -f  3O  -  2SO3 
2SO  +  40 


SULPHUR  HEPTOXIDE,  S207,  AND  PERSULPHURIC  ACID,  H2S208. 

225  When  a  mixture  of  dry  oxygen  and  sulphur  dioxide  is 
subjected  to  the  silent  electrical  discharge,  combination  takes 
place  and  sulphur  heptoxide  is  produced.1 

A  substance  of  the  same  composition  is  formed  at  the  positive 
pole  during  the  electrolysis  of  sulphuric  acid  of  40  per  cent,  and 
1  Berthelot,  Ann.  Chim.  Phys.  (5)  14,  345,  363. 


PERSULPHURIC  ACID  411 

also  when  hydrogen  peroxide  is  mixed  with  concentrated 
sulphuric  acid. 

The  anhydrous  substance  is  a  viscid  liquid  which  solidifies  at 
0°  forming  granules,  needles  or  scales.  It  is  readily  volatile  and 
gradually  decomposes  on  preservation,  rapidly  on  heating,  into 
sulphur  tri oxide  and  oxygen. 

When  brought  into  water,  it  decomposes  with  evolution  of 
oxygen,  according  to  the  equation  : — 

4H20  +  2S2O7  =  4H2SO4  +  02. 

The  corresponding  acid  has  not  been  isolated,  but  its  salts  have 
been  prepared  by  Marshall l  and  also  studied  by  Berthelot.2 

The  potassium  salt,  K2S2O8,  may  be  obtained  by  passing  a 
current  of  3  to  3J  amperes  through  a  saturated  solution  of 
potassium  hydrogen  sulphate.  The  solution  is  placed  in  a 
platinum  basin,  cooled  externally  by  a  current  of  water,  and 
connected  with  the  positive  pole  of  the  battery ;  the  negative 
pole  consists  of  a  platinum  wire  and  is  placed  in  a  porous  cell 
filled  with  dilute  sulphuric  acid  and  suspended  in  the  solution 
of  potassium  hydrogen  sulphate.  The  potassium  salt  separates 
out  in  the  course  of  24  to  48  hours  as  a  mass  of  white  crystals. 
These  may  be  recrystallised  by  dissolving  in  warm  water  and 
allowing  to  cool,  and  then  form  large,  tabular,  apparently 
asymmetric  crystals.  One  hundred  parts  of  water  at  0°  dissolve 
177  parts  of  the  salt,  the  solution  being  neutral  to  test  paper. 
On  preservation  decomposition  commences  after  some  time  and 
becomes  rapid  when  the  solution  is  heated,  oxygen  being  evolved 
and  potassium  sulphate  formed.  When  the  dry  salt  is  heated 
it  loses  oxygen  and  sulphur  trioxide  and  is  converted  into  the 
sulphate  : — 

2K2S208  =  2K2S04  +  2S03  +  02. 

The  molecular  formula  of  the  salt 3  is  found,  from  a  determina- 
tion of  the  electrical  conductivity  of  its  dilute  aqueous  solution, 
to  be  most  probably  K2S2O8  and  not  KSO4.  The  solution  of 
potassium  persulphate  has  strongly  oxidising  properties ;  thus 
ferrous  sulphate  is  rapidly  converted  into  the  ferric  salt,  solutions 
of  silver,  copper,  manganese,  cobalt  and  nickel  salts,  all  yield 
precipitates  of  the  higher  oxides  on  warming,  hydrochloric 

i  Journ.  CJiem.  Soc.  1891,  i.  771,  a  Ann.  Ghim.  Phys.  [6]  26,  526. 

3  Lowenherz,  Ckem.  Zcit.,  16,  838. 


412  THE  NON-METALLIC  ELEMENTS 

acid  evolves  chlorine,  potassium  iodide  is  decomposed  with 
liberation  of  iodine,  and  alcohol  is  converted  into  aldehyde. 

The  ammonium  salt,  (NH4)2S2O8,  is  prepared  in  a  similar 
manner  to  the  potassium  salt ;  it  forms  lozenge-shaped,  ap- 
parently monosymmetric  tablets,  and  is  much  more  soluble  than 
the  potassium  salt,  100  parts  of  water  at  0°  dissolving  58'2 
parts.  The  barium  salt,  BaS208  +  4H2O,  is  freely  soluble  in 
water,  whilst  the  lead  salt,  PbS.2O8-|-3H20,  is  deliquescent. 

All  the  persulphates  appear  to  be  soluble  in  water,  the 
potassium  salt  being  less  soluble  than  any  other  which  has  yet 
been  examined. 


THIOSULPHURIC  ACID.    H2S203. 

226  This  compound  is  better  known  under  its  old  name  of 
"  hyposulphurous  acid,"  with  which  name  however,  we  now 
designate  the  body  obtained  by  the  reduction  of  sulphurous 
acid.  Thiosulphuric  acid  has  not  been  prepared  in  the  free 
state,  but  it  forms  a  series  of  stable  salts  which  are  known  as 
the  thiosulphates  (hyposulphites).  When  dilute  sulphuric  acid 
is  added  to  a  solution  of  a  thiosulphate,  the  solution  remains,  to 
begin  with,  perfectly  clear  ;  but  sulphur  soon  begins  to  separate 
out  as  a  white,  very  finely  divided  powder,  and  the  solution 
is  found  to  contain  sulphurous  acid,  sulphur  dioxide  being  also 
evolved  ;  the  thiosulphuric  acid  undergoes  on  liberation  the 
following  decomposition  :  — 


This  decomposition  is  quantitative  when  the  sulphur  dioxide 
is  removed  from  the  solution,  but  when  it  is  retained,  a  limit 
short  of  complete  decomposition  is  attained,  and  other  products, 
among  which  is  pentathionic  acid,  are  formed.1 

Thiosulphuric  acid  is  formed  in  small  amount  by  the  action 
of  sulphurous  acid  upon  flowers  of  sulphur  at  the  ordinary  tem- 
perature, more  rapidly  at  80—  90°.  2 

The  thiosulphates  are  formed  in  various  ways  :  thus,  for 
instance,  sodium  thiosulphate  (formerly  called  hyposulphite  of 
soda),  which  was  first  prepared  by  Chaussier  in  1799,  but  after- 

1  Colefax,  Journ.  Chcm.   Soc.  1892,  i.  176. 

2  Jo  urn.  Chem.  Soc.  1892,  i.  199. 


THIOSULPHURIC  ACID  413 

wards  more  carefully  examined  by  Vauquelin,  is  formed  when 
sulphur  dioxide  is  passed  into  a  solution  of  sodium  sulphide; 
thus  :  — 

(a)  SO,  +  H2O  +  Na2S  =  Na2S03  +  SH2. 

(b)  SCX  +  2SH2  =  2H9O  +  3S. 

(c)  Na2S03  +  S  =  Na2S2O3. 

The  same  salt  is  also  formed  according  to  equation  (c)  when  a 
solution  of  sodium  sulphite  is  boiled  with  flowers  of  sulphur 
and  this  is  the  method  by  which  the  salt  is  usually  prepared 
(Vauquelin).  Sodium  thiosulphate  is  also  formed  when  iodine 
is  added  to  a  solution  of  sodium  sulphite  and  sodium  sulphide  ; 
thus  :  1— 

Na2S03  +  Na2S  +  12  =  Na2S2O3  +  2NaI. 

Thiosulphates  are  also  produced  by  the  oxidation  of  sulphides, 
either  by  the  oxygen  of  the  air  or  by  means  of  suitable 
oxidising  agents. 

These  various  methods  of  preparation  point  out  that  thiosul- 
phuric  acid  is  formed  by  the  addition  of  sulphur  to  sulphurous 
acid,  just  as  sulphuric  acid  is  formed  by  the  addition  of  oxygen 
to  the  same  substance.  Thiosulphuric  acid  may,  therefore,  be 
regarded  as  sulphuric  acid  in  which  one  atom  of  oxygen  is 

r  OTT 

replaced  by  sulphur,  and  its  formula  is  accordingly  S02-<  ^TT' 

The  decompositions  which  its  salts  undergo  bear  out  this  inter- 
pretation of  its  composition.2  Thus  the  decomposition  of  the 
free  acid  into  sulphur  and  sulphurous  acid  has  already  been 
mentioned  ;  and  when  a  solution  of  sodium  thiosulphate  is 
treated  with  sodium  amalgam,  sodium  sulphite  and  sodium 
sulphide  are  formed  (Spring);  thus:  — 

Na2S203  +  2Na  =  Na2SO3  +  Na2S. 

Again,  when  silver  thiosulphate  is  warmed  with  water,  black 
sulphide  of  silver  separates  out,  and  the  solution  contains  free 
sulphuric  acid  ;  thus  :  — 


And,  moreover,  when  a  solution  of  sodium  thiosulphate  is  treated 
1  Spring,  Her.  7,  1157.  2  Schorlemraer,  Journ.  Chcm.  Soc.  1869,  256. 


414  THE  NON-METALLIC  ELEMENTS 

with  cobalt  chloride,  black   sulphide  of  cobalt  is  precipitated  ; 
thus  :  — 


S02  +  CoCl  +  H20  =  S0  +  CoS  +  2NaCl. 


This  formula  is  also  borne  out  by  the  fact  that  the  isomeric 
sodium  potassium  sulphites  (p.  372)  are  converted  by  the  action 
of  ammonium  polysulphides  into  isomeric  thiosulphates,  which 
differ  in  solubility  and  specific  gravity,  and  yield  different 
products  when  acted  upon  by  ethyl  iodide.1 

Sulphite.  Thiosulphate. 

XOK  XOK 

S02<  SO  / 
J  \Na  xSNa 

/ONa  /ONa 

S02<  S02 
\K 


The  soluble  thiosulphates  generally  crystallise  well,  and  contain 
water  of  crystallisation,  the  last  molecule  of  which  is  very 
difficult  to  remove  by  heat,  being  generally  lost  at  such  a  high 
temperature  that  the  decomposition  of  the  salt  has  already  com- 
menced. Hence  it  was  formerly  supposed  that  the  thiosulphates 
all  contained  hydrogen. 

The  thiosulphates  exhibit  a  great  tendency  to  form  double 
salts  ;  those  of  the  thiosulphates  insoluble  in  water  are  found 
to  dissolve  in  an  aqueous  solution  of  sodium  thiosulphate, 
which  also  has  the  power  of  dissolving  other  insoluble  salts, 
such  as  silver  chloride,  silver  bromide,  silver  iodide,  lead  iodide, 
lead  sulphate,  calcium  sulphate,  &c  ;  thus  :  — 

.O.+  AgCl=  J 

Sodium  silver  thiosulphate  forms  distinct  crystals,  having  the 
composition  AgNaS203-f  H2O,  and  these  are  distinguished 
by  possessing  a  sweet  taste.  The  use  of  sodium  thiosulphate 
for  fixing  prints  in  photography,  first  suggested  by  Sir  John 
Herschel,  probably  depends  on  the  formation  of  this  salt.  The 
silver  chloride  with  which  the  photographic  paper  is  impregnated 
when  exposed  to  the  light  becomes  blackened,  the  chloride 
1  Sch  wicker,  Ber.  22,  1733. 


DITHIONIC  ACID  415 


undergoing  a  chemical  change,  after  which  it  is  insoluble  in 
sodium  thiosulphate.  In  order,  therefore,  to  fix  such  a  photo- 
graphic print,  it  is  only  necessary,  after  exposure  to  light,  to  soak 
the  paper  in  a  bath  of  the  thiosulphate ;  the  unaltered  chloride 
of  silver  dissolves,  and  the  picture,  on  washing,  is  found  to  be 
permanent. 

Several  other  complex  thiosulphates  of  silver  and  sodium  are 
also  known. 

The  thiosulphates  are  distinguished  from  the  sulphites, 
inasmuch  as  when  dilute  hydrochloric  acid  or  sulphuric  acid 
is  added  to  their  solution  not  only  is  sulphur  dioxide  given 
off  as  a  gas,  but  free  sulphur  is  deposited  as  a  white  powder. 
On  adding  a  thiosulphate  solution  to  a  silver  or  lead  salt,  a 
white  precipitate  soluble  in  excess  of  the  thiosulphate  solution 
is  produced,  whilst  with  a  mercuric  salt,  a  white  precipitate  of 
the  insoluble  thiosulphate  is  first  thrown  down,  but  this  quickly 
becomes  dark,  and  finally  black,  from  its  decomposition  into 
a  metallic  sulphide  and  sulphuric  acid.  Solutions  of  mercurous 
salts  give,  with  the  thiosulphates,  dense  black  precipitates,  and 
when  a  thiosulphate  is  boiled  with  an  ammoniacal  solution  of 
a  ruthenium  salt,  the  solution  becomes  of  such  an  intensely 
dark  red  colour  that  in  the  concentrated  condition  it  appears 
almost  black.  Ferric  chloride  added  to  a  solution  of  a  thio- 
sulphate gives  a  dark  violet  coloration,  which  soon  disappears, 
sulphur  being  deposited. 


DITHIONIC  ACID.    H2S206. 

227  This  acid,  formerly  called  hyposulphuric  acid,  was  dis- 
covered by  Welter  and  Gay-Lussac  in  1819.  The  manganese  salt 
of  the  acid  is  prepared  by  passing  sulphur  dioxide  into  water 
containing  manganese  dioxide  in  suspension  ;  thus  :  — 

2  S02  +  Mn02  =  MnS2O6. 

At  the  same  time  a  portion  of  the  manganese  is  converted  into 
manganese  sulphate  ;  thus  :  — 


In  order  to  obtain  the  dithionate  free  from  sulphate,  advantage 
is  taken  of  the  solubility  of  barium  dithionate  in  water  ;  baryta 


416  THE  NON-METALLIC  ELEMENTS 

•water,  Ba  (OH.)9,  is  added  until  all  the  metal  is  precipitated  as 
manganese  hydroxide,  Mn  (OH)2,  and  all  the  sulphuric  acid  is 
thrown  down  as  insoluble  barium  sulphate,  BaSO4 ;  on  evapora- 
ting and  cooling,  barium  dithionate,  BaS206+2H20,  crystallises 
out,  and  when  this  is  decomposed  by  the  requisite  quantity  of 
dilute  sulphuric  acid,  a  solution  of  dithionic  acid  is  obtained. 
This  solution  may  be  concentrated  in  vacuo  over  sulphuric  acid 
until  it  has  a  specific  gravity  of  T347,  but  on  attempting  to 
concentrate  it  further,  the  acid  is  resolved  into  sulphur  dioxide 
and  sulphuric  acid  ;  thus : — 

H2S206  =  S02  +  H2S04. 

Dithionic  acid  is  also  formed  in  small  quantity  by  the  oxidation 
of  sulphurous  acid  by  means  of  potassium  permanganate.1  In 
order  to  obtain  the  salts  of  this  acid  we  may  add  the  cor- 
responding base  to  the  acid  solution,  or  they  may  be  obtained 
more  simply  by  adding  a  soluble  sulphate  to  barium  dithionate. 
Only  the  normal  salts  are  known,  most  of  which  crystallise 
well,  and  they  contain,  with  the  exception  of  the  potassium 
salt,  water  of  crystallisation.  Their  aqueous  solutions  are  not 
oxidised  in  the  cold  either  by  atmospheric  oxygen,  by  nitric 
acid,  or  by  potassium  permanganate ;  though  when  they  are 
heated  with  these  oxidising  agents  they  are  converted  into 
the  sulphates.  On  heating  they  decompose  partially  at  100°, 
and  entirely  at  a  higher  temperature,  into  sulphur  dioxide, 
and  a  sulphate  which  remains  behind ;  when  sodium  amalgam 
is  added  to  a  solution  of  sodium  dithionate,  two  molecules 
of  sodium  sulphite  are  formed  (R.  Otto) ;  thus  : — 

SO2'ONa  Na.S02.ONa. 

+  2Na  = 
S02'ONa  Na.S02.ONa. 

The  dithionates  are  distinguished  from  the  thiosulphates,  inas- 
much as  they  evolve  sulphurous  acid  when  heated  with  hydro- 
chloric acid  without  separation  of  sulphur,  whilst  the  solution 
afterwards  contains  a  sulphate. 


TRITHIONIC  ACID.    H2S306. 

228  In  1842,  Langlois  obtained  the  potassium  salt  of  this  acid 
1  Ann.  CMm.  Phys.  (3)  55,  374. 


TRITH10NIC  ACID  417 


by  gently  heating  a  solution  of  acid  potassium  sulphite  with 
sulphur  ;  thus  :  — 

S2  +  6KHS03  =  2K2S306  +  K2S2O3  +  3H2O. 

The  same  salt  is  also  produced  when  a  solution  of  potassium 
thiosulphate  is  saturated  with  sulphur  dioxide  ;  thus  :  — 

3SO2  +  2K2S2O3  =  2K2S3O6  +  S. 

Further  changes  also  occur,  the  trithionate  combining  with 
the  sulphur  which  is  liberated  to  form  tetra-  and  pentathionate 
of  potassium.  (Debus.)  The  potassium  salt  is  moreover  formed 
when  potassium  silver  thiosulphate  is  heated  with  water; 
thus  :  — 


When  iodine  is  added  to  a  solution  of  sodium  thiosulphate  and 
sodium  sulphite,  sodium  trithionate  is  likewise  formed  (Spring)  ; 
thus  :  — 

Na2S2O3  +  Na2SO3  +  I2  =  Na2S3O6  +  2NaI  ; 

whilst  a  solution  of  the  trithionate  treated  with  sodium 
amalgam  or  caustic  potash,  decomposes  again  into  sulphite  and 
thiosulphate. 

In  order  to  prepare  the  free  trithionic  acid,  hydro-fluosilicic 
acid  is  added  to  a  solution  of  the  potassium  salt,  when  the 
insoluble  hydro-fluosilicate  of  potassium  is  precipitated.  The 
aqueous  acid  thus  obtained  has  no  smell,  but  has  a  strong  acid 
.and  bitter  taste  ;  it  may  be  concentrated  in  a  vacuum  up  to 
a  certain  point,  but  it  is  an  unstable  compound,  and  at  the 
ordinary  temperature  easily  decomposes  even  in  dilute  solution 
into  sulphur,  sulphur  dioxide,  and  sulphuric  acid.  The  only  one 
of  the  trithionates  which  is  well  known  is  the  potassium  salt. 
This  on  heating  decomposes  into  sulphur,  sulphur  dioxide  and 
potassium  sulphate  and  its  solution  undergoes  the  same  decom- 
position on  standing  :  — 

K2S306  =  K2S04  +  S02  +  S. 

In  this  case  again  secondary  reactions  occur,  tetra-  ana  penta- 
thionates  being  formed  by  the  combination  of  the  trithionate 
with  the  sulphur.  Its  solution  is  not  precipitated  by  barium 

28 


418  THE  NON-METALLIC  ELEMENTS 

chloride  in  the  cold,  though  on  heating  barium  sulphate 
separates  out  and  sulphur  dioxide  is  evolved,  whilst  silver 
nitrate  gives  a  yellow  precipitate  which  very  quickly  becomes 
black  on  standing. 

Sulphuretted  hydrogen  has  no  action  upon  solutions  of  the 
free  acid  but  decomposes  the  potassium  salt  with  formation  of 
sulphate  and  thiosulphate  of  potassium  and  deposition  of 
sulphur  (Debus) : 

2K2S306  +  5H2S  =  K2S04  +  K2S2O3  +  5H2O  -f  8S. 

The  constitutional  formula  (p.  423)  of  potassium  trithionate  is 
probably  the  following  (Debus)  : 

K.S02.0 
KO.SCL 


TETRATHIOXIC  ACID.    H2S406. 

229  Fordos  and  Gelis  in  1843  first  prepared  this  acid  and  its 
salts.  They  obtained  the  sodium  salt  by  adding  iodine  to  an 
aqueous  solution  of  sodium  thiosulphate  ;  thus  :  — 

(ONa 

T  2  )  S 

I*  =    S02     O 


Salts  of  this  acid  are  also  formed  by  the  action  of  copper 
sulphate  upon  barium  thiosulphate  and  by  the  action  of  sul- 
phuric acid  upon  a  mixture  of  lead  thiosulphate  and  peroxide. 
In  order  to  prepare  the  free  acid,  iodine  in  excess  is  added  very 
gradually  to  thiosulphate  of  barium  suspended  in  a  very  small 
quantity  of  water,  the  iodide  of  barium  and  excess  of  iodine 
being  removed  by  shaking  up  the  semi-solid  mass  with  strong 
alcohol,  leaving  a  white  crystalline  mass  of  barium  tetrathionate, 
BaS406.  This  may  then  be  dissolved  in  a  small  quantity  of 
water  and  recrystallised,  whilst  from  this  pure  salt  trhe  acid 
may  be  prepared  by  adding  exactly  sufficient  sulphuric  acid 
to  decompose  it  completely. 

Tetrathionic  acid  is  a  colourless,  inodorous,  very  acid  liquid, 
which,  when  dilute,  may  be  boiled  without  undergoing  decom- 


TETRATHIONIC  ACID  419 

position,  but  in  the  concentrated  state  is  easily  decomposed  into 
sulphurous  and  sulphuric  acids  and  sulphur.  The  tetrathionates 
are  all  soluble  in  water,  but  their  solutions  cannot,  as  a  rule,  be 
evaporated  without  decomposition  into  sulphur  and  a  trithionate. 
A  solution  of  the  potassium  salt  decomposes  gradually  on  pre- 
servation, sulphur  dioxide  being  evolved  but  no  sulphur 
deposited.  The  decomposition  proceeds  in  two  stages,  tri-  and 
pentathionate  of  potassium  being  first  formed,  and  the  tri- 
thionate then  decomposing  in  the  usual  way  into  potassium 
sulphate,  sulphur  dioxide  and  sulphur  which  converts  a  portion 
of  the  trithionate  into  pentathionate.  (Debus.) 


(1)  2K2S406  =  K2S3064K2S506. 

(2)  3K2S306  =  2K2S04  +  2SO2  +  K2S5O6. 

Dry  potassium  tetrathionate  undergoes  no  alteration  on  pre- 
servation. Sodium  amalgam  decomposes  the  compound  into 
two  molecules  of  thiosulphate,  and  the  same  decomposition 
occurs  on  the  addition  of  potassium  sulphide  ;  thus  :  — 

K2S406  +  K2S  --=  2K2S203  +  S. 

Potassium  tetrathionate  does  not  combine  with  ordinary  sulphur 
but  combines  with  nascent  sulphur  to  form  a  pentathionate. 
Thus  the  free  acid  is  decomposed  by  an  excess  of  sulphuretted 
hydrogen  into  water  and  sulphur  :  — 

H2S406  +  5H2S  =  6H20  +  9S. 

When  however  a  slow  current  of  sulphuretted  hydrogen  is 
passed  into  a  solution  of  the  potassium  salt,  the  sulphur 
combines  with  a  portion  of  the  salt  and  forms  potassium 
pentathionate.  Sulphur  dioxide  reacts  with  potassium  tetra- 
thionate to  form  a  trithionate  and  a  pentathionate,  the  latter 
being  probably  produced  by  a  secondary  reaction  from  thio- 
sulphuric  acid  or  its  anhydride  :  — 


PENTATHIONIC  ACID.    H2S5O6. 

230  Wackenroder,  in  1845,  was  the  first  to  draw  attention  to 
the  existence  of  this  acid  in  the  liquid  obtained   by  passing 


420  THE  NON-METALLIC  ELEMENTS 

sulphuretted  hydrogen  into  a  solution  of  sulphur  dioxide ;  the 
clear  liquid  obtained  after  the  removal  of  the  precipitated 
sulphur  was  looked  upon  by  him  and  his  successors  as  a  solu- 
tion of  the  acid  in  water.  The  reaction  was  represented  by 
the  equation  : — 

5H2S  +  5S02  =  H2S5O6  +  5S  +  4H2O. 

Later  observers  succeeded  in  preparing  crystalline  pentathion- 
ates  from  this  liquid  and  thus  placed  the  existence  of  the  acid 
beyond  doubt.  The  investigations  of  Debus1  have  shown 
that  the  liquid  in  question,  known  as  Wackenroder's  solution, 
has  a  very  complex  composition,  which  varies  according  to  the 
method  of  preparation  The  liquid  is  best  prepared  by  passing 
sulphuretted  hydrogen  slowly  for  two  hours  into  480  cc.  of 
a  saturated  solution  of  sulphurous  acid  at  a  temperature  near 
0°,  allowing  the  whole  to  stand  for  two  days  in  a  closed  bottle 
and  repeating  the  operation  until  all  the  sulphurous  acid  has 
disappeared.  The  liquid  is  then  filtered  from  the  sulphur 
which  has  separated  out  and  is  thus  obtained  as  a  semi-trans- 
parent milky  fluid.  The  solution  contains  oily  drops  of  sulphur 
in  suspension  and  a  considerable  amount  of  sulphur  in  a  soluble 
form  (p.  346),  which  can  be  precipitated  by  the  addition  of 
potassium  nitrate  or  by  concentrating  the  fluid  on  the  water - 
bath.  The  concentration  of  the  clear  liquid  cannot  be  carried 
further  than  a  sp.  gr.  of  1*32  in  this  way  without  evolution  of 
sulphur  dioxide  and  deposition  of  sulphur,  but  may  be  continued 
over  sulphuric  acid  in  vacuo  beyond  this  point. 

An  analysis  of  the  solution  thus  obtained  (sp  gr.  1-32)  shows 
that  its  composition  is  approximately  that  of  pentathionic  acid, 
H2S5O6.  When  however  the  liquid  is  carefully  treated  with 
about  one  third  of  an  equivalent  of  caustic  potash  and  then 
allowed  to  evaporate  spontaneously,  a  mixture  of  potassium 
tetra-  and  pentathlon ates  is  obtained  which  can  be  separated 
by  recrystallisation  from  lukewarm  water.  The  mother  liquor 
on  evaporation  yields  a  salt  which  contains  more  sulphur  than 
a  pentathionate  and  is  probably  potassium  hexathionate  (p.  422). 
In  addition  to  these  substances,  Wackenroder's  solution,  pre- 
pared in  the  manner  described,  contains  sulphuric  acid  and 
traces  of  trithionic  acid,  the  amount  of  which  becomes  much 

1  Journ.  Ghem.  Soc.   1888,  278 — 357  (where  references  to  the  literature  of  the 
subject  will  be  found). 


PENTATHIONIC  ACID  421 


greater  when  an  insufficient  quantity  of  sulphuretted  hydrogen 
is  used. 

The  pentathionates  are  decomposed  by  caustic  alkalis,  sulphur 
being  deposited  and  a  tetrathionate  formed,  and  they  are  there- 
fore best  prepared  from  Wackenroder's  solution  by  treating  it 
with  the  acetate  of  the  metal  and  allowing  the  solution  to  evap- 
orate spontaneously.  The  potassium  salt,  2K2S5O6+3H2O,  crys- 
tallises in  short  rhombic  prisms  or  six-sided  plates.  It  forms  a 
clear,  neutral  solution  in  two  parts  of  water  and  can  be  recrys- 
tallised  from  water  at  50°  made  acid  with  a  little  sulphuric  acid. 
The  solution  decomposes  on  standing,  more  rapidly  on  heating, 
sulphur  being  deposited  and  a  tetrathionate  formed,  but  is  more 
stable  in  the  presence  of  a  little  sulphuric  acid  ; 


The  salt  itself  decomposes  on  keeping  if  any  moisture  be  present, 
but  if  dried  by  washing  the  fine  powder  with  dilute  alcohol,  and 
then  preserved  over  sulphuric  acid,  it  remains  unaltered  for  years. 
On  ignition  it  is  converted  into  potassium  sulphate,  sulphur 
dioxide  and  sulphur  :  — 


Potassium  pentathionate  is  also  formed  when  a  slow  current  of 
sulphuretted  hydrogen  is  passed  into  a  solution  of  the  tetra- 
thionate (p.  419).  The  barium  salt,  BaS5O6  +  3H,O,  a.nd  the 
copper  salt,  CuS5O6  +  4H2O,  are  also  crystalline. 

The  pentathionates  may  be  distinguished  from  the  lower 
members  of  the  series  by  the  facts  that  ammoniacal  silver  nitrate 
solution  produces  a  brown  coloration  which  rapidly  becomes 
darker,  a  black  precipitate  being  finally  formed,  and  that  caustic 
potash  produces  an  immediate  precipitate  of  sulphur,  tetrathionate 
being  also  formed,  whilst  ammonia  only  gives  this  reaction  on 
standing.  They  are  not  affected  by  hydrochloric  acid,  ferric 
chloride  or  barium  chloride. 

A  solution  of  pure  pentathionic  acid,  prepared  by  the  decom- 
position of  the  potassium  salt,  is  converted  by  sulphuretted 
hydrogen  into  water  and  sulphur  :  — 

H2S508  +  5H2S  =  6H20  +  10S, 


THE  NON-METALLIC  ELEMENTS 


whilst  the   potassium  salt  yields  a  thiosulphate  and  a  trithi- 
onate  : — 

3K2S5O6  +  3H2S  =  K2S2O3  +  2K2S3O6  +  3H2O  +  10S. 

Sulphur  dioxide  reacts  both  with  the  acid  and  its  salts  to  form 
trithionic  acid  and  probably  thiosulphuric  anhydride : — 


HEXATHIONIC  ACID,  H2S0O6. 

231  This  substance  is  only  known  in  the  form  of  its  potassium 
salt  obtained  from  the  mother  liquors  of  potassium  pentathionate. 
This  salt,  which  has  not  been  obtained  in  a  perfectly  pure  state, 
forms  a  warty  crust  and  yields  an  aqueous  solution,  which  de- 
composes even  in  the  presence  of  acid.  It  reacts  with  caustic 
potash  and  ammoniacal  silver  nitrate  solution  in  the  same  way 
as  the  pentathionates,  but  gives  an  immediate  precipitate  of 
sulphur  with  ammonia. 


FORMATION  OF  WACKENROBER'S  SOLUTION. 

It  seems  probable  that  in  the  preparation  of  Wackenroder's 
solution,  the  direct  product  is  tetrathionic  acid  : — 

H2S  +  3S02  =  H2S406. 

This  substance  is  then  acted  upon  by  sulphurous  acid  with 
formation  of  trithionic  acid  and  thiosulphuric  anhydride,  S2O2 
(p.  419).  Moreover  the  tetrathionic  acid  is  converted  by  the 
sulphuretted  hydrogen  into  water  and  sulphur,  and  a  number 
of  secondary  reactions  then  go  on,  by  means  of  which  the  thio- 
sulphuric anhydride  gives  up  some  of  its  sulphur  to  the  tetra- 
thionic acid,  forming  pentathionic  acid,  whilst  the  trithionic 
acid  combines  with  nascent  sulphur,  forming  tetra,  penta-and 
hexathionic  acids. 

When  the  action  of  the  sulphuretted  hydrogen  is  allowed  to 
continue  until  all  action  is  at  an  end,  water  and  sulphur  are 
found  to  be  the  final  products : — 


SELENIUM  423 


CONSTITUTION  OF  THE  ACIDS  OF  WACKENRODER'S  SOLUTION. 

Potassium  pentathionate  is  oxidised  by  bromine  according  to 
the  following  equation  : — 

K2S506  +  4Br2  +  6H20  =  2KBr  +  2S  +  3H2SO4  +  6HBr. 

Hence  two  of  the  sulphur  atoms  are  differently  combined  in 
the  molecule  from  the  other  three ;  these  two  atoms  are  pre- 
cipitated in  the  above  reaction  as  free  sulphur,  whilst  the  other 
three  are  converted  into  sulphuric  acid  and  are  therefore 
probably  already  combined  with  oxygen  in  the  molecule  of  pen- 
tathionic  acid  and  in  those  of  tetra-  and  trithionic  acid,  which 
stand  in  so  close  a  relation  to  it.  Trithionic  acid  moreover 
strongly  resembles  sulphurous  acid  in  its  power  of  combining 
with  sulphur  to  form  other  acids  and  therefore  probably  contains 
the  group  K.S02,  which  is  characteristic  of  sulphurous  acid. 
The  following  formulae  exhibit  all  these  relations  and  are  also  in 
-accordance  with  the  various  decompositions  of  these  substances ; 
they  may  therefore  be  assigned  to  the  salts  of  these  acids 
(Debus)  :— 

K.  SO2.   O 

Potassium  Trithionate     ...  | 

KO.  S02.  S 

KS.  S02.  O 
Potassium  Tetrathionate.     .     . 

KO.  SO2.  S 

KS2.  S02.  O 
Potassium  Pentathionate.     .     . 

KO.  S02.  S 

KS3.  S02.  O 
Potassium  Hexathionate     .     . 

KO.  S02.  S 


SELENIUM.     Se  =  78-5. 

232  We  owe  the  discovery  of  this  element  to  Berzelius.1  He 
first  found  it  in  the  deposit  from  the  sulphuric  acid  chambers  at 
Gripsholm  in  Sweden,  in  the  year  1817,  and  it  is  to  him  that 
we  are  indebted  for  the  knowledge  of  its  most  important  com- 

1  Schweigg  Journ.  23,  309,  430  ;  Fogg.  Ann.  7,  242  ;  8,  423. 


424  THE  NON-METALLIC  ELEMENTS 

pounds.  The  name  selenium  is  derived  from  %€\ijvrj,  the  moon, 
on  account  of  its  analogy  with  the  element  tellurium  (tellus,  the 
earth),  discovered  shortly  before. 

Although  selenium  is  somewhat  widely  distributed,  it  occurs 
only  in  small  quantities.  It  is  found  together  with  sulphur  in  the 
islands  of  Volcano  and  Hawai,  and  occurs,  chiefly  combined  with 
certain  metals,  at  Clausthal  and  Zorge  in  the  Harz,  in  La  Plata, 
the  Argentine  and  Mexico,  as  the  mineral  clausthalite,  PbSe ; 
a  selenide  of  copper  and  lead,  PbSe  +  Cu2Se;  lehrbachite, 
PbSe  +  HgSe;  selenide  of  silver,  Ag2Se  ;  selenide  of  copper, 
Cu2Se.  We  also  find  it  as  onofrite,  HgSe  +  4HgS,  in  Mexico  ; 
whilst  eucairite,  Cu.2Se  -f  Ag2Se,  and  crookesite  (CuTlAg).2Se, 
occur  at  Skrikerum  in  Sweden.  Selenium  is  also  found  in  very 
small  quantity  in  many  other  minerals,  especially  in  certain 
iron-pyrites  and  copper-pyrites,  and  where  these  are  used 
for  the  manufacture  of  sulphuric  acid,  a  red  deposit  containing 
selenium  is  found  in  the  chambers. 

Preparation. — In  order  to  prepare  selenium  from  this  deposit, 
it  is  mixed  with  equal  parts  of  sulphuric  acid  and  water  to  a 
thin  paste  and  then  boiled ;  nitric  acid  or  potassium  chlorate 
being  added  until  the  red  colour  disappears.  In  this  way  a 
solution  of  selenic  acid,  H2SeO4,  is  obtained,  which  is  diluted 
with  water,  filtered,  and  then  heated  with  a  quarter  its  volume 
of  fuming  hydrochloric  acid  until  three-quarters  of  the  liquid 
has  evaporated.  By  this  process  chlorine  is  evolved  and 
selenious  acid,  H2SeO3,  formed.  The  cold  solution  is  then 
poured  off  from  the  solid  matter  and  saturated  with  sulphur 
dioxide,  when  selenium  separates  out  as  a  red  powder.  The 
selenium  thus  obtained  contains  lead  and  other  metals,  from 
which  it  may  be  separated  either  by  distillation  or  by  fusing  it 
with  a  mixture  of  nitre  and  sodium  carbonate,  by  which 
means  sodium  selenate  is  formed,  and  this  is  then  again  treated 
with  hydrochloric  acid  and  sulphur  dioxide  as  above  described 
(Wohler).1  Selenium  may  also  be  easily  obtained  from  the 
chamber  deposit  by  heating  it  on  a  water-bath  with  a  concen- 
trated solution  of  cyanide  of  potassium  until  it  assumes  a  pure 
grey  colour.  On  the  addition  of  hydrochloric  acid  to  the  filtered 
solution  selenium  is  deposited  in  cherry-red  flakes.  This  also 
contains  both  copper  and  lead,  and  these  impurities  are  removed 
either  by  the  process  as  described  above  or  by  evaporating  the 
selenium  to  dryness  with  nitric  acid  and  reducing  the  aqueous 
1  Handbook  of  Inorganic  Analysis  (1854),  154. 


PKOPERTIES  OF  SELENIUM  425 

solution  of  selenium  dioxide  by  means  of  sulphur  dioxide 
(Nilson). 

Properties. — Selenium,  like  sulphur,  exists  in  several  allotropic 
modifications,  one  class  being  soluble,  the  other  insoluble  in 
carbon  bisulphide  (Berzelius,  Hittorf). 

Soluble  selenium  is  obtained  as  a  finely  divided  brick-red 
amorphous  powder,  when  a  cold  solution  of  selenious  acid  is 
precipitated  by  a  current  of  sulphur  dioxide  ;  and  as  a  black 
crystalline  powder  when  this  gas  is  passed  through  a  hot 
solution.  Other  reducing  agents,  such  as  iron,  zinc,  stannous 
chloride  or  phosphorous  acid,  also  precipitate  soluble  selenium 
from  solutions  of  selenious  acid.  Selenium  crystallizes  from 
solution  in  carbon  bisulphide  in  small  dark-red  translucent 
crystals,  which  are  isomorphous  with  the  monosymmetric  form 
of  sulphur,  and  have  a  specific  gravity  of  4'5.T  Like  sulphur, 
selenium  is  polymorphous,  the  crystals  which  are  deposited 
from  a  cold  saturated  solution  differing  in  type  from  those  just 
described,  although  belonging  to  the  same  system.  A  third 
variety,  isomorphous  with  the  crystals  of  tellurium  and  belong- 
ing to  the  hexagonal  system,  is  obtained  at  temperatures  above 
130°.2  When  selenium  is  fused  and  allowed  to  cool  quickly  it 
solidifies  to  a  dark  brownish-black  glassy  translucent  amorphous 
brittle  mass,  which  is  also  soluble  in  carbon  bisulphide,  and  has 
a  specific  gravity  of  4'3.  Soluble  selenium  has  no  definite 
melting  point,  softening  gradually  on  heating. 

The  insoluble  or  metallic  selenium  is  obtained  by  cooling 
melted  selenium  quickly  to  210°  and  then  keeping  the  melted 
mass  at  this  temperature  for  some  time.  The  selenium  at  length 
solidifies  to  a  granular  crystalline  mass,  the  temperature  rising 
suddenly  in  the  act  of  solidification  to  217°.  The  solid  mass 
thus  obtained  has  a  specific  gravity  of  4'5,  and  is  insoluble  in 
carbon  bisulphide.  This  change  from  the  soluble  to  the  insoluble 
condition  also  takes  place,  but  more  slowly,  at  lower  tempera- 
tures ;  thus,  if  a  mass  of  soluble  selenium  be  placed  in  an  air-bath 
at  100°,  the  change  to  the  insoluble  variety  takes  place  gradually, 
and  the  temperature  rises  up  to  217°.  If  a  concentrated  solution 
of  potassium  or  sodium  selenide  be  exposed  to  the  air,  black 
selenium  separates  out  in  microscopic  crystals  which  have  a 
specific  gravity  of  4 '8,  and  are  likewise  insoluble  in  carbon 
bisulphide.  The  insoluble  or  metallic  selenium  has  a  constant 
melting  point  of  217°,  and  when  quickly  cooled  is  converted 

1  Kammelsberg,  Ber.  7,  669.  2  Muthmann,  Zeit.  Kryst,  17,  336. 


426  THE  NON-METALLIC  ELEMENTS 


into  amorphous  soluble  selenium.  The  change  of  vitreous  into 
crystalline  selenium  is  accompanied  by  an  evolution  of  2'79 
omits  of  heat.  Both  modifications  of  selenium  are  soluble  in 
chloride  of  selenium  and  separate  out  from  this  solution  in  the 
form  of  metallic  selenium. 

In  addition  to  these  forms,  a  modification  of  selenium  is  also 
known  which  is  soluble  in  water.1  This  colloidal  selenium  is 
formed  when  a  current  of  sulphur  dioxide  is  passed  into  a 
solution  of  selenious  acid.  It  is  a  dark  red  powder  which  is 
completely  soluble  in  water,  forming  a  red  fluorescent  solution, 
but  wrhich  gradually  becomes  insoluble  on  preservation.  The 
solution  may  be  boiled  without  undergoing  any  change,  but 
the  selenium  is  deposited  on  the  addition  of  acids  or  salts. 
The  solution  on  spontaneous  evaporation  deposits  the  selenium 
as  a  red  transparent  film. 

According  to  the  experiments  of  Carnelley2  and  Williams, 
selenium  boils  at  680°,  and  forms  a  dark-red  vapour  which  con- 
denses either  in  the  form  of  scarlet  flowers  of  selenium,  or  in 
dark  shining  drops  of  the  melted  substance.  As  in  the  case 
of  sulphur,  the  vapour  density  of  selenium  diminishes  very 
rapidly  with  the  temperature ;  thus  at  860°  the  vapour  density 
is  110-7,  whilst  at  1420°  it  has  a  density  of  81'5,  closely 
corresponding  to  the  normal  vapour  density  of  78'5. 

Metallic  selenium  conducts  electricity,  and  exposure  to  light 
increases  its  conducting  power.3  The  peculiar  effect  of  light 
is  best  exhibited  on  selenium  which  has  been  exposed  for  a 
considerable  time  to  a  temperature  of  210°,  until  it  has  attained  a 
granular  crystalline  condition.  When  selenium  in  this  condition 
is  heated  its  electrical  resistance  is  increased,  whilst  on  exposing 
it  to  the  action  of  diffused  daylight,  the  electrical  resistance 
instantly  diminishes  ;  this,  however,  is  only  a  temporary  change, 
for  on  cutting  off  the  light,  the  electrical  resistance  of  the 
selenium  slowly  increases,  and  after  a  short  time  reaches  the 
amount  exhibited  before  the  exposure.  This  remarkable  pro- 
perty of  selenium  may  possibly  be  made  use  of  for  photometrical 
purposes. 

When  selenium  is  heated  in  the  air  it  burns  with  a  bright 

1  Schulze,  Jr.  Pr.  Chem.  [2],  32,  390. 

2  Journ.  Chem.  Soc.  1879,  i.  563. 

3  Sale,  Proc.  Roy.  Soc.  1873,  21,  283  ;    W.  G.  Adams,  Proc.  Roy.  Soc.  1875, 
•23,  535  ;  W.  Siemens,  Berlin,  Ber.  Akad.  1875.     S.   Bid  well,  Phil.  Mag.  (5), 
30   178. 


SELENIURETTED   HYDROGEN  427 

blue  flame,  forming  an  oxide  of  selenium  to  which  is  due  the 
characteristic  odour,  resembling  that  of  rotten  horse-radish, 
noticed  when  selenium  burns.  The  emission  spectrum  of 
selenium  is  seen  when  a  small  bead  of  the  element  is  held  in 
.a  non-luminous  gas  flame ;  it  is  a  channelled  spectrum  highly 
-characteristic  and  beautiful,  consisting  of  a  very  large  number 
of  bright  bands,  which  in  the  green  and  blue  are  arranged  at 
regular  intervals.1  According  to  Salet,2  selenium,  like  sulphur, 
gives  two  emission  spectra,  one  consisting  of  lines  and  the  other 
•of  bands.  The  absorption  spectrum  of  selenium  has  been 
examined  by  Gernez.3 

The  atomic  weight  of  selenium  has  been  determined  by 
Pettersson  and  Ekman,  who  determined  the  composition  of  the 
•oxide  Se02  and  of  silver  selenite  Ag2Se03  and  obtained  the 
number  78*5  ;4  their  numbers  agree  with  the  older  determina- 
tions of  Berzelius.5 


SELENIUM  AND  HYDROGEN. 

HYDROGEN  SELENIDE  OR  SELENIURETTED  HYDROGEN.    H2Se. 

233  This  gas  is  formed  when  selenium  vapour  and  hydrogen 
-are  heated  together,  and  the  amount  which  is  thus  produced  is 
a  function  of  the  temperature.  The  quantity  formed  increases 
when  the  temperature  is  raised  from  250°  to  520°,  but  above 
this  point  the  amount  gradually  diminishes.  When  selenium 
is  heated  in  a  closed  tube  filled  with  hydrogen,  it  sublimes  in 
the  cool  part  of  the  tube  in  the  form  of  beautiful  glittering 
crystals,  and  these  increase  in  number  until  the  whole  of  the 
selenium  has  volatilized.  The  formation  of  these  crystals 
depends  upon  the  decomposition  by  heat  of  the  seleniuretted 
hydrogen  which  is  formed ;  for  when  selenium  is  heated  in  a 
tube  filled  with  an  indifferent  gas,  only  red  amorphous  selenium 
is  found  to  sublime. 

Seleniuretted  hydrogen  is  easily  obtained  by  the  action  of 
dilute  hydrochloric  acid  on  potassium  selenide,  K2Se,  or  on  iron 
.selenide,  FeSe,  which  is  prepared  by  adding  selenium  to  heated 

1  W.  M.  Watts,  Index  of  Spectra,  p.  56. 

2  Compt.  Rend.  73,  559  and  742.  3  Compt.  Rend.  74,  1190. 
4  Ber.  9,  1290.  5  Pogg.  Ann.  8,  21. 


428  THE  NON-METALLIC  ELEMENTS 


iron.  It  is  also  formed  when  organic  materials  such  as  colo- 
phonium  are  heated  with  selenium.  It  is  a  colourless,  inflam- 
mable gas,  possessing  a  smell  which,  to  begin  with,  resembles 
that  of  sulphuretted  hydrogen,  but  afterwards  is  found  to  have 
a  much  more  persistent  and  intolerable  odour,  a  small  quantity 
affecting  the  mucous  membrane  in  a  remarkable  degree,  attacking 
the  eyes,  and  producing  inflammation  and  coughing  which  last 
for  days. 

Berzelius  describes  the  effects  as  follows:1 — "In  order  to  become 
acquainted  with  the  smell  of  this  gas  I  allowed  a  bubble  not 
larger  than  a  pea  to  pass  into  my  nostril ;  in  consequence  of  its 
action  I  so  completely  lost  my  sense  of  smell  for  several  hours 
that  I  could  not  distinguish  the  odour  of  strong  ammonia,  even 
when  held  under  my  nose.  My  sense  of  smell  returned  after 
the  lapse  of  five  or  six  hours,  but  severe  irritation  of  the  mucous 
membrane  set  in  and  lasted  for  a  fortnight." 

In  order  to  ascertain  the  composition  of  seleniuretted  hydrogen 
metallic  tin  is  heated  in  a  measured  volume  of  the  gas,  when 
tin  selenide  is  formed,  and  a  volume  of  hydrogen  is  liberated 
equal  to  that  of  the  original  gas. 

Hydrogen  selenide  is  more  soluble  in  water  than  the  corre- 
sponding sulphur  compound,  yielding  a  colourless  solution  which 
reddens  blue  litmus  paper,  colours  the  skin  a  reddish -brown 
tint,  and  possesses  the  foetid  odour  of  the  gas.  Exposed  to  the 
air,  the  aqueous  solution  absorbs  oxygen  with  the  separation  of 
red  selenium.  When  added  to  solutions  of  salts  of  most  of  the 
heavy  metals,  it  produces  precipitates  of  the  insoluble  selenides 
in  an  analogous  manner  to  sulphuretted  hydrogen.  Sulphur  also 
decomposes  it,  forming  sulphuretted  hydrogen  and  precipitating 
selenium. 


SELENIUM  AND  CHLORINE. 

234  Selenium,  like  sulphur,  forms  several  compounds  with 
chlorine. 

SELENIUM  MONOCHLORIDE.    Se2Cl2. 

When  a  current  of  chlorine  is  passed  over  selenium,  the  latter 
melts  and  is  converted  into  a  brown  oily  liquid,  which  is  selenium 

1  Lehrbuch,  5  Aufl.  2,  213. 


SELENIUM  MONOBROMIDE  429 

monochloride.  It  is  more  readily  prepared  by  passing  hydro- 
chloric acid  gas  into  a  solution  of  selenium  in  fuming  nitric 
acid.1  Selenium  monochloride  is  slowly  decomposed  by  water 
according  to  the  equation  : 

2Se2Cl2  +  3H20  =  H2SeO3  +  3Se  +  4HC1. 


SELENIUM  TETRACHLORIDE.     SeCl4. 

This  body  is  obtained  by  the  further  action  of  chlorine  upon 
the  monochloride,  as  well  as  when  selenium  dioxide  is  heated 
with  phosphorus  pentachloride  (Michaelis) ;  thus  : — 

3Se02  +  3PC15  =  3SeCl4  +  P2O5  +  POC13. 

The  tetrachloride  is  a  light  yellow  solid  body,  which  on  heat- 
ing volatilizes  without  previously  melting,  subliming  in  small 
crystals.  It  also  crystallizes  from  solution  in  phosphorus 
oxychloride  in  the  form  of  bright  shining  cubes.  It  dissolves 
in  water  with  formation  of  hydrochloric  and  selenious  acids ; 
thus : — 

SeCl4  +  3H20  =  4HCl  +  H2SeOs. 

When  its  vapour  is  heated  its  density  diminishes,  dis- 
sociation taking  place.2 


SELENIUM  AND  BROMINE. 

SELENIUM  MONOBROMIDE.    Se2Br2. 

235  Equal  parts  of  bromine  and  selenium  combine  together 
with  evolution  of  heat  to  form  a  black  semi-opaque  liquid,  having 
a  specific  gravity  at  15°  of  3'6.  It  has  a  disagreeable  smell 
resembling  that  of  chloride  of  sulphur,  and  colours  the  skin 
a  permanent  red-brown  tint.  On  heating  it  is  decomposed, 
and  when  brought  into  contact  with  water,  selenium,  selenious 
acid  and  hydrobromic  acid  are  formed  ;  thus : — 

2Se2Br2  +  3H2O  =  3Se  +  H2SeO3  +  4HBr. 


1  Divers  and  Shimose,  Ber.  17,  866. 

2  Chabrie,  Bull.  Soc.  Ghim.  [3],  2,  803. 


430  THE  NON-METALLIC  ELEMENTS 


SELENIUM  TETKABROMIDE.     SeBr4. 

This  compound  is  formed  by  the  further  action  of  bromine  on 
the  monobromide.  It  is  an  orange  yellow  crystalline  powder, 
best  obtained  by  adding  bromine  to  a  solution  of  selenium 
monobromide  in  carbon  bisulphide.  It  is  very  volatile,  vapor- 
ising between  75°  and  80°  with  partial  decomposition  and 
subliming  in  black  six-sided  scales.  It  possesses  a  disagree- 
able smell  similar  to  that  of  chloride  of  sulphur,  decomposes 
in  contact  with  moist  air  into  bromine  and  the  monobromide^ 
and  dissolves  in  an  excess  of  water  with  formation  of  hydro- 
bromic  and  selenious  acids. 

The  chlorobromides  of  selenium,  SeCl3Br  and  SeClBr3,  are 
orange  yellow  crystalline  substances  (Evans  and  Ramsay). 


SELENIUM  AND  IODINE. 

SELENIUM  MONO-IODIDE.     Se2I2. 

236  The  compounds  of  selenium  and  iodine  resemble  those 
of  selenium  with  chlorine  and  bromine.  The  mono-iodide  is  ob- 
tained when  the  two  elements  are  brought  together  in  the  right 
proportions  (Schneider),  and  forms  a  black  shining  crystalline 
mass,  melting  between  68°  and  70°  with  the  evolution  of  a  small 
quantity  of  iodine.  When  more  strongly  heated,  it  decomposes 
into  iodine  which  volatilizes,  and  selenium  which  remains 
behind,  and  is  decomposed  by  water  in  a  similar  way  to  the 
corresponding  bromide. 

SELENIUM  TETRA-IODIDE.    SeI4. 

It  is  a  granular  dark  crystalline  mass,  which  melts,  at  from  75° 
to  80°,  to  a  brownish-black  liquid,  translucent  in  thin  films ; 
when  more  strongly  heated  it  decomposes  into  its  elements. 


SELENIUM  AND   FLUORINE. 

237  When   the  vapour  of  selenium    is   passed  over  melted 
fluoride   of  lead,  a  fluoride   of  selenium  sublimes  in   crystals. 


SELENIOUS  ACID  431 

These  are  soluble  in  hydrofluoric  acid,  and  are  decomposed  by 
water  (Knox).  When  selenium  is  exposed  to  gaseous  fluorine  it 
fuses  and  finally  takes  fire,  a  white  crystalline  substance  being 
formed.  This  is  soluble  in  hydrofluoric  acid,  but  is  decomposed 
by  water  (Moissan).1 


SELENIUM    AND   OXYGEN. 
OXIDES  AND  OXY- ACIDS  OF  SELENIUM. 

238  Only  one  oxide  of  selenium,  the  dioxide.  SeO2,  is  with  cer- 
tainty known  to  exist  in  the  free  state.     A  lower  oxide  is  stated 
by  Berzelius  to  be  formed  when  selenium  burns  in  the  air,  and  is 
probably  the  cause  of  the  peculiar  smell  then  observed,  which  is 
so  penetrating  that  if  1  mgrm.  of  selenium  be  burnt  in  a  room 
the  smell  is  perceptible  in  every  part. 

Selenium  also  forms  two  oxy-acids — selenious  acid,  H2SeO3 
and  selenic  acid,  H2SeO4. 

SELENIUM  DIOXIDE.    SeO2. 

239  When  selenium  is  placed  in  a  bent  tube,  as  shown  in  Fig. 
125  and  strongly  heated  in  a  current  of  oxygen,  contained  in  the 
gasholder  and  dried  by  passing  over  pumice-stone  saturated  with 
sulphuric  acid,  it  takes  fire  and  burns  with  a  bright  blue  flame,  a 
white  sublimate   of  solid  selenium  dioxide  being  deposited  in 
the  cool  part  of  the  tube.     Thus  obtained  it  forms  long  four- 
sided,  white,  needle-shaped  crystals,  which  do  not  melt  when 
heated  under  the  ordinary  atmospheric  pressure,  but  evaporate, 
when  heated  to  about  300°,  yielding  a  greenish  yellow-coloured 
vapour  possessing  a  powerful  acid  smell  (Berzelius). 

SELENIOUS  ACID,  H2SeO3. 

240  Selenious  acid  is  formed  when  selenium  is  heated  with 
nitric  acid,  or  when  five  parts  of  the  dioxide  are  dissolved  in  one 
part  of  hot  water.     On  cooling,  clear,  long,  colourless,  prismatic, 
nitre-shaped  crystals  of  selenious  acid  separate  out.    These  have  a 
strong  acid  taste,  and,  when  heated,  decompose  into  selenium 

1  Ann.  Chim.  Phys.  [6]  24,  239. 


432 


THE  NON-METALLIC  ELEMENTS 


dioxide  and  water.  If  sulphur  dioxide  be  allowed  to  pass  into 
a  hydrochloric  acid  solution  of  selenious  acid,  selenium  is 
deposited  as  a  red  powder.  This  decomposition  takes  place 
but  slowly  in  the  cold  and  in  absence  of  light ;  but  when  the 
liquid  is  heated  or  in  the  sunlight,  the  change  occurs  quickly. 
Organic  substances  also  bring  about  this  reduction,  and  the 
colourless  solution  of  the  pure  acid  soon  becomes  tinged  when 
exposed  to  the  air,  owing  to  the  presence  of  the  dust  in  the 
atmosphere.  Selenious  acid  is  distinguished  from  sulphurous 


FIG.  125. 


acid  by  this  reaction,  for  sulphurous  acid  gradually  absorbs 
oxygen  from  the  air. 

Selenious  acid  is  a  dibasic  acid,  and  forms  not  only  acid 
and  normal  salts,  but  also  salts,  containing  acid  selenites  united 
with  selenious  acid  ;  thus,  HKSeO8  +  H9SeO3. 

The  normal  selenites  of  the  alkali  metals  are  easily  soluble  in 
water  ;  those  of  the  alkaline  earth  metals  and  the  heavy  metals 
are  insoluble ;  whilst  all  the  acid  salts  are  soluble  compounds. 
When  a  selenite  is  heated  on  charcoal  in  the  reducing  flame,  a 
characteristic  horse-radish-like  smell  is  emitted,  and  when 


SELENIC  ACID  433 


heated  in  a  glass  tube  with  sal-ammoniac,  selenium  sublimes. 
The  selenites  are  further  distinguished  by  the  fact  that  red 
selenium  is  precipitated  when  sulphur  dioxide  is  led  into  their 
solution  in  water  or  in  hydrochloric  acid.  The  salts  are  very 
poisonous. 


SELENYL  CHLORIDE,  OR  SELENIUM  OXYCHLORIDE.    SeOCl2. 

241  This  chloride  of  selenious  acid  is  formed  by  the  action  of 
selenium  dioxide  on  selenium  tetrachloride  (Weber) ; *  thus : — 

SeO+2SeCI4  =  2SeOCl2. 

and  when  the  oxide  is  heated  with  sodium  chloride  : 2 — 
2Se02  +  2NaCl  =  SeOCl2  +  Na2SeO3. 

It  is  a  yellow  liquid  which  fumes  on  exposure  to  air,  and 
which,  when  cooled  below  10°,  deposits  crystals  having  a 
specific  gravity  of  2'44.  It  boils  at  179°'5  (Michaelis),  and 
decomposes  with  water  in  the  same  way  as  all  acid  chlorides. 
When  mixed  with  thionyl  chloride,  selenium  tetrachloride  and 
sulphur  dioxide  are  formed  ;  thus  : — 

SeOCl.2  +  SOC12  =  SeCl4  +  SO2. 


SELENYL  BROMIDE.     SeOBr2. 

Formed  by  the  action  of  selenium  tetrabromide  on  selenium 
dioxide.  The  two  substances  are  melted  together,  and  the 
compound,  on  cooling,  crystallizes  in  long  needles. 


SELENIC  ACID.    H2Se04. 

242  This  acid,  discovered  in  1827  by  Mitscherlich,  is  formed 
by  the  action  of  chlorine  on  selenium  or  on  selenious  acid  in 
presence  of  water ;  thus  : — 

Se  +  3C12  +  4H20  =  H2SeO4  +  6HC1. 

By  the  same  reaction  selenites  may  be  converted  into  selenates. 
Bromine  may  for  this  purpose  be  employed  instead  of  chlorine. 

1  Pogg.  Ann.  108,  615. 

2  Cameron  and  Macallan,  Gkton.  News,  59,  267. 
29  \ 


434  THE  NON-METALLIC  ELEMENTS 


Potassium   selenate   is    also  obtained  when  selenium  is  fused 
with  nitre. 

In  order  to  prepare  selenic  acid,  a  solution  of  sodium 
selenite  is  treated  with  silver  nitrate  ;  and  to  the  precipitate 
obtained,  suspended  in  water,  bromine  is  added  (J.  Thomsen) ; 
thus : — 

Ag2SeO3  +  H2O  4  Br2  =  2AgBr  +  H2SeO4. 

The  aqueous  solution  of  selenic  acid  can  be  concentrated  by 
evaporation  in  the  air,  but  it  cannot  be  thus  completely  freed 
from  water,  as  at  a  temperature  of  about  280°  it  begins  to  de- 
compose into  oxygen,  selenium  dioxide  and  water.  If  heated 
in  vacuo  to  180°  and  then  cooled,  the  whole  solidifies  to  a 
crystalline  mass,  consisting  of  the  pure  acid.1  The  acid 
crystallizes  in  long  hexagonal  prisms,  similar  to  those  formed 
by  sulphuric  acid  and  melts  at  58°.  It  combines  eagerly  with 
water,  forming  a  monohydrate  which  melts  at  25°  and,  like 
sulphuric  acid,  chars  many  organic  substances.  The  specific 
gravity  of  the  solid  acid  is  2*6273  whilst  that  of  the  liquid  at 
15°  is  2-3557. 

The  concentrated  solution  of  the  acid  is  a  colourless,  very 
acid  liquid,  which  is  miscible  with  water  in  all  proportions,  heat 
being  thereby  evolved.  The  heated  aqueous  acid  dissolves  gold 
and  copper  with  formation  of  selenious  acid,  whilst  iron,  zinc, 
and  other  metals  dissolve  with  evolution  of  hydrogen  and  pro- 
duction of  selenates.  This  acid  is  not  reduced  either  by  sulphur 
dioxide  or  by  sulphuretted  hydrogen,  and  differs  in  this  respect, 
therefore,  remarkably  from  selenious  acid.  When  boiled  with 
hydrochloric  acid  it  decomposes  with  the  evolution  of  chlorine 
and  formation  of  selenious  acid ;  thus  : — 

H2Se04  +  2HC1  =  H2SeO3  +  C12  +  H2O. 

This  mixture  dissolves  gold  and  platinum,  these  metals  com- 
bining with  the  chlorine  thus  set  at  liberty. 

The  selenates  exhibit  the  closest  analogy  with  the  sulphates 
so  far  as  regards  amount  of  water  of  crystallization,  crystalline 
form,  and  solubility.  Barium  selenate,  like  the  sulphate,  is  com- 
pletely insoluble  in  water,  and  is  employed  for  this  reason  in  the 
quantitative  determination  of  selenic  acid ;  it  is,  however, 
distinguished  from  barium  sulphate  inasmuch  as  when  boiled 
with  hydrochloric  acid  the  insoluble  selenate  is  decomposed  into 

1  Cameron  and  Macallan,  Chcm.  News,  59,  219. 


SELENOSULPHURIC  ACID  435 

the  soluble  selenite,  whereas  barium  sulphate  remains  unchanged. 
All  the  other  selenates  are  also  reduced  to  selenites  by  means 
of  hydrochloric  acid,  and  this  reaction  serves  as  a  means  of 
recognizing  these  compounds. 


SELENIUM  AND  SULPHUR. 

243  Several  substances  have  been  at  various  times  described  as 
compounds  of  selenium  and  sulphur,1  but  they  have  all  proved 
to  be  mixtures  of  the  two  elements.2 


SELENOSULPHUR  TRIOXIDE.    SeS03. 

It  has  long  been  known  that  when  selenium  is  dissolved 
in  fuming  sulphuric  acid,  a  beautiful  green  colour  is  produced, 
and  it  has  been  proved  that  this  is  due  to  the  formation 
of  the  above  compound.  This  substance  is  best  prepared  by 
dissolving  selenium  in  freshly  distilled  well-cooled  sulphur 
trioxide.  The  compound  then  separates  out  in  the  form  of 
tarry  drops  which  soon  solidify  to  prismatic  crystals,  having  a 
dirty  green  colour,  and  yielding  a  yellow  powder  when 
broken  up.  The  compound  dissolves  in  sulphuric  acid  with  a 
green  colour,  and  is  decomposed  on  addition  of  water  with 
separation  of  selenium  and  formation  of  sulphuric,  sulphurous 
and  selenious  acids.  On  heating,  the  body  does  not  melt  but 
decomposes  into  selenium,  selenium  dioxide,  and  sulphur  dioxide 
(Weber).3 

f  OH 

SELENOSULPHURIC  ACID.    SO2  -j  gejj 

This  compound,  which  corresponds  to  thiosulphuric  acid,  was 
discovered  by  Cloez,4  and  like  this  latter  acid  is  not  known  in 
the  free  state.  Its  potassium  salt  is  obtained  when  a  solution  of 
sulphurous  acid  is  mixed  with  one  of  potassium  selenide,  or, 
together  with  the  salts  of  selenotrithionic  acid,  when  selenium 

1  Rathke,   J.  Pr.    Chem.   108,    235;   Ditte,    Compt.    Rend.    73,    625,    660; 
Gerichten,  Ber.  7,  26. 

2  Divers,  Chem.  News,  51,  24  ;  Bettendorf  and  von  Rath,  Pogg.  Ann.  139, 
329. 

3  Pogg.  Ann.  156,  513.  4  Bull.  Soc.  Chim.  1861,  Il2. 


436  THE  NON-METALLIC  ELEMENTS 

is  dissolved  in  a  solution  of  normal  potassium  sulphite.1  The 
selenosulphates  are  isomorphous  with  the  thiosulphates,  and  are 
decomposed  by  all  acids,  even  by  sulphurous  acid,  with  the 
separation  of  red  selenium. 


SELENOTRITHIONIC  ACID.     H2SeS2O6. 

The  potassium  salt  of  this  acid  is  formed  when  a  solution  of 
potassium  selenosulphate  is  mixed  with  an  excess  of  normal 
potassium  sulphate,  and  a  concentrated  solution  of  selenious 
acid,  thus  :  — 

QO  J 

S(M 


The  potassium  salt  forms  monosymmetric  prisms  and  is  stable 
in  the  air.  According  to  Schulze,2  the  free  acid  is  formed  when 
selenious  acid  is  treated  with  an  excess  of  sulphur  dioxide. 

The  same  chemist  has  found  that  when  selenious  acid  is 
kept  in  excess,  an  acid  of  the  formula  H2SSe2O6  is  obtained. 

SulpJioselenoxytetrachloride,  C1SO2  (OSeCl3),  is  obtained  by  the 
action  of  selenium  tetrachloride  upon  chlorosulphonic  acid  as  a 
mass  of  white  crystals3  ;  it  melts  at  165°  and  boils  at  183°. 


TELLURIUM.     Te  =  124  (?). 

244  Tellurium  occurs  in  small  quantities  in  the  free  state  in 
nature,  and  by  early  mineralogists  was  termed  aurum  paradoxum 
or  metallum  problematum,  in  consequence  of  its  metallic  lustre. 
In  178 2  native  tellurium  was  more  carefully  examined  byMiiller 
von  Reichenstein.  He  came  to  the  conclusion  that  it  contained 
a  peculiar  metal,  and  at  his  suggestion,  Klaproth4  in  1798 
made  an  investigation  of  the  tellurium  ores  confirming  the  views 
of  the  former  experimenter  that  it  contained  a  new  metal,  to 
which  he  gave  the  name  of  tellurium  from  telhis,  the  earth. 

1  Schaffgotsch,  Pogg.  Ann.  90,    66  ;  Rathke,  J.  Pr.  Chem.  95,  1  ;  Uelsmaun, 
AnnalenUQ,  122.  2  J.  Pr.  Chem.  [2]  32,  405. 

3  Clausnizer,  Ber.  H,  2007.  4  Crell.  Ann.  1,  91. 


TELLURIUM  437 


Berzelius1  in  1832,  made  a  more  exhaustive  investigation  of 
tellurium ;  he  likewise  considered  the  substance  itself  to  be  a 
metal,  but  its  compounds  were  found  to  correspond  so  closely  with 
those  of  sulphur  and  selenium,  that  tellurium  was  placed  in  the 
sulphur  group. 

Tellurium  belongs  to  the  rarer  elements.  It  occurs  in 
Transylvania,  Hungary,  California,  Virginia,  Brazil,  and  Bolivia, 
in  small  quantities  in  the  native  state,  but  it  is  generally 
found  in  combination  with  metals  as  graphic  tellurium,  or 
sylvanite  (AgAu)  Te2;  black  or  leaf  tellurium,  or  nagyagite 
(AuPb)2  (TeSSb)3;  silver  telluride,  Ag2Te ;  tetradymite,  or 
bismuth  telluride,  Bi2Te3,  &c. 

Preparation. — In  order  to  prepare  pure  tellurium,  tellurium- 
bismuth  containing  about  60  per  cent  of  bismuth,  36  per  cent, 
of  tellurium,  and  about  4  per  cent,  of  sulphur,  is  mixed  with 
an  equal  weight  of  pure  carbonate  of  soda,  and  then  the  mixture 
rubbed  up  with  oil  to  a  thick  paste,  and  heated  strongly  in  a 
well-closed  crucible.  The  mass  is  then  lixiviated  with  water, 
and  the  filtered  solution,  which  contains  sodium  telluride  and 
sodium  sulphide,  is  exposed  to  the  air,  when  the  tellurium  grad- 
ually separates  out  as  a  grey  powder,  which  after  washing  and 
drying  may  be  purified  by  distillation  in  a  current  of  hydrogen 
(Berzelius). 

In  order  to  obtain  the  tellurium  from  graphic,  or  from  black 
tellurium,  the  ore  is  treated  with  hydrochloric  acid  in  order 
to  free  it  from  antimony,  arsenic,  and  other  bodies.  The 
residue  is  then  boiled  with  aqua  regia,  and  the  filtrate  eva- 
porated to  drive  off  the  excess  of  nitric  acid ;  ferrous  sulphate 
is  then  added,  which  precipitates  the  gold,  and  the  tellurium 
is  thrown  down  in  the  filtrate  by  means  of  sulphur  dioxide 
(v.  Schrotter). 

The  crude  tellurium  is  best  purified  by  treating  it  with  aqua 
regia,  expelling  the  excess  of  nitric  acid  by  means  of  hydro- 
chloric acid  and  then  diluting  somewhat  so  that  the  tellurous 
acid  remains  in  solution  whilst  the  chloride  of  lead  is  precipitated. 
From  the  filtrate,  the  tellurium  is  precipitated  by  means  of 
sulphurous  acid.  The  material  thus  obtained,  which  may 
contain  traces  of  selenium,  lead  and  other  metals,  is  then  fused 
in  small  portions  with  potassium  cyanide,  the  mass  extracted 
with  water  in  the  absence  of  air,  and  the  tellurium  thrown 

1  Pogg.  Ann.  28,  392  ;  32,  1  and  577. 


438  THE  NON-METALLIC  ELEMENTS 

down  from  the  clear  filtrate  by  a  current  of  air.  It  is  finally 
melted  and  distilled  in  a  current  of  hydrogen.1 

Properties. — Tellurium  is  a  silver-white  body  possessing 
a  metallic  lustre,  and  crystallizing  in  rhombohedra.  Tellurium 
is  a  very  brittle  substance,  and  can  therefore  be  easily  powdered. 
Its  specific  gravity  is  6*24 :  it  melts  at  452°  (Carnelley  and 
Williams),  and  boils  at  a  still  higher  temperature,  and  may 
accordingly  be  easily  purified  by  distillation  in  a  stream  of 
hydrogen  gas. 

Amorphous  tellurium  is  obtained  when  a  solution  of  tellurous 
acid  is  precipitated  by  means  of  sulphurous  acid.  On  heating 
it  is  converted  with  evolution  of  heat  into  the  crystalline 
variety.2 

Tellurium  burns  when  heated  in  the  air  with  a  blue  flame, 
evolving  white  vapours  of  tellurium  dioxide.  It  is  insoluble  in 
water  and  carbon  bisulphide,  but  dissolves  in  cold  fuming  sul- 
phuric acid,  imparting  to  the  solution  a  deep-red  colour,  which 
is  probably  due  to  the  formation  of  a  compound  analogous  to 
sulphur  sesquioxide,  namely,  STe03,  the  tellurium  being  pre- 
cipitated on  the  addition  of  water.  On  heating  the  sulphuric 
acid  solution,  the  tellurium  is  oxidized  to  tellurous  acid,  sulphur 
dioxide  being  given  off.  In  the  same  way  it  rapidly  undergoes 
oxidation  in  the  presence  of  nitric  acid. 

Hydrochloric  acid  does  not  attack  tellurium ;  caustic  potash 
dissolves  it  with  formation  of  potassium  telluride  and  tellurite. 
When  heated  nearly  to  the  melting-point  of  glass,  tellurium 
emits  a  golden-yellow  vapour,  which  gives  an  absorption  spec- 
trum, consisting  of  fine  lines  stretching  from  the  yellow  to  the 
violet.3  The  emission  spectrum  of  tellurium  has  been  mapped 
by  Salet  4  and  Ditte.5 

According  to  Deville  and  Troost  the  vapour  of  tellurium 
possesses  a  specific  gravity  of  9*0  at  1390°,  which  number 
corresponds  to  a  molecular  weight  of  about  259.6  The  mole- 
cule of  tellurium  therefore  contains  two  atoms  and  has  the 
formula  Te2. 

245  Atomic  Weight  of  Tellurium. — Considerable  doubt  exists 
as  to  the  value  of  the  atomic  weight  of  tellurium.  The  earlier 
observers  obtained  the  number  128,  and  this  was  confirmed  in 

1  Brainier,  Monatsh.  10,  414. 

2  Fabre  and  Berthelot,  Compt.  Rend.  104,  1405. 

3  Gernez,  Compt.  Rend.  74,  H90.  4  Compt.  Rend.  73,  559,  742. 
5  Compt.  Rend.  73,  262.                                      6  Compt.  Rend,  56,  871. 


TELLURETTED   HYDROGEN  439 


1879  by  Wills,1  who  analysed,  the  double  bromide  of '  potassium 
and  tellurium  K2TeBr6,  and  converted  the  element  into  tellurous 
acid  by  means  of  nitric  acid  and  of  aqua  regia.  The  close  relation 
of  tellurium  to  sulphur  and  selenium  however  makes  it  highly 
probable,  from  considerations  connected  with  the  arrangement 
of  the  elements  according  to  the  periodic  system  (Vol.  II., 
Part  2,  p.  503),  that  the  atomic  weight  of  tellurium  is  less 
than  that  of  iodine  (125*9).  Brauner 2  has  therefore  thoroughly 
re-examined  the  question,  but  finds  that  all  reliable  methods 
of  determination  (analysis  of  tellurous  acid  and  of  tellurium 
tetrabromide)  lead  to  about  126'5.  He  expresses  the  opinion 
that  the  substance  at  present  known  as  tellurium  contains 
some  substance  of  higher  atomic  weight.  In  this  lie  is  con- 
firmed by  the  fact  that  when  the  tellurium  is  previously  sub- 
limed in  hydrogen  its  atomic  weight  is  found  to  be  considerably 
lower  (126'5)  than  when  it  is  sublimed  in  any  indifferent  gas 
(136'7).  He  supposes  therefore  that  the  impurity  is  partially 
removed  by  the  hydrogen. 

TELLURIUM  AND  HYDROGEN. 

TELLURIUM  HYDRIDE,  OR  TELLURETTED  HYDROGEN.    H2Te. 

246  This  compound,  discovered  by  Davy  in  1810,  is  a  colour- 
less gas  possessing  a  foetid  smell  similar  to  that  of  sulphuretted 
hydrogen.  It  is  formed  in  small  quantities  when  tellurium  is 
heated  in  hydrogen  gas.  If  this  is  allowed  to  take  place  in  a 
sealed  tube,  the  same  phenomenon  presents  itself  as  is  observed 
with  selenium,  and  the  tellurium  sublimes  in  long  glittering 
prisms. 

In  order  to  prepare  telluretted  hydrogen,  tellurium  is  heated 
with  zinc,  and  the  zinc  telluride  thus  formed,  decomposed  by 
hydrochloric  acid  ;  thus  : — 

ZnTe  +  2HCl-Zn012  +  H2Te. 

Tellurium  hydride  is  easily  combustible,  burning  with  a  blue 
flame.  It  is  soluble  in  water,  and  this  solution  absorbs  oxygen 
from  the  air,  tellurium  being  deposited.  Like  sulphuretted 
hydrogen,  telluretted  hydrogen  precipitates  many  of  the  metals 
from,  their  solutions  in  the  form  of  tellurides.  The  soluble 
tellurides,  suck  as  those  of  the  alkaline  metals,  form  brownish 

1  Journ.  Chcm.  Soc.  1879,  i.  704.  '2  Monatsh.  10,  411. 


440  THE  NON-METALLIC  ELEMENTS 


red  solutions  from  which  tellurium  is  deposited  on  exposure  to 
the  air. 

The  density  of  tellurium  hydride,  like  that  of  seleniuretted 
hydrogen,  has  not  been  as  yet  directly  determined,  but  Bineau 
has  shown  that  both  gases,  when  they  are  heated  with  certain 
metals,  give  up  all  their  selenium  or  tellurium,  leaving  a 
residue  of  hydrogen  which  occupies  the  same  volume  as  the 
original  gas.  Hence  each  molecule  of  these  gases  contains  two 
atoms  of  hydrogen  combined,  as  analysis  shows,  with  78'5  and 
about  124  parts  of  these  elements.  Consequently  tellurium 
hydride  contains  one  part  by  weight  of  hydrogen  and  about 
sixty-two  parts  by  weight  of  tellurium. 


TELLURIUM   AND    CHLORINE. 

TELLURIUM  BICHLORIDE.    TeCl2. 

247  This  compound  is  formed  together  with  the  tetrachloride 
when  chlorine  is  passed  over  melted  tellurium.  It  may  be 
separated  from  the  less  volatile  tetrachloride  by  distillation; 
and  thus  obtained,  it  forms  an  indistinctly  crystalline,  almost 
black  mass  which  gives  a  greenish  yellow  powder,  melts  at 
175°,  boils  at  3270,1  and  yields  a  deep  red-coloured  vapour, 
having  the  density  6*89.2 

Water  decomposes  the  compound  with  separation  of  tellurium 
and  formation  of  tellurous  acid ;  thus : — 

2TeCl2  +  3H20  =  Te  f  H2Te03  +  4HC1. 


TELLURIUM  TETRACHLORIDE,  TeCl4, 

Is  formed  by  the  further  action  of  chlorine  on  the  preceding 
compound.  It  is  a  white  crystalline  body  which  melts  at  2240,3 
forming  a  yellow  liquid,  which  when  more  strongly  heated 
becomes  at  last  of  a  dark  red  colour  and  boils  at  380 °,4  without 
decomposition.  It  is  extremely  hygroscopic  and  is  decomposed 
when  thrown  into  cold  water,  an  insoluble  oxychloride  being 

1  Carnelley  and  Williams.  Journ.  Chem.  Soc.  1879,  i.  563  ;  1880,  i.  125. 
3  Michaelis,  Ber.  20,  2488. 

3  Carnelley  and  Williams,  Journ.  Chem.  Soc.  1880,  i.  125. 

4  Michaelis,  Ber.  20,  2491. 


BROMIDES  OF  TELLURIUM  441 


formed  together  with  tellurous  acid.  Hot  water,  on  the  other 
hand,  gives  rise  only  to  the  formation  of  the  latter  compound. 
Like  the  corresponding  tetrachlorides  of  sulphur  and  selenium, 
it  forms  crystalline  compounds  with  a  large  number  of  metallic 
chlorides.  The  vapour  density  of  the  tetrachloride  is  9*2 
(Michaelis). 


TELLURIUM   AND    BROMINE. 
TELLURIUM  DIBROMIDE,  TeBr2, 

Is  best  obtained  by  heating  the  tetrabromide  with  tellurium. 
On  heating  it  volatilizes  in  the  form  of  a  violet  vapour,  which 
condenses  to  black  needles,  melting  at  280°,  and  again  solidify- 
ing to  a  crystalline  mass  ;  it  boils  at  339°. 


TELLURIUM  TETRABROMIDE,  TeBr4. 

In  order  to  prepare  this  compound,  finely  divided  tellurium 
is  added  to  bromine  which  has  been  cooled  down  to  00<  It  is 
a  dark  yellow  solid  body  which  can  be  sublimed  without  de- 
composition. It  dissolves  in  a  small  quantity  of  water  with  a 
yellow  colour,  but  when  added  to  a  large  quantity  of  water  the 
solution  becomes  colourless  from  the  formation  of  hydrobromic 
and  tellurous  acids.  It  boils  at  420°.  It  forms  double  salts 
with  the  bromides  of  the  alkali  metals. 


TELLURIUM   AND   IODINE. 
TELLURIUM  DI-IODIDE.    TeI2. 

Is  obtained  as  a  black  crystalline  mass  by  heating  iodine 
and  tellurium  together  in  the  proper  proportions. 


TELLURIUM  TETRA-IODIDE,  TeI4; 

Is  formed  by  the  action  of  hydriodic  acid  on  tellurous  acid ; 
thus : — 

H2TeO3  +  4HI  =  TeI4  +  3H2O. 


442  THE  NON-METALLIC  ELEMENTS 

It  forms  an  iron-gray  crystalline  mass  which  melts  when  gently 
heated,  and  when  heated  more  strongly  decomposes  with  separa- 
tion of  iodine.  It  is  but  slightly  soluble  in  cold  water,  and  is 
decomposed  by  boiling  water. 


TELLURIUM    AND    FLUORINE. 

TELLURIUM  TETRAFLUORIDE.     TeF4, 

Is  prepared  by  heating  the  dioxide  with  hydrofluoric  acid  in  a 
platinum  retort.  It  distils  over  as  a  colourless,  transparent,  very 
deliquescent  mass. 

When  a  solution  of  tellurous  acid  in  hydrofluoric  acid  is  con- 
centrated, crystals  of  the  formula  TeF4  +  4H2O  separate  out. 
Powdered  tellurium  becomes  incandescent  on  exposure  to 
fluorine  gas,  the  tetrafluoride  being  formed.1 


TELLURIUM   AND    OXYGEN. 

OXIDES  AND  OXYACIDS  OF  TELLURIUM. 

248  Three   oxides  of  tellurium  are  known,   TeO,   Te02  and 
Te03  as  well  as  the  acids,  H2Te03,  and  H2Te04  corresponding 
to  the  last  two  of  these. 

TELLURIUM  MONOXIDE,  TeO. 

249  This  substance  is  left  behind  as  a  brownish  black  amor- 
phous mass  when  tellurium  sulphoxide,  TeS03,  is  heated  to  230° 
in  vacuo,  sulphur  dioxide  being  evolved.  On  heating  in  the  air  or 
on  exposure  to  moist  air  it  is  gradually  oxidised.     Sulphuric  acid 
forms  with  it  a  red  solution,  from  which  a  crystalline  mass  of 
tellurium  sulphate,  Te(S04)9,   soon  separates.      The   monoxide 
when    heated     in    hydrochloric   acid    gas    is    converted    into 
tellurium  dichloride. 

TELLURIUM  DIOXIDE,  Te02, 

250  Occurs  in  the  impure  state  in  nature  as  tellurite  or  tellu- 
rium ochre,  at  Facebay  in  Transylvania.     It  is  formed  by  the 

1  Moissan,  Ann.  Ghim.  Phys.  [6]  24,  239. 

2  Divers  and  Shimose,  Ber.  16,  1004  ;  Journ.  Chem.  Soc.  1883,  i.  319. 


TELLUKOUS   ACID  443 


combustion  of  tellurium  in  the  air,  and  separates  out  in  small 
octahedra  when  tellurium  is  dissolved  in  warm  nitric  acid.  It 
is  only  very  slightly  soluble  in  water,  and  the  solution  does  not 
redden  blue  litmus  paper.  On  heating  it  melts  to  a  lemon- 
yellow  liquid  which  boils  on  further  heating  without  decom- 
position, whilst  on  cooling  it  solidities  to  a  white  crystalline 
mass.  Although  this  is  an  acid-forming  oxide,  it  also  exhibits 
basic  properties,  inasmuch  as  it  combines  with  certain  acids 
forming  an  unstable  class  of  salts,  which  are  decomposed  by 
water.  The  nitrate  on  ignition  leaves  a  residue  of 


TELLUROUS  ACID,     H2Te03, 

251  Is  obtained  by  pouring  a  solution  of  tellurium  in  dilute 
nitric  acid  into  water.  It  separates  out  in  the  form  of  a  very 
voluminous  precipitate,  which  when  placed  over  sulphuric  acid 
dries  to  a  light  white  powder.  It  is  but  slightly  soluble  in 
water,  and  the  solution  possesses  a  bitter  taste.  Tellurous  acid 
like  sulphurous  acid,  is  dibasic,  and  therefore  forms  two  series  of 
salts  :  thus  we  have,  normal  potassium  tellurite,  K2Te03,  and  acid 
potassium  tellurite,  KHTe03.  Other  more  complicated  series 
of  salts  exist  such  as : — 


K2Te,05,  K2Te409,  K2Te6O 


13' 


The  tellurites  of  the  alkali  metals  are  soluble  in  water,  and  are 
formed  by  the  solution  of  the  acid  in  an  alkali,  or  by  fusing 
the  dioxide  with  an  alkali.  The  tellurites  of  the  alkaline  earth 
metals  are  only  slightly  soluble,  and  those  of  the  other  metals 
are  insoluble  in  water,  but  soluble  in  hydrochloric  acid. 

Tellurous  acid  is  converted  into  telluric  acid  by  the  action  of 
potassium  permanganate  both  in  acid  and  alkaline  solution. 
The  amount  of  tellurous  acid  in  a  solution  may  be  estimated  in 
this  manner.2 

TELLURIUM  TRIOXIDE.    TeO3. 

252  This  oxide  is  prepared  by  heating  crystallized -telluric  acid 
to  nearly  a  red  heat.  If  it  is  heated  too  strongly,  a  small  quantity 
of  dioxide  is  formed  with  evolution  of  oxygen,  but  this  can  be 
separated  by  treatment  with  hydrochloric  acid,  in  which  it  dis- 

1  Klein,  Ann.  Chim.  Phys.  [6],  10,  108.  2  Brauner  Monatsh.  12,  29. 


444  THE  NON-METALLIC  ELEMENTS 


solves  whilst  the  trioxide  is  insoluble.  Tellurium  trioxide  is  an 
orange  yellow  crystalline  mass,  which,  when  strongly  heated 
decomposes  into  oxygen  and  the  dioxide. 


TELLURIC  ACID.    H2TeO4. 

253  When  tellurium  or  the  dioxide  is  fused  with  carbonate  of 
potash  and  saltpetre,  potassium  tellurate  is  formed  ;  thus  : — 

Te2  +  K2C03  +  2KN03  -  2K2TeO4  +  N2  +  CO. 

The  same  salt  is  produced  when  chlorine  is  passed  through 
an  alkaline  solution  of  a  tellurite  ;  thus  : — 

K,Te03  +  2KOH  +  C12  =  K2TeO4  +  2KC1  -f  H2O. 

On  dissolving  the  fused  mass  in  water,  and  adding  a  solution  of 
barium  chloride,  the  insoluble  barium  tellurate  is  precipitated ; 
this  is  purified  by  washing  with  water,  and  afterwards  decom- 
posed by  the  exact  amount  of  sulphuric  acid  necessary.  The 
clear  acid  solution,  filtered  from  the  sulphate  of  barium,  gives  on 
evaporation  crystals  of  the  hydra  ted  acid  having  the  composition 
H2Te04  +  2H2O.  These  are  sparingly  soluble  in  cold,  but 
readily  soluble  in  hot  water.  When  the  crystals  are  heated  to 
160°  they  lose  their  water  of  crystallization,  and  the  telluric  acid 
remains  as  a  white  powder  nearly  insoluble  in  cold,  but  readily 
dissolving  in  hot  water,  forming  a  solution  which  deposits  the 
crystalline  hydrate. 

The  Tellurates. — Amongst  the  tellurates  only  those  of  the 
alkali-metals  are  more  or  less  readily  soluble  in  water,  those  of  the 
remaining  metals  being  either  sparingly  soluble  or  insoluble  in 
water,  although  generally  dissolving  readily  in  hydrochloric  acid. 

Certain  of  the  tellurates  are  found  to  exist  in  two  modifica- 
tions, viz.  : — 

(a)  As  colourless  salts,  soluble  in  water  or  in  acids. 
(&)    As  yellow  salts,  insoluble  in  water  and  in  acids. 

Besides  these  modifications  we  are  acquainted  not  only  with 
normal  and  acid  tellurates,  but  with  several  other  series  of 
acid  salts  ;  thus  we  have  :— 

(1)  Normal  potassium  tellurate,  K2TeO4  +  5H2O,  obtained 
upon  evaporation  of  a  solution,  either  in  the  form  of  crystalline 
crusts,  or  as  a  gum-like  residue,  both  being  soluble  in  water. 


THE  TELLURATES  445 


(2)  K.2TeO4  +  TeO,  +  4H..O,  a  salt  sparingly  soluble  in  cold, 
but  dissolving  freely  in  hot  water,  and  crystallizing  to  a  woolly 
mass. 

(3)  K2TeO4  +  3TeO3+4H2O  (or  2KHTeO4  +  2H2TeO4  +  H2O), 
a  salt  sparingly  soluble  in  water,  obtained  by  adding  nitric  acid 
to  salt  No.  2. 

The  yellow  modification  of  this  salt  is  obtained  when  salt  No.  2 
is  heated  to  redness.  On  adding  water  to  the  residue,  normal 
potassium  tellurate  dissolves  out,  and  a  tetratellurate  remains 
behind,  being  insoluble  in  water  and  in  dilute  acids. 

Barium  tellurate,  BaTeO4  -f  3H2O,  is  a  white  powder,  not  pre- 
cipitated in  dilute  solutions  as  it  is  not  quite  insoluble  in  water. 
The  di-  and  tetra-tellurate  of  barium,  as  well  as  calcium  and 
strontium  tellurates,  are  similar  white  precipitates,  whilst 
magnesium  tellurate  is  rather  more  soluble.  All  the  other 
tellurates  are  insoluble  in  water. 

When  a  tellurate  is  heated  to  redness,  oxygen  is  evolved  and 
a  tellurite  formed,  and  this  reduction  also  occurs,  with  the 
evolution  of  chlorine,  when  a  tellurate  is  heated  with  hydro- 
chloric acid  ;  thus  : — 

K2Te04  +  2HC1  =  K2TeO3  +  H2O  +  CL, 

254  Tellurium  Oxy chloride,  TeOCl2. — This  substance1  is 
obtained  by  heating  the  compound  of  tellurium  dioxide  with 
hydrochloric  acid,  Te02  +  2HC1,  above  90°, 

Te02,  2HC1  =TeOCl2  +  H2O. 

On  further  heating  it  decomposes  into  the  tetrachloride  and 
the  dioxide. 

2TeOCl2  =  TeCl4  +  Te02. 

Tellurium  Oxylromide,  TeOBr2. — This  compound  is  obtained 
in  a  similar  manner  to  the  oxychloride  and  is  a  faintly  yellow 
coloured  mass.  (Ditte). 


TELLURIUM    AND    SULPHUR. 

TELLURIUM  BISULPHIDE,  TeS2. 

255  A  disulphide,  TeS2,  and  a  trisulphide,  TeS3,  were  pre- 
pared by  Berzelius  by  the  action  of  sulphuretted  hydrogen  upon 
solutions    containing    an    alkali    tellurite    and    telluric     acid 
1  Ditte,  Compt.  Rend.  83,  336,  446. 


446  THE  NON-METALLIC  ELEMENTS 

respectively.  Both  of  these  compounds  yield  up  nearly  all  their 
sulphur  to  carbon  bisulphide  and  are  therefore  probably 
mixtures.1 

Salts  of  the  formula  3K2S,  TeS2  (potassium  thio-tellurite)  and 
K2TeS4  (thio-tellurate)  are  however  known. 


TELLURIUM  SULPHOXIDE.     TeS03. 

256  This  compound  is  obtained  by  the  direct  union  of  tellurium 
with  sulphur  trioxide.2  It  is  a  red  amorphous,  transparent 
solid,  which  melts  at  30°.  On  heating  for  some  time  to  35°,  it 
becomes  of  a  light  reddish  brown  colour.  Water  decomposes  it 
with  formation  of  tellurium  and  sulphuric  acid,  along  with 
other  products.  On  heating  to  230°,  it  loses  sulphur  dioxide 
and  forms  tellurium  monoxide. 


NITROGEN.     N  =  13-94. 

257  Dr.  Rutherford,  Professor  of  Botany  in  the  University  of 
Edinburgh,  showed  in  the  year  1772  that  when  animals  breathe 
in  a  closed  volume  of  air,  it  not  only  becomes  laden  with  impure 
air  from  the  respiration,  but  contains,  in  addition,  a  constituent 
which  is  incapable  of  supporting  combustion  and  respiration. 
He  prepared  this  constituent  by  treating  air  in  which  animals 
had  breathed  with  caustic  potash,  by  means  of  which  the  fixed 
air  (carbonic  acid)  can  be  removed.  The  residual  air  was  found 
to  extinguish  a  burning  candle,  and  did  not  support  the  life  of 
animals  which  were  brought  into  it.3 

In  the  same  year  Priestley  found  that  when  carbon  is  burnt 
in  a  closed  bell-jar  over  water,  one-fifth  of  the  common  air  is 
converted  into  fixed  air  which  can  be  absorbed  by  milk  of  lime; 
a  residual  (phlogisticated)  air  incapable  of  supporting  either 
combustion  or  respiration  being  left.  Priestley,  however,  did  not 
consider  that  this  air  was  a  constituent  of  the  atmosphere,  and 
it  is  to  Scheele  that  we  owe  the  first  statement,  contained  in  his 
treatise  on  Air  and  Fire,  that  the  "  air  must  be  composed  of 
two  different  kinds  of  elastic  fluids."  The  constituent  known 
as  mephitic  or  phlogisticated  air  was  first  considered  to  be  a 

1  Becker,  Annalen,  180,  257. 

2  Weber,  J.  Pr  Chem.  [2],  25,  218  ;    Divers  and  Shimose,  Ber.  16,   1009. 

3  Rutherford,  De  aerc  Mephitico.  Edinb.  1772. 


PREPARATION  OF   NITROGEN  447 

simple  body  by  Lavoisier,  who  gave  to  this  gas  the  name 
azote  (from  a,  privative,  and  farf,  life,  by  which  it  is  still  usually 
designated  in  France).  Chaptal  first  suggested  the  name 
nitrogen,  which  it  now  generally  bears  (from  vlrpov,  saltpetre, 
and  yevvdw  I  give  rise  to),  because  it  is  contained  in  saltpetre. 

Nitrogen  is  found  in  the  free  state  in  the  atmosphere,  of 
which  it  forms  four-fifths  by  bulk,  and  occurs  also  in  combi- 
nation in  many  bodies  such  as  ammonia,  in  the  nitrates,  and  in 
many  organic  substances  which  form  an  essential  part  of  the 
bodies  of  vegetables  and  animals. 

258  Preparation. — The  simplest  method  for  preparing  nitrogen 
is  to  remove  the  oxygen  from  the  air.  This  can  be  done  in  a 
variety  of  ways  : — 

(1)  A  small  light  porcelain  basin  is  allowed  to  swim  on  the 


FIG.  126. 

water  of  a  pneumatic  trough,  a  small  piece  of  phosphorus 
brought  into  the  basin  and  ignited,  and  the  basin  then  covered 
by  a  large  tubulated  bell-jar  (see  Fig.  126).  The  phosphorus 
burns  with  the  deposition  of  a  white  cloud  of  phosphorus 
pentoxide,  which,  however,  soon  dissolves,  whilst  on  cooling, 
one-fifth  of  the  contents  of  the  bell-jar  is  found  to  be  filled 
with  water.  The  colourless  residual  gas  is  nitrogen ;  this  may 
be  easily  proved  by  first  equalising  the  level  of  the  water  inside 
and  outside  the  bell-jar,  after  which,  on  opening  the  stopper 
and  plunging  a  burning  taper  into  the  bell-jar,  the  flame  is 
seen  to  be  instantly  extinguished.  The  nitrogen  thus  obtained 
is  never  perfectly  pure,  as  it  always  contains  small  quantities 
of  oxygen  which  have  not  been  removed  by  the  combustion  of 
the  phosphorus.  In  presence  of  aqueous  vapour,  phosphorus 


448 


THE  NON-METALLIC  ELEMENTS 


slowly  absorbs  the  oxygen  of  the  air,  at  temperatures  above 
15°,  whilst  the  sulphides  of  the  alkali  metals  as  well  as  moist 
sulphide  of  iron  (obtained  by  heating  flowers  of  sulphur  with 
iron  filings)  act  in  a  similar  way. 

(2)  In  order  to  obtain  pure  nitrogen,  air  contained  in  a  gas- 
holder is  allowed  to  pass  through  tubes,  T  and  T',  Fig.  127, 
containing  caustic  potash  and  sulphuric  acid  for  the  purpose  of 
purifying  the  air  from  carbon  dioxide  and  drying  it.  The  air 
thus  purified  is  passed  over  turnings  of  pure  metallic  copper 
contained  in  a  long  glass  tube  (e  f)  which  is  heated  to  redness 
in  a  charcoal  furnace ;  copper  oxide  is  thus  formed,  and  pure 
nitrogen  passes  over  and  is  collected  in  the  pneumatic  trough. 


Fro.  127. 


(3)  Copper  also  rapidly  absorbs  oxygen  from  the  air  at  the 
ordinary  temperature  in  presence  of  a  solution  of  ammonia ;  in 
order  to  obtain  nitrogen  by  this  method  a  slow  current  of  air  is 
passed  through  a  tall  cylinder  containing  copper  turnings  over 
which  a  solution  of  ammonia  is  allowed  to  drop  continuously. 
The   oxygen    is    absorbed    and    the    resulting   nitrogen,  after 
washing  with  water  to  remove  traces  of  ammonia,  is  collected 
in   the  usual   manner.     The   last   traces   of    oxygen   are   best 
removed  by  a  solution  of  chromous  chloride,  CrCl2,  and  this 
method,  when  carried  out  with  other  precautions,  yields  abso- 
lutely pure  nitrogen.1 

(4)  Pure  nitrogen  can  also  be  prepared  by  heating  a  con- 

1  Threlfall,  Phil.  Mag.  [5],  35,  1. 


PREPARATION  OF  NITROGEN 


449 


centrated  solution  of  ammonium  nitrite,  the  following  reaction 
taking  place  : 


It  is  more  convenient  to  employ  a  mixture  of  potassium  nitrite 
and  ammonium  chloride,  which  by  double  decomposition  yield 
potassium  chloride  and  ammonium  nitrite,  the  latter  then 
decomposing  into  nitrogen  and  water.  As  potassium  nitrite 
frequently  contains  free  alkali  it  is  advisable  to  add  some 
potassium  bichromate  in  order  to  neutralise  the  latter  ;  a 
solution  of  one  part  of  potassium  nitrite,  one  part  of  ammonium 


FIG.  128. 

chloride,  and  one  part  of  potassium  bichromate  in  three  parts 
of  water  gives  good  results. 

(5)   By  action  of  chlorine  upon   ammonia,  pure  nitrogen  is 
also  formed ;  thus  : — 

8NH3  +  3C12  =  N2  +  6NH4C1. 

The  chlorine  evolved  in  a  large  flask  passes  into  a  three- 
necked  Woulffe's  bottle  containing  a  strong  aqueous  solution  of 
ammonia.  The  nitrogen  gas  which  is  here  liberated  is  collected 
in  the  ordinary  way  over  water,  as  shown  in  Fig.  128.  Care 
must,  however,  be  taken  in  this  preparation  that  the  ammonia 
30 


450  THE  NON-METALLIC  ELEMENTS 

is  always  present  in  excess,  otherwise  chloride  of  nitrogen  may 
be  formed,  and  this  is  a  highly  dangerous  body,  which  explodes 
most  violently  (p.  477). 

(6)  On  heating  ammonium  bichromate,  nitrogen  gas,  chromium 
sesquioxide  and  water,  are  formed  ;  thus  : — 

(NH4)2Cr207  =  N2  +  0203  +  4H20. 

Nitrogen  may  be  obtained  by  this  reaction  at  a  cheaper  rate 
by  heating  a  mixture  of  potassium  bichromate  and  sal-ammoniac 
instead  of  the  ammonium  bichromate,  which  is  a  somewhat 
expensive  salt ;  thus  :  — 

K2Cr207  +  2NH4C1  =  N2+Cr203  +  2KC1  +  4H2O. 

259  Properties. — Pure  nitrogen  is  a  colourless,  tasteless,  inodor- 
ous gas,  which  is  distinguished  by  its  inactive  properties  ;  hence  it 
is  somewhat  difficult  to  ascertain  its  presence  in  small  quantities. 
As  has  been  said,  it  does  not  support  combustion,  nor  does 
it  burn  nor  render  lime-water  turbid.  It  combines  directly 
with  but  very  few  non-metals,  although  indirectly  it  can  easily 
be  made  to  form  compounds  with  most  of  these  elements, 
and  many  of  its  compounds,  such  as  nitric  acid,  ammonia, 
chloride  of  nitrogen,  &c.,  possess  characteristic  and  remarkable 
properties.  It  combines  directly  with  a  number  of  the  metals, 
such  as  calcium,  barium,  and  magnesium,  yielding  compounds 
termed  nitrides.  The  specific  gravity  of  nitrogen  is  0*97209, 
and  one  litre  at  0°,  and  under  the  normal  pressure,  weighs 
1-25749  grm.  (Rayleigh). 

Nitrogen  was  first  liquefied  by  Cailletet  by  compressing  it 
to  200  atmospheres  and  allowing  it  suddenly  to  expand.  It 
has  since  been  obtained  in  much  larger  quantities  by 
Wroblewski l  and  Olszewski 2  in  the  manner  already  described 
(p.  84).  Under  a  pressure  of  33  atmospheres  at  —146°  (its 
critical  temperature)  it  forms  a  colourless  liquid,  which  boils 
under  atmospheric  pressure  at  —  194°  and  has  a  sp.  gr.  of 
0'885.  If  allowed  to  evaporate  in  a  vacuum  the  temperature 
is  further  lowered  and  at  —  214°  the  nitrogen  solidifies,  forming 
a  crystalline  mass.  If  compressed  nitrogen  be  cooled  by  means 
of  boiling  oxygen  and  the  pressure  then  somewhat  diminished, 

1  Compt.  Rend.  97,  1553  ;  98,  982  ;  102,  1010  ;  Monatsh.  6,  204. 

2  Compt.  Rend.  99,  133  ;  100,  350 ;  Ann.  Chim.  Phys.  31,  58. 


PROPERTIES  OF  NITROGEN  451 

the   nitrogen   separates   out    in    crystalline   flakes    resembling 
snow. 

Nitrogen  is  but  slightly  soluble  in  water,  one  volume  of  water 
absorbing  only  0 '02348  of  the  gas  at  4°,  and  a  smaller  quantity 
at  higher  temperatures.1  The  co-efficient  of  the  solubility  of 
nitrogen  (c)  from  0-20°  may  be  found  by  the  following  inter- 
polation formula : — 

c  =  0-023481  - 

The  gas  is  rather  more  soluble  in  alcohol  than  in  water. 

There  are  two  characteristic  spectra  of  nitrogen  both  obtained 
bypassing  the  spark  from  an  induction  coil  through  a  Geissler's 
tube  containing  a  small  quantity  of  highly  rarefied  nitrogen  gas. 
The  nitrogen  spectrum  commonly  obtained  in  this  way  is  a 
channelled  one,  exhibiting  a  large  number  of  bright  bands 
especially  numerous  in  the  violet.  If  the  spark  is  produced 
by  high  tension,  as  when  a  Leyden  jar  is  used,  a  spectrum  of 
numerous  fine  lines  distributed  throughout  the  length  of  the 
spectrum  will  be  obtained  (Plucker). 

Under  ordinary  conditions  nitrogen  is  incombustible,  but  a 
mixture  of  nitrogen  and  oxygen  can  be  made  to  ignite  under 
certain  circumstances.  Thus  if  a  powerful  current  of  electricity 
be  passed  through  the  primary  of  a  large  induction  coil,  an 
arching  flame  is  seen  to  issue  from  each  secondary  pole, 
provided  these  are  not  too  far  apart,  the  two  flames  joining 
in  the  centre  ;  the  flame  is  due  to  the  combination  of  the 
nitrogen  and  oxygen  of  the  air  to  form  oxides  of  nitrogen. 
When  the  flame  has  once  been  formed  the  distance  between 
the  secondary  poles  may  be  considerably  increased  without 
extinguishing  the  flame,  and  the  latter  may  then  be  blown  out 
and  reignited  by  a  taper.  The  flame  does  not  extend  to  the 
surrounding  air  because  heat  is  absorbed  in  the  combination, 
and  this  can  only  therefore  occur  when  energy  is  supplied  to 
the  mixture  from  an  external  source.2 

Assimilation  of  Free  Nitrogen  ~by  Plants. — As  already  men- 
tioned nitrogen  forms  an  essential  constituent  of  a  very  large 
number  of  animal  and  vegetable  substances  and  is  necessary 
for  the  maintenance  of  animal  and  vegetable  life.  It  was  how- 
ever for  a  long  time  believed  that  members  of  the  vegetable 

1  Winkler,  Ber.  24,  3605. 

2  Spottiswoode,  Proc.  Roy.  Soc.  31,  173  ;  Crookes,  Chcm.  News,  1892,  i.  302. 


452  THE  NON-METALLIC  ELEMENTS 

kingdom  were  unable  to  take  up  the  free  nitrogen  of  the  air, 
and  that  they  were  dependent  for  their  supply  of  this  element 
on  combined  nitrogen  contained  in  the  atmosphere  and  in  the 
soil  chiefly  in  the  form  of  nitric  acid  and  ammonia.  About  the 
year  1886  it  was  shown  that  certain  leguminous  plants,  such 
as  the  white  lupine,  when  grown  in  air  free  from  ammonia  and 
other  nitrogen  compounds,  contain  more  nitrogen  than  was 
originally  present  in  the  seed  and  in  the  soil  in  which  they  were 
sown,  and  they  must,  therefore,  have  obtained  the  excess  from 
the  nitrogen  of  the  air ;  since  then  it  has  been  shown  certain 
algae,  fungi  and  mosses  behave  in  a  similar  manner,  and  it  is 
not  improbable  that  further  investigation  will  show  that  this 
property  is  possessed  by  many  other  varieties.  The  assimilation 
is  brought  about  by  the  action  of  certain  micro-organisms, 
which  absorb  the  free  nitrogen  from  the  air,  forming  compounds 
of  nitrogen  which  are  then  assimilated  by  the  plant.1  Other 
classes  of  micro-organisms  also  play  a  considerable  part  in 
the  assimilation  of  nitrogen  by  plants,  as  the  latter  are 
incapable  of  directly  assimilating  ammonia,  although  they  can 
take  up  nitric  acid ;  the  micro-organisms  in  the  soil  bring 
about  the  conversion  of  ammonia  into  nitric  acid,  one  variety 
of  these  converting  the  former  into  nitrous  acid,  and  another 
converting  the  nitrous  into  nitric  acid.2 


NITROGEN  AND  HYDROGEN. 

AMMONIA.     NH3  =  16-94. 

260  Ammonia  is  found  in  the  atmosphere  in  combination  with 
carbonic  acid  forming  a  small  but  essential  constituent  of  the  air. 
It  likewise  occurs  in  combination  with  nitric  and  nitrous  acids 
in  rain-water,  and,  especially  as  sal-ammoniac,  NH4C1,  and  as 
sulphate  of  ammonia,  (NH4).,SO4,  deposited  on  the  sides,  the 
craters,  and  in  the  crevices  of  the  lava  streams  of  active  volcanoes, 
as  well  as  mixed  with  boric  acid  in  the  fumaroles  of  Tuscany. 
Many  samples  of  rock-salt  also  contain  traces  of  ammoniacal 

1  For  further  details  see  Lawes  and  Gilbert,  Phil.   Trans.  180  (B)  1  ;  Journ. 
Roy.  Agnc.  Soc.  1891,  657  ;  Proc.  Roy.   Soc.  46,  85  ;  Schloesing,  Compt.  Rend. 
113,  776  ;  Marshall  Ward,  Nature,  49,  511. 

2  Warington,  Journ.  Chem.  Soc.  1891,  i.  484. 


AMMONIA  453 


compounds,  and  all  fertile  soil  contains  this  substance,  which 
is  likewise  found,  although  in  small  quantities,  widely  distributed 
in  rain  and  in  running  water,  as  well  as  in  sea- water,  in  clays, 
marls,  and  ochres.  Ammoniacal  salts  are  also  found  in  the 
juices  of  plants  and  in  most  animal  fluids,  especially  in  the 
urine. 

Ammonia  was  known  to  the  early  alchemists  in  the  form  of 
the  carbonate  under  the  name  of  spiritus  salis  urince.  In  the 
fifteenth  century  it  was  known  that  the  same  body  may 
be  obtained  by  the  action  of  an  alkali  upon  sal-ammoniac ; 
and  Glauber,  in  consequence,  termed  this  body  spiritus  volatilis 
salis  armoniaci.  Sal-ammoniac,  which  was  known  to  Geber, 
appears  to  have  been  brought  in  the  seventh  century  from 
Asia  to  Europe,  and  was  known  under  the  name  of  sal-armon- 
iacum.  It  is  possible  that  this  sal-ammoniac  was  derived 
from  the  volcanoes  of  Central  Asia.  Geber,  however,  describes 
the  artificial  production  of  the  salt  by  heating  urine  and  common 
salt  together.  In  later  times,  sal-ammoniac  was  brought  into 
Europe  from  Egypt,  where  it  was  prepared  from  the  soot 
obtained  by  burning  camel's  dung.  Its  original  name  was 
altered  to  sal-armoniacum,  and  then  again  changed  to  sal- 
ammoniacum.  This  last  name  served  originally  among  the 
Alexandrian  alchemists  to  describe  the  common  salt  (chloride 
of  sodium)  and  native  sodium  carbonate,  which  was  found  in 
the  Libyan  desert  in  the  neighbourhood  of  the  ruins  of  the  temple 
of  Jupiter  Ammon.  Boyle  says  in  his  "  Memoirs  for  the  Natural 
History  of  Human  Blood  : "  l — "  Though  the  sal-armoniac  that  is 
made  in  the  East  may  consist  in  great  part  of  camel's  urine,  yet 
that  which  is  made  in  Europe,  and  commonly  sold  in  our  shops, 
is  made  of  man's  urine."  Later  on,  sal-ammoniac  was  obtained 
by  the  dry  distillation  of  animal  refuse,  such  as  hoofs,  bones,  and 
horns  ;  the  carbonate  of  ammonia  thus  obtained  being  neutralized 
with  hydrochloric  acid.  From  this  mode  of  preparation  ammonia 
was  formerly  termed  spirits  of  hartshorn. 

Up  to  the  time  of  Priestley,  ammonia  was  known  only  in 
the  state  of  aqueous  solution,  termed  spirits  of  hartshorn, 
or  spiritus  volatilis  salis  ammoniaci.  Stephen  Hales,  in  1727 
observed  that  when  sal-ammoniac  is  heated  with  lime  in  a 
vessel  closed  by  water,  no  air  is  given  out,  but,  on  the  contrary, 
water  is  drawn  into  the  apparatus;  Priestley,  in  1774,  repeated 
this  experiment,  with  the  difference,  however,  that  he  used 
1  Boyle,  op.  4,  597,  1684. 


454  THE  NON-METALLIC  ELEMENTS 

mercury  to  close  his  apparatus.  He  thus  discovered  ammonia 
gas,  to  which  he  gave  the  name  of  alkaline  air.  He  also  found 
that,  when  electric  sparks  are  allowed  to  pass  through  this 
alkaline  air,  its  volume  undergoes  a  remarkable  change,  and 
the  residual  air  is  found  to  be  combustible.  Berthollet,  following 
up  this  discovery  in  1785,  showed  that  the  increase  of  volume 
which  ammonia  gas  thus  undergoes  is  due  to  the  fact  that 
it  is  decomposed  by  the  electric  spark  into  hydrogen  and 
nitrogen.  This  discovery  was  confirmed,  and  the  composi- 
tion of  the  gas  more  accurately  determined  by  Austin  (1788), 
H.  Davy  (1800),  and  Henry  (1809).  It  was  shown  by  them 
that,  in  the  reaction  above  described,  two  volumes  of  ammonia 
are  resolved  into  three  volumes  of  hydrogen  and  one  of 
nitrogen. 

It  has  been  proved  by  Donkin  that  ammonia  can  be  syntheti- 
cally prepared  by  the  direct  combination  of  its  elements,1  the 
silent  electric  discharge  being,  for  this  purpose,  passed  through 
a  mixture  of  nitrogen  and  hydrogen.  If  a  mixture  of  3  vols. 
of  hydrogen  and  1  voJ.  of  nitrogen  is  subjected  to  the  action  of 
the  electric  discharge  in  a  eudiometer,  containing  above  the 
mercury  a  little  dilute  sulphuric  acid,  the  whole  volume  finally 
disappears,  the  two  gases  slowly  combining  to  form  ammonia, 
which  dissolves  in  the  acid  as  fast  as  it  is  formed.  It  is  like- 
wise obtained  together  with  nitrous  acid  by  passing  nitrogen 
over  a  mixture  of  platinum  black  and  alkali.2 

Ammonia  is  also  formed  : — 

(1)  By  the  putrefaction   or  decay  of  the    nitrogenous  con- 
stituents of  plants  and  animals. 

(2)  By  the   dry  distillation  of  the   same   bodies ;  that   is,  by 
heating  these  substances  strongly  out  of  contact  with  air. 

(3)  By  the  action  of  nascent  hydrogen  on  the  salts  of  nitric 
or  nitrous  acid. 

It  is  to  the  first  of  these  processes  that  we  owe  the  existence 
of  ammonia  in  the  atmosphere,  whilst  the  second  serves  for  the 
production  of  ammonia  and  its  compounds,  especially  of  sal- 
ammoniac,  on  the  large  scale. 

At  the  present  day  almost  all  the  sal-ammoniac  and  other 
ammonium  salts  are  prepared  from  the  ammoniacal  liquor  which 
is  obtained  as  a  by-product  in  the  manufacture  of  coal-gas. 
Coal  consists  of  the  remains  of  an  ancient  vegetable  world, 
and  contains  about  2  per  cent,  of  nitrogen,  some  of  which,  in 

1  W.  F.  Donkin,  Proc.  Roy.  Soc.  21,  281.  2  Loew,  Ber.  23,  1443. 


PREPARATION  OF  AMMONIA  455 

the  process  of  the  dry  distillation  of  the  coal  carried  on  in  the 
manufacture  of  coal-gas,  is  obtained  in  the  form  of  ammonia 
dissolved  in  the  water  and  other  products  formed  at  the  same 
time. 

In  order  to  prepare  sal-ammoniac  from  this  liquor,  which 
contains  very  little  if  any  free  ammonia,  but  chiefly  the  sulphide, 
carbonate,  sulphite  and  thiosulphate  of  ammonia,  it  is  boiled 
with  milk  of  lime  to  liberate  the  whole  of  the  ammonia.  This 
ammonia  distils  over,  and  the  distillate  is  neutralized  with 
hydrochloric  acid,  sal-ammoniac  being  formed  as  follows  : — 

NH3  +  HC1  =  NH4C1. 

The  solution  is  then  evaporated  to  dryness  and  the  salt  purified 
by  sublimation. 

In  most  cases,  however,  the  ammonia  is  passed  into  sulphuric 
acid,  thus  forming  ammonium  sulphate,  (NH4)2S04,  which  is 
much  more  readily  purified,  and  from  which  most  of  the  other 
ammoniacal  compounds  are  prepared. 

Preparation. — If  any  one  of  these  ammoniacal  salts  be 
heated  with  an  alkali,  such  as  potash  or  soda,  or  with  an  alkaline 
earth,  such  as  lime,  the  ammonia  is  set  free  as  gas.  In  order  to 
prepare  the  gas  it  is  only  necessary,  therefore,  to  heat  together 
sal-ammoniac  and  slaked  lime ;  thus  : — 

2NH4C1  +  Ca(OH)2  =  2NH3  +  CaCl2  +  2H20. 

In  order  to  ensure  the  decomposition  of  all  the  sal-ammoniac,  a 
large  excess  of  lime  is  usually  employed.  One  part  by  weight 
of  powdered  sal-ammoniac  is  for  this  purpose  mixed  with  two 
parts  of  caustic  lime  slaked  to  a  fine  dry  powder ;  these  are  well 
mixed  together,  and  then  introduced  into  a  capacious  flask,  placed 
on  a  piece  of  wire  gauze  and  heated  by  a  Bunsen-lamp.  The 
ammonia  gas,  which  comes  off  when  the  mixture  is  gently  heated, 
is  then  dried  by  allowing  it  to  pass  through  a  cylinder  filled  with 
small  lumps  of  quick-lime,  or  by  placing  a  layer  of  the  latter 
over  the  mixture  of  ammonium  chloride  and  slaked  lime  in  the 
flask,  and  the  gas  thus  dried  may  be  collected  either  over 
mercury,  or,  like  hydrogen,  by  upward  displacement  in  an 
inverted  dry  cylinder  as  shown  in  Fig.  129,  inasmuch  as  this 

8'47 
gas   is   fTTocn  =  0'586  times  as  light  as  air,  one  litre  weighing 

at  0°  and  under  760  mm.  pressure,  O76193  grams. 


456 


THE  NON-METALLIC  ELEMENTS 


261  Properties. — Pure  ammonia  is  a  colourless  gas,  possessing, 
like  its  aqueous  solution,  a  peculiar  pungent  alkaline  odour  and 
caustic  taste.  In  the  solid  state,  however,  it  possesses  but  a 
very  faint  smell.  Ammonia  gas  turns  red  litmus-paper  blue, 
like  the  alkalis,  neutralizes  acids,  and  forms  with  them  a  series 
of  stable  compounds,  termed  the  ammonium  salts.  It  is  a 
very  stable  compound,  but  at  1300°  gradually  decomposes  into 
its  constituents.1 

Ammonia  is  not  combustible,  and  a  flame  is  extinguished  if 


FIG.    129. 

plunged  into  the  gas.  If,  however,  ammonia  be  mixed  with 
oxygen,  the  escaping  gas  maybe  ignited,  and  burns  with  a  pale 
yellow  flame,  with  formation  of  water,  nitrogen  gas,  and  nitric 
acid,  HNO3.  Another  method  of  showing  the  combustibility 
of  ammonia  is  to  put  a  jet  of  this  gas  into  the  air  holes  of  an 
ordinary  Bunsen -burner,  in  which  a  flame  of  coal-gas  is  already 
burning  ;  the  flame  becomes  at  once  coloured  yellow,  and  in- 
creases greatly  in  dimensions.  A  third  experiment  of  this  nature 
is,  to  allow  a  stream  of  oxygen  gas  to  bubble  through  a  small 
quantity  of  strong  aqueous  ammonia  placed  in  a  flask  and 
warmed  as  shown  in  Fig.  130 ;  on  bringing  a  light  in  contact 
1  Crafts,  Compt.  Rend.  90,  309. 


PROPERTIES  OF  AMMONIA 


457 


with  the  mixed  gases  issuing  from  the  neck   of  the  flask  they 
will  be  seen  to  burn  with  a  large  yellow  flame. 

Ammonia  gas  was  first  liquefied  by  Faraday,  in  1823, 
by  heating  a  compound  of  silver  chloride  with  ammonia, 
placed  in  one  limb  of  a  strong  hermetically  sealed  bent  tube 
whilst  the  other  limb  was  placed  in  a  freezing  mixture.  The 
compound  of  silver  chloride  and  ammonia  is  obtained  by 
saturating  dry  precipitated  silver  chloride  with  ammonia  gas:  it 
has  the  formula  AgCl(NH3)2  and  fuses  at  38°,  whilst  at  about 


FIG.  130. 

115°  it  begins  to  part  with  its  ammonia.  The  gas  thus  collects 
in  the  tube  until  the  pressure  is  reached  under  which  it  begins  to 
condense  as  a  clear,  highly  refracting  liquid.  When  the  silver 
chloride  cools,  the  ammonia  is  again  absorbed,  the  original  com- 
pound being  re-formed.  Liquid  ammonia  is  also  easily  obtained 
by  leading  the  gas  into  a  tube  plunged  in  a  freezing  mixture 
composed  of  crystallized  calcium  chloride  and  ice,  and  having  a 
temperature  of  —  40J  and  forms  a  colourless  highly  refracting 
liquid,  boiling  at  —  33°'7  (Bunsen).  When  the  temperature  of 
the  liquid  is  lowered  to  below  -  75°  in  a  bath  of  solid  carbonic 


458 


THE  NON-METALLIC  ELEMENTS 


acid   and  ether  placed  in  vacuo,  a  mass   of  white    translucent 
crystals  of  solid  ammonia  is  obtained  (Faraday). 

The  co-efficient  of  expansion  of  liquid  ammonia  is  0*00204,  and, 
therefore,  larger  than  that  of  most  liquids  having  a  higher  boiling 
point ;  its  specific  gravity  compared  with  water  at  0J  is  Q'6234.1 
The  tension  of  liquid  ammonia  at  0°  is  4*4  atmospheres,  at 
15°'5,  6'9  atmospheres,  and  at  28°,  10  atmospheres. 


FIG.  131. 

The  condensation  of  ammonia  by  pressure  and  the  production 
of  cold  by  its  evaporation  can  easily  be  shown  by  the  following 
experiment.  The  apparatus  required  for  this  purpose  consists 
essentially  of  two  strong  glass  tubes  (a  and  &,  Fig.  131),  which 
are  closed  below  and  are  connected  together  by  the  tubes  (c  c) 
and  (d  d).  The  tube  (d  d)  ends  at  (I)  in  a  narrower  tube 
(mm),  which  is  at  this  point  melted  into  the  tube  (a).  The  tube 

1  Joly,  Annalen,  117,  181. 


PEOPERTIES  OF  AMMONIA 


459 


(a)  is  three-fourths  filled  with  an  alcoholic  solution  of  ammonia 
saturated  at  8°,  and  then  placed  in  the  cylinder  (A).  The 
syphon  tube  (g)  and  the  tube  (//),  which  reach  to  the 
bottom  of  the  cylinder,  are  fixed  in  position  through  the  cork. 
In  order  now  to  perform  the  experiment,  the  cylinder  (A)  is 
nearly  filled  with  warm  water ;  the  glass  stopcock  (h)  is 
opened,  and  the  tube  (6)  placed  in  ice-cold  water.  The  water 
contained  in  the  flask  is  now  quickly  boiled,  and  thus  the 
water  in  (A)  is  rapidly  heated  to  100°,  and  the  ammonia  gas 
driven  out  of  solution  until  by  its  own  pressure  it  liquefies  in  (6). 
As  soon  as  the  condensation  of  liquid  ammonia  ceases,  the 
ebullition  is  stopped  and  a  portion  of  the  hot  water  is  with- 
drawn from  the  cylinder  by  means  of  the  syphon  (g),  cold  water  is 
allowed  to  enter  the  cylinder,  and  after  a  while  this  is  replaced 


FIG.  132. 

by  ice-cold  water.  The  cylinder  (B)  is  now  removed,  when  the 
liquefied  ammonia  begins  to  evaporate  and  is  again  absorbed  by 
the  alcohol,  though  only  slowly.  But,  on  closing  the  stopcock  (h\ 
the  gas  above  the  alcohol  is  quickly  absorbed,  and  thus  the 
equilibrium  is  disturbed.  The  ammonia  now  passes  rapidly 
through  the  tube  (m  m),  and  is  absorbed  so  quickly  that  the 
liquid  ammonia  in  (b)  begins  to  boil,  by  which  the  temperature 
is  so  much  lowered,  that  if  a  test-tube  containing  water  is  placed 
outside  (b)  it  is  soon  filled  with  ice. 

Ammonia  may  be  used  for  the  artificial  production  of  ice. 
For  this  purpose  an  apparatus  (Fig.  132)  has  been  invented  by 
M.  Carre.  It  consists  of  two  strong  iron  vessels  connected  by  a 
vent-pipe  of  the  same  metal.  The  cylinder  (A)  contains  water 
saturated  with  ammonia  gas  at  0°.  When  it  is  desired  to  procure 


460 


THE  NON-METALLIC  ELEMENTS 


ice,  the  vessel  (A)  containing  the  ammonia  solution,  which  we  may 
term  the  retort,  is  gradually  heated  over  a  large  gas-burner.  The 
ammonia  gas  is  thus  driven  out  of  solution,  and  as  soon  as  the 
pressure  in  the  interior  of  the  vessel  exceeds  that  of  seven 
atmospheres,  it  condenses  in  the  double- walled  receiver  (B). 
When  the  greater  portion  of  the  gas  has  thus  been  driven  out  of 
the  water,  the  apparatus  is  reversed,  the  retort  (A)  being  cooled 
in  a  stream  of  cold  water,  whilst  the  liquid  which  it  is  desired 
to  freeze  is  placed  in  a  cylinder  which  fits  into  the  interior 
portion  of  the  hollow  cylinder.  A  reabsorption  of  the 
ammonia  by  the  water  now  takes  place,  and  a  consequent 
evaporation  of  the  liquefied  ammonia  in  the  receiver.  This 
evaporation  is  accompanied  by  the  absorption  of  heat  which 
becomes  latent  in  the  gas.  Thus  the  receiver  is  soon  cooled 
down  far  below  the  freezing  point,  and  the  liquid  contained 
in  the  vessel  (D)  is  frozen.  For  the  production  of  larger 
quantities  of  ice,  a  continuous  ammonia  freezing  machine  of  more 
complicated  construction,  but  arranged  on  the  same  principle, 
has  been  devised  by  M.  Carre,  in  which  10  kilos,  of  ice  can 
be  prepared  by  the  combustion  of  1  kilo,  of  coal.1 

262  Ammonia  gas  is  very  soluble  in  water  ;  one  gram,  of  water 
absorbs  at  0°  and  under  normal  pressure,  0*875  grms.  or  1148  cc. 
of  the  gas.  The  solubility  at  different  temperatures  and  under 
a  pressure  of  760  mm.  of  mercury  is  given  in  the  following 
table  (Roscoe  and  Dittmar) : — 


Temp. 

Gram. 

Temp. 

Gram. 

Temp. 

Gram. 

Temp. 

Gram. 

0° 

0-875 

16° 

0-582 

32° 

0-382 

i  48° 

0-244 

2° 

0-833 

18° 

0-554 

34° 

0-362 

50° 

0-229 

4° 

0-792 

20° 

0-526 

36° 

0-343 

52° 

0-214 

6° 

0-751 

22° 

0-499 

38° 

0-324 

54° 

0-200 

8° 

0-713 

24° 

0-474 

40° 

0-307 

56°    i  0-186 

10° 

0-679 

26° 

0-449 

42° 

0-290 

12° 

0-645 

28° 

0-426 

44° 

0-275 

14° 

0-612 

30° 

0-403 

46° 

0-259 

The  same  observers  have  found  that  the  absorption  of  ammonia 

1  Ice-making  machines  depending  on  the  same  principle  are  now  in  use  in 
which  liquefied  sulphur  dioxide,  or  ether,  are  employed  instead  of  aqueous 
ammonia.  2  Journ.  Chem.  Soc.  1860,  128. 


SOLUBILITY  OF  AMMONIA  461 

in  water  does  not  follow  the  law  of  Dalton  and  Henry  at  the 
ordinary  atmospheric  temperature,  inasmuch  as  the  quantity 
absorbed  does  not  vary  directly  as  the  pressure.  Sims  l  has 
shown  that  at  higher  temperatures  the  deviations  from  the  law 
become  less  until  at  100°  the  gas  follows  the  law,  the  quantity 


FIG.  133. 

absorbed  being  directly  proportional  to  the  pressure,  taken  of 
course  under  pressures  higher  than  the  ordinary   atmospheric 
pressure,  inasmuch  as  boiling  water  does  not  dissolve  any  of  the 
gas  under  the  pressure  of  one  atmosphere. 
1  Journ.  Chem.  Soc.  1862,  17. 


462 


THE  NON-METALLIC  ELEMENTS 


In  order  to  show  the  great  solubility  of  ammonia  gas  in  water 
the  same  apparatus  may  be  employed  which  was  used  for 
exhibiting  the  solubility  of  hydrochloric  acid  in  water  (Fig. 
133),  the  lower  balloon  being  filled  with  water  slightly  coloured 
with  red  litmus  solution. 

The  liquor  ammoniae  of  the  shops,  a  solution  of  the  gas  in 
water, is  prepared  (as  shown  in  Fig.  134)  by  passing  the  gas,  which 


FIG  134. 

has  been  previously  washed,  into  a  flask  containing  water  kept 
cool  by  being  placed  in  a  large  vessel  of  cold  water,  considerable 
heat  being  evolved  in  the  condensation  of  the  gas. 

The  commercial  liquor  ammonise  is  now  frequently  prepared 
directly  from  the  ammoniacal  liquor  of  the  gas-works  instead  of 
from  sal-ammoniac.  For  this  purpose  the  liquor  is  heated 
together  with  milk  of  lime  in  an  iron  boiler  having  a  capacity 


COMPOSITION  OF  AMMONIA  463 

of  1,000  gallons.  The  gas  which  is  evolved  is  first  passed 
through  a  long  system  of  cooling  tubes  before  it  enters  the 
washing  and  condensing  apparatus.  The  condenser  consists  of 
a,  series  of  tubes  filled  with  charcoal,  by  means  of  which  any 
remaining  empyreumatic  impurities  are  removed.  By  this 
method,  if  the  system  of  tubes  be  sufficiently  long,  and  by  the 
use  of  a  sufficient  number  of  wash-bottles,  a  perfectly  pure 
liquor  ammonise  can  be  obtained. 

The  fact  that  great  heat  is  evolved  in  the  production  of  the 
saturated  solution  of  ammonia  is  rendered  evident  by  the 
following  experiment.  -  If  a  rapid  current  of  air  be  passed 
through  a  cold  concentrated  solution  of  ammonia,  the  gas  will 
be  driven  out  of  solution,  and  an  amount  of  heat  will  be  absorbed 
exactly  equal  to  that  which  was  given  off  when  the  solution  of 
the  gas  was  made,  in  consequence  of  which  the  temperature 
of  the  liquid  will  be  seen  to  fall  below  -  40°.  A  small  quantity 
of  mercury  may  thus  be  frozen.  Ammonia  is  very  soluble 
in  alcohol,  and  this  solution,  which  is  frequently  used  in  the 
laboratory,  is  most  conveniently  prepared  by  gently  warming  a 
concentrated  aqueous  solution  of  ammonia  and  passing  the  gas 
thu$  evolved  into  alcohol.--  This  method  may  also  be  employed 
for  preparing  the  gas  in  place  of  heating  sal-ammoniac  with 
lime,  or  the  concentrated  solution  may  be  allowed  to  drop  from 
a  tap  funnel  on  to  lumps  of  caustic  soda  placed  in  a  cylindrical 
glass  vessel,  by  which  means  a  regular  and  continuous  evolution 
of  the  gas  is  obtained. 

.  The  following  table  gives  the   percentage  of  ammonia  con- 
tained in  aqueous  solutions  of  different  specific  gravity  (Carius) 

Specific  Gravity.  Per  Cent.  NH3. 

0-8844    '  36-0 

0-8864  35-0 

0-8976  30-0 

0-9106  25-0 

0-9251  20-0 

0-9414  15-0 

0-9593  10-0 

0-9790  5-0 

263  Composition  of  Ammonia. — In  order  to  determine  the 
composition  of  ammonia  the  arrangement  shown  in  Fig.  135  is 
employed.  The  ammonia  gas  is  placed  in  the  closed  limb  of 


464  THE  NON-METALLIC  ELEMENTS 

the  syphon  eudiometer,  after  which  the  mercurial  column  in  both 
limbs  is  brought  to  the  same  height,  the  volume  accurately 
read  off,  and  a  series  of  electric  sparks  from  the  induction  coil 
allowed  to  pass  through  the  gas  until  its  volume  undergoes  no 
further  alteration.  The  tube  and  gas  are  next  allowed  to  cool, 
and  the  pressure  in  both  limbs  again  adjusted.  On  the  volume 
being  again  measured  it  is  seen  to  have  loubled.  Oxygen  is 
next  added  in  such  proportion  that  the  mixture  shall  contain 
no  more  than  35  per  cent,  of  the  explosive  mixture  of  oxygen 
and  hydrogen  (2  volumes  of  hydrogen  to  1  of  oxygen),  and 
an  electric  spark  is  passed  through  the  mixture.  From  the 
alteration  of  volume  which  takes  place,  the  proportion  of 
hydrogen  to  nitrogen  can  readily  be  deduced,  as  is  seen  from 
the  following  example  : — 


Volume  of  ammonia 20*0 

„         nitrogen  and  hydrogen    ...  40 '0 

After  the  addition  of  oxygen 157*5 

After  the  explosion 112*5 

Hence  45  volumes  have  disappeared,  of  which  30  consisted  of 
hydrogen  ;  consequently  two  volumes  of  ammonia  contain  three 
volumes  of  hydrogen  and  one  volume  of  nitrogen. 

That  the  relation  between  hydrogen  and  nitrogen  in  ammonia 
gas  is  in  the  proportion  of  three  volumes  of  the  former  to  one 
of  the  latter  can  be  shown  by  the  following  experiment  thus 
clearly  described  by  Prof.  Hofmann : — l 

A  glass  tube  for  holding  chlorine,  having  a  small  stoppered 
portion  separated  from  the  rest  of  the  tube  by  a  glass  stopcock 
and  used  for  receiving  solution  of  ammonia,  and  admitting  it, 
drop  by  drop,  to  the  chlorine,  constitute  the  requisite  apparatus 
(Fig.  136).  The  glass  tube  is  from  1  to  1*5  metre  long,  sealed 

1  Introduction  to  Modern  Chemistry. 


COMPOSITION  OF  AMMONIA 


465 


at  one  end,  open  at  the  other,  and  marked  off,  by  elastic 
caoutchouc  rings  slipped  over  it  and  clipping  it  firmly,  into 
three  equal  portions. 

The  apparatus  is  thus  employed.  The  long  chlorine-tube  having 
been  filled  with  lukewarm  water  and  inverted  over  a  pneumatic 
trough,  with  its  mouth  immersed  below  the  water-level, 
is   filled    with    chlorine    in    the    usual    way    (see    Fig. 
137).    When  full,  it  is  still  allowed  to  stand  for  about 


FIG.  136. 

fifteen  minutes  over  the  chlorine  delivery-tube,  so  that  its 
interior  surface  may  be  quite  freed  from  the  chlorine- 
saturated  water  that  would  else  remain  adherent  to  it.  The 
stopcock  is  now  closed  and  the  chlorine  thus  shut  in  the 
tube.  This  is  then  removed  from  the  trough,  and  turned  round 
so  that  the  stoppered  end  is  uppermost.  The  small  space 
between  the  stopcock  and  the  end  is  next  two-thirds  filled  with 
a  strong  solution  of  ammonia,  and  a  single  drop  of  the  ammonia- 
solution  is  suffered  to  fall  into  the  chlorine  tube,  the  stopcock 
being  opened  for  a  moment  for  this  purpose  (Fig.  138).  The 
entrance  of  this  drop  into  the  atmosphere  of  chlorine  is  marked 
by  a  small,  lambent,  yellowish-green  flame  at  the  point  where 
the  drop  enters  the  gas.  Drop  by  drop,  at  intervals  of  a  few 
seconds,  the  ammonia  solution  is  allowed  to  fall  into  the 
chlorine-tube,  the  ammonia  of  each  drop  being  converted,  at  the 
instant  of  its  contact  with  the  chlorine,  into  hydrochloric  acid 

31 


466 


THE  NON-METALLIC  ELEMENTS 


and  nitrogen  with  a  flash  of  light  and  the  formation  of  a  dense 
white  cloud.  The  addition  of  ammonia  must  be  continued  till 
the  whole  of  the  chlorine  present  is  supplied  with  hydrogen 
at  the  expense  of  ammonia.  To  insure  the  ammoniacal  solu- 
tion being  added  in  excess,  a  column  of  three  or  four  centimetres 
is  abundantly  sufficient.  The  result  is  that  the  hydrochloric 
acid  formed  combines  with  the  excess  of  ammonia  to  form  a 
compound,  which  makes  its  appearance  as  a  white  deposit  lining 
the  interior  of  the  chlorine  tube.  This  deposit,  being  soluble, 
is  readily  washed  down  and  dissolved  by  agitating  the  liquor  in 


FIG.  137. 

the  tube,  which  now  contains  the  whole  of  the  nitrogen 
separated. 

We  are  now  sure  of  two  points,  viz.,  that  the  whole  of  the 
chlorine  has  been  converted  into  hydrochloric  acid  at  the 
expense  of  the  ammonia ;  and,  that  we  possess  within  our  tube 
the  whole  of  the  nitrogen  thus  set  free. 

It  becomes  our  next  object  to  withdraw  the  excess  of  the 
ammonia.  For  this  purpose  dilute  sulphuric  acid,  which  fixes 
ammonia,  is  introduced  by  means  of  the  portion  of  the  tube 
previously  employed  to  admit  ammonia. 


COMPOSITION  OF  AMMONIA  467 

The  nitrogen,  being  thus  freed  from  all  intermixed  gaseous 
bodies,  has  only  now  to  be  brought  to  mean  atmospheric 
temperature  and  pressure  in  order  that  it  may  be  ready  for 
measurement. 

To  equalize  the  pressure  within  and  without  the  tube,  the 
bent  syphon  tube  (Fig.  139)  is  employed.  One  end  of  this 
communicates  with  the  interior  of  the  tube,  while  the  other 
plunges  beneath  the  surface  of  water  subject  to  atmospheric 
pressure.  That  this  pressure  exceeds  that  of  the  gas  in  the  tube 
is  at  once  seen  by  the  flow  of  water  through  the  syphon  into 


FIG.  138. 

the  tube.  As  the  water-level  in  the  tube  rises,  the  nitrogen, 
previously  expanded,  gradually  approaches  its  normal  volume, 
which  it  exactly  attains  when  the  flow  ceases,  showing  the 
pressure  within  and  without  to  be  in  equilibrium.  Both  tem- 
perature and  pressure  being  now  at  the  mean,  all  the  requisite 
conditions  are  fulfilled  for  obtaining  an  exact  knowledge  of  the 
true  volume  of  nitrogen;  and  this,  on  inspection,  is  found  to 
exactly  fill  one  of  the  three  divisions  marked  off  at  the  outset  on 
our  tube.  Now,  bearing  in  mind  that  we  started  with  the  three 
divisions  full  of  chlorine,  and  that  we  have  saturated  this 
chlorine  with  hydrogen  supplied  by  the  ammonia ;  bearing  in 


468 


THE  NON-METALLIC  ELEMENTS 


mind',  moreover,  that  hydrogen  combines  with  chlorine,  bulk  for 
bulk,  it  is  evident  that  the  one  measure  of  nitrogen  which 
remains  in  the  tube  has  resulted  from  the  decomposition  of  a 
quantity  of  ammonia  containing  three  measures  of  hydrogen. 
It  is,  therefore,  clearly  proved  by  this  experiment  that  ammonia 


FIG.  139. 


is  formed  by  the  union  of  three  volumes  of  hydrogen  with  one 
volume  of  nitrogen  (Hofmann). 

Another  method  of  demonstrating  the  same  fact  is  by  the 
electrolysis  of  a  strong  solution  of  ammonia.  For  this  purpose, 
the  solution,  mixed  with  a  little  ammonium  sulphate  to  increase 
its  conductivity,  is  introduced  into  a  Hofmann  apparatus 
(Fig.  76,  p,  250),  and  subjected  to  the  action  of  a  moderately 
strong  current  of  electricity ;  hydrogen  is  evolved  at  the  neg- 


COMBINATION  OF  AMMONIA  WITH  ACIDS  469 

ative  pole  and  nitrogen  at  the  positive  pole,  the  volume  of  the 
former  being  three  times  as  large  as  that  of  the  latter. 

264  Detection  and  Estimation  of  Ammonia.  —  The  method 
adopted  for  the  detection  and  estimation  of  small  traces  of 
ammonia  with  Nessler's  reagent  has  already  been  described 
under  Natural  Waters  (see  p.  300).  If  the  quantity  of  ammonia 
or  of  ammonium  salt  be  larger,  it  may  be  detected  by  the 
peculiar  smell  of  the  gas,  by  its  alkaline  reaction,  and  by  the 
formation  of  white  fumes  in  presence  of  strong  hydrochloric 
acid.  These  fumes  consist  of  ammonium  chloride  NH4C1,  and 
are  formed  by  the  direct  combination  of  the  ammonia  and 
hydrogen  chloride  : 


If,  however,  the  gases  are  mixed  together  in  a  perfectly  dry 
condition,  no  reaction  takes  place,  but  on  addition  of  a  little 
moisture  combination  immediately  ensues.  When  an  am- 
moniacal  salt  is  present,  the  ammonia  must  be  liberated  by 
heating  the  solid  salt,  or  its  solution,  with  a  caustic  alkali.  For 
the  estimation  of  ammonia  in  quantities  larger  than  those  for 
which  Nessler's  method  is  applicable,  it  is  usual  to  distil  the 
ammonia  either  into  hydrochloric  or  sulphuric  acid  of  known 
strength,  and  then  to  ascertain,  by  volumetric  analysis  with  a 
standard  solution  of  alkali,  the  amount  of  acid  remaining  free, 
or  into  hydrochloric  acid  of  unknown  strength  to  which  a 
solution  of  chloroplatinic  acid,  H2PtCl6,  is  added.  On  evaporat- 
ing the  resulting  solution  to  dryness  on  a  water  bath,  and 
exhausting  with  alcohol,  an  insoluble  yellow  precipitate  of  am- 
monium platinochloride,  (NH4)2PtCl6,  is  left,  and  this  can  either 
be  collected  on  a  weighed  filter,  or  it  may  be  ignited  and  the 
quantity  of  the  metallic  platinum  remaining  weighed,  from 
which  the  weight  of  ammonia  is  calculated. 

In  addition  to  its  use  in  the  laboratory  and  as  a  means  of 
obtaining  artificial  cold,  ammonia  is  also  largely  employed  for 
the  preparation  of  alum,  carbonate  of  soda,  aniline  colours,  and 
in  the  manufacture  of  indigo.1 

Combination  of  Ammonia  with  Acids.  —  Mention  has  been 
frequently  made  in  the  foregoing  pages  of  the  combination 
which  takes  place  between  ammonia  and  acids.  Thus  ammonia 
unites  with  hydrochloric  acid  to  form  the  compound  NH3,HC1, 

1  On  the  Manufacture  of  Liquor  Ammonise  see  Lunge's  work  on  Coal-  Tar  and 
Ammonia,  p.  667.  London,  1887 


470  THE  NON-METALLIC  ELEMENTS 

or  NH4C1,  and  with  hydrobromic  acid  to  form  NH3HBr,  or 
NH4Br,  whilst  with  sulphuric  acid  it  yields  2NH3,H2SO4  or 
(NH4)2SO4.  In  a  similar  manner  ammonia  combines  with 
almost  all  other  acids  yielding  compounds  containing  the  group 
of  atoms  NH4.  Thus  the  compound  NH4C1  may  be  regarded  as 
hydrochloric  acid,  HC1,  in  which  the  hydrogen  atom  has  been 
replaced  by  the  group  NH4,  just  as  potassium  chloride  is  hydro- 
chloric acid  in  which  the  hydrogen  atom  is  replaced  by  a 
potassium  atom.  The  two  substances  do  in  fact  bear  a  strong 
resemblance  to  each  other  both  in  their  chemical  and  physical 
properties,  and  all  the  other  substances  obtained  by  the  com- 
bination of  ammonia  with  acids  are  similarly  related  to  the  salts 
of  potassium. 

The  groups  of  atoms  NH4  is  therefore  a  never  varying  con- 
stituent in  a  series  of  compounds,  and  behaves  in  these 
compounds  as  though  it  were  a  simple  substance :  to  such  a 
group  the  name  of  "compound  radical"  is  given,  and  as  in  this 
case  the  group  in  combination  has  the  same  effect  on  the 
properties  of  the  substance  as  a  metal,  it  is  termed  ammonium, 
the  compounds  with  acids  being  known  as  the  ammonium  salts, 
as  they  correspond  to  the  salts  of  potassium,  sodium,  &c.  They 
will  be  described  together  with  these  in  Vol.  II.,  Part  I. 

HYDRAZINE  OR  Di AMIDE,  N2H4. 

265.  This  compound  was  first  obtained  by  Curtius  in  1887 
by  the  action  of  hot  dilute  acids  on  triazo-acetic  acid,  a  sub- 
stance described  in  Vol.  III.,  Part  II.  (2nd  edition)  p.  107,  which 
has  the  composition  C3H3N6(COOH)3,  and  contains  the  group 
— N  =  N  —  three  times.1  It  has  since  been  obtained  from  other 
organic  compounds  containing  two  nitrogen  atoms  combined 
together ;  among  these  may  be  mentioned  amidoguanidine, 
which  has  the  constitution  NH2.C(NH).NH.NH2.  As  this 
substance  is  obtained  without  difficulty  from  ammonium  thio- 
cyanate,  it  affords  the  best  means  for  the  preparation  of  hydra- 
zine  in  quantity,  and  a  description  of  the  preparation  may  be 
therefore  shortly  given  here,  although  the  intermediate  com- 
pounds will  not  be  described  till  later. 

Ammonium  thiocyanate,  NH4CNS,  is  heated  for  some  time 
at  a  constant  temperature  of  170 — 180°,  and  the  residue,  which 
consists  chiefly  of  guanidine  thiocyanate,  treated  first  with 
1  Curtius,  Bev.  20,  1632. 


HYDRAZINE  471 


strong  sulphuric  acid,  then  with  a  little  fuming  sulphuric  acid, 
and  the  mixture  after  cooling  mixed  with  nitric  acid  of 
sp.  gr.  1'5,  and  the  whole  poured  into  water.  Crude  nitro-guan- 
idine,  NH2.C(NH).NH.NO2,  separates  out,  and  is  at  once  treated 
with  zinc  dust  and  just  sufficient  dilute  acetic  acid  for  its 
reduction.  The  solution  obtained  by  the  reduction  of  208 
grams  of  nitroguanidine  is  then  evaporated  to  1200  cc.,  mixed 
with  a  solution  of  260  grams  of  caustic  soda  in  500  cc.  of 
water,  and  boiled  for  8 — 10  hours.  The  cooled  liquid  is  poured 
off  from  the  sodium  hydrogen  carbonate  which  separates  out, 
and  mixed  with  260  cc.  of  concentrated  sulphuric  acid  ;  the 
greater  part  of  the  hydrazine  separates  out  as  the  sulphate, 
which  is  quite  pure  after  a  single  recrystallization.1 

Free  hydrazine  has  not  up  to  the  present  been  obtained  in 
a  pure  condition,  owing  to  the  great  readiness  with  which  it 
combines  with  water  to  form  a  hydrate;  hitherto  it  has  not 
been  found  possible  to  remove  the  water  completely  by  the 
action  of  even  the  strongest  dehydrating  agents.  If  a  mixture 
•of  the  hydrate  and  anhydrous  baryta  is  heated  in  a  sealed  tube 
at  170°,  and  the  latter  opened  after  cooling,  a  gas  is  given  off 
which  has  an  extremely  penetrating  odour,  fumes  in  the  air, 
and  probably  consists  of  free  hydrazine. 

266.  Hydrazine  Hydrate,  N2H4,H2O,  is  best  obtained  by  dis- 
tilling hydrazine  sulphate  with  a  solution  of  caustic  potash  in 
a  silver  retort,  connected  without  rubber  or  cork  to  a  silver 
condensing  tube ;  the  distillation  is  continued  till  the  last  drop 
has  passed  over,  and  the  distillate  subjected  to  fractional  dis- 
tillation, by  which  the  hydrazine  hydrate  is  readily  separated 
from  the  excess  of  water. 

Hydrazine  hydrate  forms  a  strongly  refractive  almost  odour- 
less caustic-tasting"  liquid,  which  fumes  strongly  in  the  air,  but 
may  be  kept  unaltered  in  closed  vessels ;  it  boils  without  decom- 
position at  118*5°  under  739*5  mm.  pressure,  and  solidifies  in  a 
mixture  of  solid  carbonic  acid  and  ether,  but  melts  again  below 
-40°.  It  has  a  sp.  gr.  of  1*0305  at  21°,  and  a  vapour  density 
at  100°  under  diminished  pressure,  agreeing  with  the  formula 
N2H4.H2O,  whilst  at  170°  it  is  completely  split  up  into  hydrazine 
and  water ;  if  the  temperature  be  further  increased  the  vapour 
density  also  increases,  until  at  very  high  temperatures  the  latter 
corresponds  with  a  formula  double  that  observed  at  100°.  The 
cause  of  this  abnormal  behaviour  has  not  yet  been  ascertained. 
1  Thiele,  Annalen,  270,  1. 


472  THE  NON-METALLIC  ELEMENTS 


In  aqueous  solution  a  determination  of  the  molecular  weight 
gave  numbers  corresponding  to  the  formula  N2H4.2H2O. 

Hydrazine  hydrate  when  hot  attacks  glass  strongly,  and  also 
quickly  destroys  cork  and  indiarubber  ;  it  is  a  very  strong  poison 
for  lower  organisms.1  It  is  the  strongest  reducing  agent  known, 
quickly  precipitating  all  the  more  easily  reducible  metals  from 
their  solutions  even  in  the  cold.  It  is  a  very  strong  base,  and 
like  ammonia  unites  with  acids  to  form  well-defined  salts,  most 
of  which  are  readily  soluble  in  water.  Unlike  ammonia,  how- 
ever, it  forms  more  than  one  series  of  salts,  giving  with  hydro- 
chloric acid  for  example  two  salts,  N2H4.HC1  and  N2H4.2HC1, 
and  with  hydriodic  acid  three,  N2H4.H1,  N2H4.2HI,  and 
3N2H4.2HI.2  These  will  be  described,  together  with  the 
ammonium  salts,  in  Vol.  II.,  Part  I. 

AZOIMIDE  OR  HYDRAZOIC  ACID,  N3H. 

267.  This  interesting  substance,  was  like  the  foregoing,  dis- 
covered by  Curtius,  who  obtained  it  by  the  action  of  nitrites  on 
derivatives  of  hydrazine,  the  reaction  being  analogous  to  that  of 
nitrites  on  ammonia.  As  already  mentioned,  the  interaction  of 
the  latter  substances  leads  to  the  formation  of  nitrogen,  according 
to  the  equation 

NH4C1  +  NaN02  =  N2  +  2H2O  +  NaCl, 

and  the  formation  of  azoimide  is  represented  in  a  similar 
manner  by  the  equation 

NH2  Nx 

|         +  NO.OH  =  |!    >NH  +2H20; 
NH2  N/ 

hydrazine  itself  may  be  used  if  only  a  dilute  solution  of 
azoimide  is  required,3  but  as  a  rule  it  is  preferable  to  employ 
one  of  its  organic  derivatives,  in  which  one  of  the  hydrogen 
atoms  is  replaced  by  an  organic  radical.  Curtius  in  his  ex- 
periments employed  hippurylhydrazine,  C9H8NO2HN.NH2, 
which  by  the  action  of  nitrous  acid  yields  hippurylazoimide, 

/K 

C9H8NO2.N/    1 1 ;  the  latter  when  treated  with  acids  or  alkalis  is 

1  Loew,  Ser.  23,  3203. 

2  Curtius  and  Jay,  J.  Pr.    Chem.  [2]    39,  33  ;  Curtius  and  Schultz,  J.  Pr. 
Chem.   (2),  42,  521.  3  Ber.  26    1263. 


AZOIMIDE  473 


converted  into  azoimide  and  hippuric  acid.1  The  former  distils 
over  with  the  water  on  boiling  the  solution,  which  must  first  be 
acidified  if  an  alkali  has  been  employed  for  the  hydrolysis. 

Numerous  derivatives  of  azoimide  in  which  the  hydrogen 
atom  has  been  replaced  by  an  organic  radical  had  long  been 
known,  the  most  readily  prepared  of  which  is  the  phenyl 

XN 
derivative,  C6H5.N<(  ]|  (Vol.  III.,  Part  III.,  2nd  edition,  p.  325), 

X-N 

this  substance  being  known  as  triazobenzene  or  diazoben- 
zeneimide.  This  compound  itself  cannot  be  converted  into 
azoimide  by  the  action  of  acids  or  alkalis,  but  Noelting, 
Grandmougin  and  Michel  have  found  2  that  if  certain  acid 
radicals  such  as  the  group  NO9  be  introduced  into  the  phenyl 
group,  the  resulting  azoimide  may  be  in  many  cases  converted 
into  the  mother  substance  by  the  action  of  alkalis.  Tilden 
and  Millar  have  shown3  that  diazobenzeneimide  is  converted 

/N 
into  a  nitro-derivative,  C6H4(NO2)NC    [[,  by  the  action  of  hot 


nitric  acid  of  sp.  gr.  1*4,  and  that  this  compound  on  boiling 
with  alcoholic  potash  and  subsequent  acidification  yields 
azoimide,  a  dilute  solution  of  which  is  obtained  by  distilling 
the  liquid. 

Another  convenient  source  of  azoimide  is  the  crude  amido- 
guanidine  obtained  in  the  preparation  of  hydrazine  (p.  470)  ; 
this  substance  is  converted  by  nitrous  acid  into  diazoguani- 
dine  nitrate,  NH2.C(NH).NH.N:N.NO3,  which  on  boiling  with 
alkalis  yields  azoimide  and  cyanamide;  the  former  may  be 
isolated  by  acidification  and  distillation  in  the  manner  already 
described. 

A  further  very  interesting  synthesis  of  azoimide  from  purely 
inorganic  sources  has  also  been  described  by  W.  Wislicenus,4 
who  obtained  its  sodium  salt  by  the  action  of  nitrous  oxide  on 
sodamide  : 

NaNH2  +  N2O  =  N3Na  +  H2O. 

The  water  formed  acts  on  a  further  molecule  of  sodamide 
yielding  caustic  soda  and  ammonia.  The  sodamide  is  obtained 
by  passing  ammonia  over  metallic  sodium  at  a  temperature  of 
150  —  250°,  and  as  soon  as  all  metallic  sodium  has  disappeared, 

1  Ber.  23,  3023.  2  Ber.  24,  2546  ;  25,  3328. 

3  Journ.  Chem.  Soc.  1893,  i.  256.       4  Ber.  25,  2084. 


474  THE  NON-METALLIC  ELEMENTS 

the  stream  of  ammonia  is  replaced  by  one  of  nitrous  oxide,  and 
continued  till  ammonia  is  no  longer  evolved ;  the  product  is 
then  dissolved  in  water  and  distilled  with  dilute  sulphuric  acid. 

The  dilute  solution  obtained  by  any  of  the  above  processes 
may  be  concentrated  by  fractional  distillation  until  the  solution 
contains  91  per  cent,  of  azoimide,  and  the  remainder  of  the 
water  is  then  removed  by  calcium  chloride.  Pure  azoimide 
forms  a  colourless  mobile  liquid  which  has  a  most  pene- 
trating, unbearable  odour,  boils  without  decomposition  at  37°, 
and  dissolves  readily  in  water  and  alcohol.  When  brought  in 
contact  with  a  hot  body  it  explodes  with  extreme  violence, 
giving  a  bright  blue  flash  ;  the  explosion  also  sometimes  takes 
place  at  the  ordinary  temperature,  rendering  the  substance  an 
extremely  dangerous  one  to  work  with.  Thus  on  one  occasion 
O'Oo  gram  was  introduced  into  a  barometric  vacuum,  and 
exploded  with  such  violence  that  the  glass  and  mercury  were 
reduced  to  dust  and  spread  over  the  whole  of  a  large  room,  and 
in  another  case  0*7  gram  exploded  when  taken  out  of  a  freezing 
mixture,  breaking  all  the  bottles  in  the  neighbourhood,  and 
somewhat  severely  injuring  one  of  the  investigators.1 

The  aqueous  solution  of  azoimide  behaves  as  a  strong  acid 
and  readily  dissolves  zinc,  iron,  magnesium,  and  aluminium 
with  evolution  of  hydrogen  and  formation  of  salts  of  the 
metals.  It  also  gives  salts  with  the  metals  of  the  alkalis  and 
alkaline  earths  and  forms  a  silver  salt,  AgN3,  and  a  mercurom 
salt,  HgN3,  both  of  which  are  extremely  explosive,  but  in  other 
respects  closely  resemble  the  corresponding  chlorides.  The 
aqueous  solution  of  the  acid,  as  will  be  seen  from  the  pro- 
perties already  given,  behaves  in  a  similar  manner  to  aqueous 
hydrochloric  acid.  Hence  the  group  N3  must  be  itself 
electronegative,  and  is  analogous  in  its  chemical  properties 
with  the  halogens. 

The  salts  formed  by  azoimide  with  the  metals  of  the  alkalis 
of  the  alkaline  earths  are  not  nearly  so  explosive  as  those 
formed  with  the  heavy  metals,  and  azoimide  itself  when  in 
dilute  aqueous  solution  may  with  reasonable  care  be  handled 
without  danger. 

Other  compounds  of  nitrogen  and  hydrogen  have  been 
prepared  by  Curtius ;  these  consist  of  the  salts  formed  by 
ammonia  and  hydrazine  with  azoimide,  and  will  be  described 
with  the  other  ammonium  and  hydrazine  salts. 

1  Curtius  and  Radenhausen,  J.  Pr.  Chem.  [2],  43,  207. 


HYDROXYLAMINE  475 


HYDROXYLAMINE,  NH3O. 

268.  This  compound  was  discovered  in  1865  by  Lossen, l 
but  until  1891  was  only  known  in  the  form  of  salts  or  in 
aqueous  solution.  Lossen  obtained  it  by  the  action  of  nascent 
hydrogen  on  nitric  oxide,  the  reaction  taking  place  as  follows  : 

2NO+6H=2NH3O. 

Since  that  time  it  has  been  obtained  in  other  ways,  such  for 
example  as  the  reduction  of  nitric  acid  by  metals  under  suitable 
conditions,2  as  a  product  of  decomposition  of  the  fulminates  3 
(Vol.  III.,  part  L,  p.  524),  and  by  the  action  of  sulphuretted 
hydrogen  on  silver  nitrite.4 

By  the  interaction  of  nitrites  and  sulphites  under  suitable 
conditions,  salts  of  hydroxylaminedisulphonic  acid  (p.  520)  are 
formed,  and  these  on  heating  with  water  are  converted  into 
hydroxylamine  sulphate.  To  prepare  it  by  this,  the  most  con- 
venient method,  a  concentrated  aqueous  solution  of  sodium  hy- 
drogen sulphite  (2  mols.)  is  added  to  a  similar  solution  of  sodium 
nitrite  (1  mol.),  the  temperature  not  being  allowed  to  rise  much 
above  0° ;  a  sufficient  quantity  of  potassium  chloride  is  then 
added  to  convert  the  whole  of  the  disulphonic  acid  into  the 
less  soluble  potassium  salt,  which  separates  out  in  compact 
crystals.  The  latter  are  then  heated  in  neutral  aqueous  solution 
at  100°  for  a  long  time,  or  for  a  short  time  at  130°,  and  the 
resulting  hydroxylamine  sulphate  separated  from  the  sparingly 
soluble  alkaline  sulphates  by  fractional  crystallisation.5 

For  a  longtime  free  hydroxylamine  was  only  known  in  solution, 
but  in  1891  the  anhydrous  compound  was  prepared  almost 
simultaneously  by  Lobry  de  Bruyn,6  and  Crismer.7  The  former 
prepared  it  by  dissolving  hydroxylamine  hydrochloride  in 
absolute  methyl  alcohol,  adding  a  solution  of  sodium  methylate, 
NaOCH3,  in  the  same  solvent,  separating  the  sodium  chloride 
formed,  and  distilling  off  the  greater  portion  of  the  methyl 
alcohol  under  100  mm.  pressure  ;  the  residue  is  then  distilled 
in  small  portions  under  40  mm.  pressure  with  the  addition 
of  a  little  vaseline  to  prevent  frothing.  As  soon  as  solid 

1  Annalen  Suppl,  6,  240.  2  Journ.  Chem.  Soc.  1883,  i.  443. 

3  J.  Pr.  Chem.  [2],  25,  233  ;  Ber.  19,  993. 

4  Journ.  Chem.  Soc.  1887,  i.  48.  5  Raschig,  Annalen,  241,  161 
6  Rec.  Trav.  Chim.  10,  100  ;  H,  18.  7  Bull.  Soc.  Chim.  [3],  6,  793. 


476  THE  NON-METALLIC  ELEMENTS 


hydroxylamine  passes  over,  the  receiver  is  changed  and  cooled 
to  0°,  care  being  taken  that  the  hydroxylamine  vapour  is 
not  exposed  to  air  at  60 — 70°  for  any  length  of  time,  as  violent 
explosions  then  take  place.  Crismer  prepared  the  base  by 
heating  a  double  compound  which  it  forms  with  zinc  chloride, 
ZnCl,,  2NH2OH. 

Hydroxylamine  forms  white  inodorous  scales  or  hard  needles, 
has  a  sp.  gr.  of  about  1'3,  melts  at  33'05°,  and  boils  at  58°  under 
22  mm.  pressure,  the  density  of  its  vapour  agreeing  with  the 
formula  NH3O.  When  heated  to  90 — 100°  it  decomposes,  and 
detonates  at  a  higher  temperature.  It  inflames  in  a  current  of 
chlorine  gas  and  combines  violently  with  bromine  and  iodine,  but 
without  evolution  of  light.  In  the  pure  state  it  is  stable,  but  in 
presence  of  alkali  it  gradually  decomposes,  the  alkali  dissolved 
from  the  less  resistant  forms  of  glass  being  sufficient  to  bring 
about  this  change.  Strong  oxidizing  agents  such  as  potassium 
permanganate  or  bichromate  decompose  it  with  production  of 
flame  or  explosion,  and  sodium  likewise  attacks  it  with  produc- 
tion of  flame.  On  exposure  to  the  air  it  liquefies  owing  to  the 
absorption  of  moisture,  and  then  undergoes  oxidation  with  form- 
ation of  a  solid  substance  containing  nitrous  acid  and  ammonia  ; 
it  dissolves  readily  in  water  and  to  a  less  extent  in  ethyl  and 
methyl  alcohol  and  in  boiling  ether,  and  separates  from  the  last- 
named  solution  in  acicular  crystals  on  cooling.  The  solutions 
have  a  strongly  alkaline  reaction,  and  cause  the  separation  of 
the  more  easily  reduced  metals  from  their  salts.  From  a 
solution  of  copper  sulphate  it  precipitates  red  cuprous  oxide, 
this  reaction  being  sufficiently  delicate  to  recognize  one  part  of 
hydroxylamine  in  1 00,000  parts  of  water.  If  sodium  nitroprusside 
be  added  to  a  neutralized  solution  of  hydroxylamine,  and  then 
a  little  caustic  soda,  the  whole  assumes  on  boiling  a  beautiful 
magenta  red  colour.1  When  hydroxylamine  or  its  salts  are  treated 
with  nitrous  acid  decomposition  takes  place  rapidly,  nitrous  oxide 
being  evolved.  The  reaction  proceeds  in  two  stages,  hyponitrous 
acid  being  first  formed  according  to  the  equation : 

HO.NH2  +  ON.OH  =  HO.N.-N.OH  +  H20. 

This  substance  then  splits  up  into  its  anhydride,  nitrous  oxide, 
and  water.2 

The  salts  of  hydroxylamine  will  be  more  fully  described  after 

1  Angeli  Gazzetta,  23,  ii,  102. 
Wislicenus,  Ber.  26,  771  ;  Thnm,  Monats.  14,  294. 


NITROGEN  CHLORIDE  477 


the  ammonium  salts  in  Vol.  II.,  Part  I.  They  all  decompose 
on  heating  with  effervescence,  the  nitrate  yielding  nitric  oxide 
and  water. 

NH30,  HN08  =  2NO  +  2H2O. 

The  behaviour  of  hydroxylamine  towards  organic  substances 
shows  that  it  must  contain  the  hydroxyl  group  OH,  and  it  is  there- 
fore ammonia  in  which  one  atom  of  hydrogen  is  replaced  by  that 


group,  its  constitutional  formula  being  N^-H 


COMPOUNDS  OF  NITROGEN  WITH  THE 
ELEMENTS  OF  THE  CHLORINE  GROUP. 

NITROGEN  CHLORIDE,  NC13. 

269.  This  dangerous  body  was  discovered  by  Dulong  l  in  1811, 
who,  notwithstanding  the  fact  that  he  lost  one  eye  and  three 
fingers  in  the  preparation  of  this  body,  yet  continued  its  in- 
vestigation. A  similar  accident  happened  in  1813  to  Faraday 
and  Davy,  who  had,  however,  been  made  aware  of  the  ex- 
plosive properties  of  this  substance.  "  Knowing  that  the  liquid 
would  go  off  on' the  slightest  provocation,  the  experimenters 
wore  masks  of  glass,  but  this  did  not  save  them  from  injury.  In 
one  case  Faraday  was  holding  a  small  tube  containing  a  few 
grains  of  it  between  his  finger  and  thumb,  and  brought  a  piece 
of  warm  cement  near  it,  when  he  was  suddenly  stunned,  and  on 
returning  to  consciousness  found  himself  standing  with  his  hand 
in  the  same  position,  but  torn  by  the  shattered  tube,  and  the 
glass  of  his  mask  even  cut  by  the  projected  fragments.  Nor  was 
it  easy  to  say  when  the  compound  could  be  relied  on,  for  it 
seemed  very  capricious  ;  for  instance,  one  day  it  rose  quickly  in 
vapour  in  a  tube  exhausted  by  the  air-pump,  but  on  the  next 
day,  when  subjected  to  the  same  treatment,  it  exploded  with  a 
fearful  noise  and  injuring  Sir  H.  Davy."  - 

The  compound  is  formed  by  passing  chlorine  into  a  warm 
solution  of  sal-ammoniac,  or  when  a  solution  of  hypochlorous 
acid  is  brought  into  contact  with  ammonia  (Balard).  If  a 
galvanic  current  be  passed  through  a  concentrated  solution  of 
sal-ammoniac,  chloride  of  nitrogen  is  formed  at  the  positive 
pole  (Bottger,  Kolbe).  The  liquid  obtained  by  the  action  of 

1  Schweigg  Journ.  8,  302.  -  Gladstone,  Life  of  Faraday,  p.  10. 


478  THE  NON-METALLIC  ELEMENTS 

chlorine  on  a  solution  of  ammonium  chloride  has  been  carefully 
examined  by  Gattermann,  who  finds  that  it  is  not  a  homogeneous 
compound,  but  is  a  mixture  of  more  or  less  completely  chlorinated 
ammonias.  If  however  it  be  mixed  with  a  little  water  and 
chlorine  passed  over  it  for  half  an  hour,  it  is  converted  into  the 
completely  chlorinated  compound  NClg.1  The  analysis  of  the 
liquid  was  carried  out  by  placing  a  weighed  quantity  in  water, 
and  carefully  adding  ammonia  solution  which  slowly  converts  it 
into  nitrogen  and  ammonium  chloride,  the  chlorine  in  the 
solution  being  then  estimated  as  silver  chloride. 

Chloride  of  nitrogen  is  a  thin  yellowish  oil,  which  evaporates 
quickly  on  exposure  to  the  air,  has  a  sp.  gr.  of  about  1*6, 
and  possesses  a  peculiar  smell,  the  vapour  attacking  the  eyes 
and  mucous  membrane  violently.  Gattermann  has  shown  that 


FIG.  140. 

it  does  not  readily  explode  spontaneously,  and  may  even  be 
subjected  to  operations  such  as  washing  without  much  risk  of 
explosion  provided  direct  sunlight  is  excluded. 

When  heated  by  itself  to  95°,  or  if  it  is  brought  in  contact 
with  certain  bodies,  such  as  phosphorus  or  turpentine,  it  explodes 
with  great  violence,  giving  out  light,  and  pulverizing  any  glass 
or  porcelain  vessels  in  which  it  may  be  contained.  In  cold 
water  it  undergoes  spontaneous  decomposition  with  the  evolution 
of  chlorine,  nitrogen,  hydrochloric  acid,  and  nitrous  acid. 

The  following  method  is  employed  for  preparing  this  dangerous 
1  JSer.  21,  751. 


NITROGEN  CHLORIDE  479 


substance  in  small  quantities,  and  for  showing  its  explosive 
properties  without  risk.  A  flask  of  about  two  litres  capacity, 
having  a  long  neck,  is  filled  with  chlorine,  and  placed  mouth 
downwards  in  a  large  glass  basin  filled  with  warm  saturated 
solution  of  sal-ammoniac  as  shown  in  Fig.  140.  Below  the  neck 
of  the  flask  is  placed  a  small  thick  leaden  saucer,  in  which  the 
chloride  of  nitrogen  is  collected.  The  solution  of  sal-ammoniac 
absorbs  the  chlorine,  and,  as  soon  as  the  flask  is  three  parts  filled 
by  the  liquid,  oily  drops  are  seen  to  collect  on  the  surface  inside 
the  flask.  These  gradually  increase,  and  at  last  drop  one  by  one 
down  into  the  leaden  saucer.  When  a  few  drops  have  collected, 
the  leaden  saucer  may  be  carefully  removed  by  a  pair  of  clean 
tongs,  another  being  placed  in  its  stead.  A  small  quantity  of  the 
chloride  of  nitrogen  may  be  exploded  by  touching  it  with  a 
feather  moistened  with  turpentine  attached  to  the  end  of  a  long 
rod.  Another  drop  of  the  oil  may  be  absorbed  by  filtering 
paper,  and  when  this  is  held  in  a  flame  a  loud  explosion  like- 
wise ensues. 


FJG.  Hi. 

The  force  of  the  explosion  may  be  rendered  still  more  evident 
by  placing  the  flask  in  a  strongly  constructed  box  with  glass 
sides,  and  when  the  drops  of  chloride  of  nitrogen  collect  on  the 
surface  of  the  solution,  passing  in  some  turpentine  from  a 
stoppered  funnel ;  the  latter  is  kept  outside  the  box  and  is  con- 
nected with  it  by  means  of  rubber  and  glass  tube  the  end  of 
which  dips  under  the  neck  of  the  flask.  When  the  turpentine 
reaches  the  surface  of  the  solution  in  the  flask  a  bright  flash  is 
seen,  and  a  violent  thunderlike  explosion  occurs,  completely 
shattering  the  flask.1 

The  formation  and  properties  of  chloride  of  nitrogen  may 
be  also  exhibited  in  the  following  way.  A  solution  of  sal- 
ammoniac,  saturated  at  a  temperature  of  28°,  is  brought  into  a 
glass  basin  (A,  Fig.  141),  and  a  cylinder  (B),  the  lower  end  of 

1  V.  Meyer,  Ber.  21,  26. 


480  THE  NON-METALLIC  ELEMENTS 


which  is  closed  by  a  piece  of  bladder,  is  also  filled  with  the 
solution  and  placed  upright  in  the  basin.  A  layer  of  oil  of 
turpentine  is  then  poured  on  to  the  top  of  the  liquid  in  the 
cylinder,  and  a  platinum  plate  (a)  in  contact  with  the  positive 
pole  of  a  battery  of  six  cells  placed  in  the  cylinder,  whilst  the 
negative  pole  (ft)  is  placed  under  the  bladder  in  the  basin.  Yellow 
oily  drops  soon  begin  to  form  on  the  surface  of  the  positive  pole, 
and  these  gradually  become  detached  from  the  pole,  and  rise  in 
the  liquid  until  they  come  in  contact  with  the  layer  of  turpentine, 
when  they  explode. 

BROMIDE  OF  NITROGEN. 

When  bromide  of  potassium  is  added  to  chloride  of  nitrogen 
under  water,  potassium  chloride  and  bromide  of  nitrogen  are 
formed.  The  latter  is  a  dark  red  very  volatile  oil,  possessing 
a  powerful  smell,  and  is  as  explosive  as  chloride  of  nitrogen 
(Millon). 

IODIDE  OF  NITROGEN. 

270  When  iodine  is  brought  into  contact  with  aqueous  or 
alcoholic  ammonia,  a  black  powder  is  formed  which,  when  dried, 
decomposes  spontaneously  with  a  very  violent  detonation  either 
when  touched,  or  when  slightly  heated,  violet  vapours  of  iodine 
being  emitted.  This  compound  gradually  decomposes  under  cold 
water,  pure  iodine  being  left  behind.  Warm  water  facilitates 
this  decomposition,  and  the  body  explodes  violently  when 
thrown  into  boiling  water. 

The  composition  of  this  black  powder  appear  to  vary  accord- 
ing to  the  process  employed  in  it  preparation.  Serullas,1 
who  first  prepared  this  body,  precipitated  an  alcoholic  solu- 
tion of  iodine  with  aqueous  ammonia ;  the  compound  thus 
obtained  possesses,  according  to  Gladstone,2  the  composition 
NHI2 ;  but  according  to  Stahlschmidt,3  its  composition  is 
represented  by  the  formula  NI3,  a  body  having  the  former 
composition  being  obtained  when  alcoholic  solutions  of  iodine 
and  ammonia  are  mixed.  Bunsen,  on  the  other  hand,  found 
that  under  these  circumstances,  and  when  absolute  alcohol  is 
used,  a  precipitate  having  the  composition  N2I3H3,  or  NH3,NI3 
is  formed,  and  that  on  precipitating  an  aqueous  solution  of 

1  Ann.  Chim.  Phys.  42,  200.  2  Joum.  Chem.  Soc.  1852,  34. 

3  Pogg.  Ann.  119,  421. 


OXIDES  OF  NITROGEN  481 

chloride  of  iodine  with  ammonia  a  black  powder  is  obtained, 
which  consists  of  NH^NIg.1  These  observations  render 
it  probable  that  under  different  circumstances  at  least  two 
distinct  iodides  of  nitrogen  are  formed,  one  being  derived 
from  ammonia  by  the  replacement  of  the  whole,  and  the  other 
by  the  replacement  of  a  portion  of  the  hydrogen  by  iodine, 
thus : — 

(a)  4NH3  +  3I2  =  NI3  +  3NH4L 
(V)  3NH3  +  2I2  =  NHI2  +  2NH4L 

Iodide  of  nitrogen  would  thus  appear  to  be  capable  of  combining 
with  ammonia,  giving  rise  to  the  compounds  described  by 
Bunsen. 

Szuhay 2  has  recently  shown  that  the  compound  obtained  by 
adding  an  excess  of  aqueous  ammonia  to  a  strong  solution  of 
iodine  in  concentrated  aqueous  potassium  iodide  has  the 
composition  NHI2,  its  formation  being  represented  by  equa- 
tion (&)  above.  When  suspended  in  water  and  mixed  with  an 
ammoniacal  solution  of  silver  nitrate  it  yields  a  black  substance 
having  the  composition  NAgI2,  which  is  likewise  explosive, 
.and  is  decomposed  by  boiling  water  and  dilute  acids.  If  it  be 
treated  with  a  solution  of  a  soluble  cyanide  the  silver  may  be 
replaced  by  other  metals,  but  the  compounds  thus  formed 
have  not  been  isolated  ;  they  regenerate  the  compound  NAgI2 
on  addition  of  a  silver  salt.  The  compound  NHI2  therefore 
possesses  acid  properties. 


NITROGEN  AND  OXYGEN. 

OXIDES  AND  OXY-ACIDS  OF  NITROGEN. 

271  As  already  mentioned  nitrogen  burns  in  oxygen  when 
both  gases  are  heated  to  a  sufficiently  high  temperature  (p.  451), 
and  direct  combination  also  takes  place  more  slowly  when  a 
series  of  electric  sparks  is  passed  through  fairly  dry  air,  whilst 
if  much  moisture  is  present  nitric  acid  is  produced.  When  a 
mixture  of  nitrogen  and  oxygen  is  passed  over  platinum  black  at 
250°,  oxides  of  nitrogen  are  also  formed,3  and  they  are  further 
produced  when  carbon  is  burnt  in  highly  compressed  air.4  We 

1  Annalen,  84,  1  2  Ber.  26,  1933. 

3  Loew,  Ber.  23,  1443.  4  Hempel,  Ber.  23,  1457. 


482  THE  NON-METALLIC  ELEMENTS 

are  acquainted  with  five  oxides  of  nitrogen,1  and  three  oxygen 
acids  corresponding  to  the  oxides  numbered  1,  3,  and  5. 

Oxides.  Acids. 

1.  Nitrous  Oxide,  or  N  )  n     „  A   . ,     HO.N  \ 

Nitrogen  Monoxide  N  /  e  lcl'    HO.N  } 

2.  Nitric  Oxide,  or  -^-Q 

Nitrogen  Dioxide 

3.  Nitrogen  Trioxide      -^-Q  !•  O    Nitrous  Acid     .     .       TT  |  0 

J  J 

4.  Nitrogen  Peroxide,  or  -^Q 

Nitrogen  Tetroxide 

5.  Nitrogen  Pentoxide  Mr.2  \  O     Nitric  Acid  ...       T|  r  O 

IM  u«  )  n  ) 


NITRIC  ACID.    HN03. 

272  In  Geber's  tract,  De  Inventione  Veritatis,  we  find  the 
following  description  of  a  mode  of  preparing  nitric  acid  or  aqua- 
fortis : — "  Sume  libram  unam  de  vitrioli  de  cypro.  et  libram  salis 
petrse,  et  unam  quartam  aluminis  Jameni,  extrahe  aquam  (the 
acid)  cum  rubidine  alembici."  That  is,  by  strongly  heating  a 
mixture  of  saltpetre,  alum,  and  sulphate  of  copper,  the  nitric 
acid  distils  over,  owing  to  the  decomposition  of  the  saltpetre  by 
the  sulphuric  acid  of  the  other  salts.  Nitric  acid  was  commonly 
prepared  and  used  as  a  valuable  reagent  by  the  alchemists, 
especially  as  a  means  of  separating  gold  and  silver.  The 
method  of  preparation  which  we  now  use  from  nitre  and  oil  of 
vitriol  appears  to  have  been  first  employed  by  Glauber,  for  long 
afterwards  the  acid  thus  obtained  was  called  Spiritus  nitri 
fumans  Glanberi. 

The  first  theory  respecting  the  composition  of  nitric  acid 
was  proposed  by  Mayow  in  1669.2  He  believed  that  the  acid 
contained  two  components,  one  derived  from  the  air  and 
having  a  fiery  nature,  and  the  other  derived  from  the  earth. 
More  than  a  century  later,  in  1776,  Lavoisier  showed  that 
one  constituent  of  nitric  acid  is  oxygen,  but  he  was  unable  to 
satisfy  himself  as  to  the  nature  of  the  other  components,  and 
it  was  reserved  for  Cavendish  3  to  prove  the  exact  composition 

1  According  to  Berthelot  and  Hautefeuille  and  Chappuis,  a  sixth  oxide,  pos- 
sessing the  composition  N206,  exists.     It  is  formed  by  the  action  of  an  electric 
discharge  on  a  mixture  of  nitrogen  peroxide  and  oxygen.     (See  Ann.  Chem.  Phys. 
[5],  22,  432  ;  Compt.  Rend.  92,  80>  134>  94,  1111,  1306.) 

2  Mayow,  De  sal-nitro  et  spiritu  nitri  aereo. 

3  Phil.  Trans.  1784,  p.  119.     Do.  1785,  p.  372. 


NITRIC  ACID  483 


and  mode  of  formation  of  this  acid  or  its  salts  by  the  direct 
combination  of  oxygen  and  nitrogen  gases  in  presence  of  water 
or  alkaline  solutions.  Priestley  had  already  observed  that  when 
a  series  of  electric  sparks  was  made  to  pass  through  common 
air  included  between  short  columns  of  a  solution  of  litmus,  the 
solution  acquired  a  red  colour  and  the  air  was  diminished  in 
volume.  Cavendish  repeated  the  experiment,  using  lime-water 
and  soap-lees  (caustic  potash)  in  place  of  the  litmus,  and  he 
concluded  that  the  lime-water  and  soap-lees  became  saturated 
with  some  acid  formed  during  the  operation.  He  proved  that  this 
was  nitric  acid  by  passing  the  electric  discharge  through  a  mix- 
ture of  pure  dephlogisticated  air  (oxygen)  and  pure  phlogisticated 
air  (nitrogen)  over  soap-lees  (caustic  potash),  when  nitre  (potas- 
sium nitrate)  was  formed.  Cavendish  J  clearly  expresses  his 


FIG.  142. 

views  in  the  following  words  : — "  We  may  safely  conclude  that 
in  the  present  experiments  the  phlogisticated  air  was  enabled, 
by  means  of  the  electric  spark,  to  unite  to,  or  form  a  chemical 
combination  with,  the  dephlogisticated  air,  and  was  thereby 
reduced  to  nitrous  acid,  which  united  to  the  soap-lees,  and 
formed  a  solution  of  nitre  ;  for  in  these  experiments  those  two 
airs  actually  disappeared,  and  nitrous  acid  was  actually  formed 
in  their  room."  The  apparatus  which  he  employed,  represented 
in  Fig.  142,  consisted  of  a  bent  syphon-tube  containing,  in  the 
bent  portion,  the  mixture  of  gases,  and  mercury  in  the  limbs, 
the  open  ends  dipping  under  mercury  in  the  glasses. 

Nitric  acid  is  also  formed  when  various  bodies  are  burnt  in  a 

mixture  of  oxygen  and  nitrogen.     Thus,  if  three  to  five  volumes 

of  the  detonating  mixture  of  oxygen  and  hydrogen  be  mixed 

with   one  volume   of  air  in  a  eudiometer  over  mercury,  and 

1  Phil.  Trans.  1785,  p.  379. 


484  THE  NON-METALLIC  ELEMENTS 

an  electric  spark  passed  through  the  mixture,  instantaneous 
combination  takes  place  and  nitric  acid  is  formed-,  the  surface 
of  the  mercury  becoming  covered  with  crystals  of  mercurous 
nitrate.1 

In  order  to  exhibit  the  direct  combination  of  oxygen  and 
nitrogen,  it  is  only  necessary  to  allow  the  sparks  from  an  in- 
duction-coil to  pass  between  two  platinum  wires  placed  in  the 
interior  of  a  large  glass  globe  containing  dry  air,  as  shown  in 
Fig.  143.  Red  fumes  of  nitrogen  peroxide  are  rapidly  formed, 
and  their  presence  may  be  distinctly  recognized  by  plunging 
a  piece  of  iodized  starch  paper  into  the  globe,  when  the  blue 
iodide  of  starch  will  at  once  be  produced.  On  pouring  a  few 


FIG.  143. 

drops  of  water  into  the  globe  and  shaking  it  up,  the  red  fumes 
are  absorbed  and  nitrous  and  nitric  acids  formed,  as  may  be 
shown  by  the  acid  reaction  of  the  liquid. 

In  a  similar  manner,  if  a  flame  of  hydrogen  be  allowed  to 
burn  in  a  large  flask  into  which  oxygen  is  led,  but  not  in  such 
quantity  as  to  displace  the  whole  of  the  air,  nitric  acid  is 
formed  in  large  quantity.2  The  same  acid  is  also  produced  when 
ammonia  burns  in  oxygen.  The  formation  of  nitric  acid  by 
the  discharge  of  electricity  through  the  air  accounts  for  the  pres- 
ence of  this  acid  in  the  atmosphere. 

Another  and  more  productive  source  of  the  nitrates  has  yet 
to  be  described.  When  nitrogenous  organic  matter  is  exposed 

1  Bunsen,  Gasometry,  p.  58. 

2  Kolbe,  Annalen,  119,  176  ;  Hofmann,  Ber.   3,  663. 


PREPARATION  OF  NITRIC  ACID  485 

to  the  air  the  nitrogen  assumes  the  form  of  ammonia  ;  but  when 
alkalis,  such  as  potash,  soda,  or  lime,  are  present,  a  further 
slow  oxidation  of  the  nitrogen  takes  place,  and  nitrates  of 
these  metals  are  formed.  Hence  these  nitrates  are  widely 
diffused  in  all  surface  soils,  especially  in  hot  countries  such 
as  India,  where  oxidation  takes  place  quickly.  Soil  in  the 
neighbourhood  of  the  Indian  villages,  which  contains  con- 
siderable amounts  of  potash,  thus  becomes  rich  in  nitre 
or  potassium  nitrate,  KNO3,  originating  from  the  decomposition 
of  the  urea  of  the  urine.  It  is  from  this  source  that  the 
largest  quantity  of  nitre  imported  into  this  country  is  obtained. 
Another  nitrate,  lime-saltpetre  or  calcium  nitrate,  Ca(NOg).2,  is 
often  found  as  an  efflorescence  on  the  walls  of  buildings,  such  as 
stables,  or  cellars  of  inhabited  houses,  and  this  source  was  made 
use  of  during  the  French  revolution  for  the  manufacture  of 
nitre.  Chili  saltpetre,  or  sodium  nitrate,  NaN03,  occurs  in 
large  deposits  in  the  province  of  Tarapaca,  a  rainless  district 
on  the  Peruvian  coast  lying  near  the  20th  degree  of  south 
latitude.  The  nitrate  of  soda  is  found  mixed  with  chloride 
of  sodium  and  other  salts,  probably  pointing  to  the  fact 
that  the  locality  has  been  covered  by  the  sea,  and  that  the 
nitrate  is  a  product  of  the  decomposition  of  sea-plants  and 
animals. 

Preparation. — In  order  to  prepare  pure  nitric  acid,  saltpetre 
which  has  been  previously  well  dried  is  pla'ced  in  a  tubulated 
retort  together  with  an  equal  weight  of  concentrated  sulphuric 
acid.  On  heating  the  mixture,  the  volatile  nitric  acid  passes 
over,  and  is  collected  in  a  well-cooled  receiver,  see  Fig.  144, 
hydrogen  potassium  sulphate  (commonly  called  bisulphate  of 
potash)  remaining  behind  in  the  retort ;  thus : — 

KN03  +  H2S04  =  HN03  +  KHSO4. 

The  distillate  thus  obtained  has  a  yellowish  colour,  caused  by 
the  presence  of  nitrogen  peroxide,  and  it  is  not  free  from  water, 
as  the  concentrated  acid,  when  heated,  decomposes  partially  into 
peroxide,  oxygen  and  water.  In  order  to  purify  the  acid  thus 
obtained  it  must  be  again  distilled  with  its  own  volume  of  con- 
centrated sulphuric  acid,  and  the  distillate  freed  from  traces  of 
the  peroxide  by  gently  warming  the  acid,  and  leading  a  current 
of  dry  air  through  it  until  it  is  cold.  Thus  prepared  it 
contains  from  99'5  to  99'8  per  cent,  of  the  anhydrous  acid, 
HN(X. 


486  THE  NON-METALLIC  ELEMENTS 

273  Properties. — Nitric  acid  is  a  colourless  liquid  fuming 
strongly  in  the  air.  It  possesses  a  peculiar  though  not  very 
powerful  smell,  and  absorbs  moisture  from  the  air  with  the 
greatest  avidity.  Nitric  acid  is  an  extremely  corrosive  sub- 
stance, which,  when  brought  in  contact  with  the  skin,  produces 
painful  wounds,  being  used  in  surgery  as  a  powerful 
cautery.  The  dilute  acid  acts  less  energetically,  and  colours 
the  skin,  nails,  wool,  silk,  and  other  organic  bodies  of  a  bright 
yellow  tint. 

When  pure  concentrated  nitric  acid  is  heated,  it  begins  to 
boil  at  86°,  and  becomes  of  a  dark-yellow  colour  owing  to 
the  decomposition  of  a  portion  of  the  acid  into  nitrogen 


FIG.  144. 

peroxide,  oxygen,  and  water.  As  soon  as  about  three-fourths 
of  the  acid  has  distilled  over,  the  residue  becomes  colour- 
less, and  then  contains  only  95*8  per  cent,  of  acid.1  If  the 
distillation  is  pushed  further,  the  boiling  point  continually  rises, 
a  strong  acid  distils  over,  and  the  residue  becomes  constantly 
weaker  until  it  contains  68  per  cent,  of  acid,  when  the  liquid  is 
found  to  boil  unaltered  at  120°'5  under  the  normal  atmospheric 
pressure,  yielding  an  acid  of  the  above  constant  composition, 
and  with  a  specific  gravity  of  T414  at  15°'5.  This  constant 
acid  is  always  obtained,  whether  a  stronger  or  a  weaker  acid  be 
subjected  to  distillation.  If  this  acid  of  constant  composition 
be  distilled  under  an  increased,  or  under  a  diminished  pres- 

1  Roseoe,  Journ.  Chem.  Soc.  1861,  i.  147. 


PROPERTIES  OF  NITRIC  ACID  487 

sure,  the  composition  of  the  residual  acid  again  undergoes  a 
change,  until  for  each  pressure  a  constant  boiling  point  is 
reached.  Thus,  under  the  pressure  of  l"22m.  of  mercury,  an 
acid  containing  68'6  per  cent,  of  HNO3  distils  without  alteration, 
whilst  under  a  pressure  of  0'070m.  an  acid  distils  over  at  a  tem- 
perature of  from  65°  to  70°,  having  a  constant  composition  of  66'7 
per  cent.  When  a  current  of  dry  air  is  passed  through  aqueous 
nitric  acid,  either  a  stronger  or  a  weaker  acid  is  volatilized, 
according  to  the  concentration  or  the  temperature  of  the  acid, 
until  at  length  a  residue  is  obtained  which  volatilizes  unchanged. 
Thus,  when  the  experiment  is  made  at  100°,  the  residual 
acid  contains  66*2  per  cent. ;  when  at  60°,  64 '5  per  cent. ; 
and  at  15°  the  residual  acid  contains  64'0  per  cent,  of  HNO3. 
From  this  it  will  be  seen  that  nitric  acid  behaves  in  a 
similar  way  in  this  respect  to  hydrochloric  and  the  other 
aqueous  acids. 

When  the  concentrated  acid  is  mixed  with  water  an  in- 
crease of  temperature  and  a  contraction  of  bulk  is  observed. 
This  attains  its  maximum  when  one  molecule  of  the  acid  is 
mixed  with  three  molecules  of  water  (Kolbe).  The  following 
table  gives  the  specific  gravities  of  aqueous  acids  at  0°  and 
15°  i—1 

Per  cent.  Sp.  gr.  Sp.  gr. 

HN03.  at  0°.  at  15°. 

100-0 1-559 1-530 

90-0 1-522 1-495 

80-0 1-484 1-460 

70-0 1-444 1-423 

60-0 1-393 1-374 

50-0 1-334 1-317 

40-0 1-267 1-251 

30-0 1-200 1-185 

20-0 1-132 1-120 

15-0 1-099 1-089 

10-0 1-070 1-060 

5-0 1-031 1-029 

It  has  been  already  remarked  that  concentrated  nitric  acid 
begins  to  decompose,  at  a  temperature  of  86°,  into  water,  oxygen, 
and  nitrogen  peroxide.  If  the  acid  be  more  strongly  heated  in 
closed  glass  tubes  this  change  takes  place  so  rapidly  that  at 
260°  the  whole  of  the  nitric  acid  is  thus  decomposed  (Carius). 
1  Kolbe,  Ann.  Chim.  Phys.  [4],  10,  140. 


488 


THE  NON-METALLIC  ELEMENTS 


In  order  to  exhibit  this  decomposition  by  means  of  heat,  the 
same  apparatus  serves  which  was  employed  for  the  decomposi- 
tion of  sulphuric  acid,  Fig.  145.  Strong  nitric  acid  is  allowed 
to  fall  on  the  hot  pumice-stone  contained  in  the  platinum 
flask.  Immediately  red  vapours  are  emitted,  and  these  are 
condensed  in  a  U-tube  placed  in  a  freezing  mixture  to  a 
brown  liquid,  NO2,  whilst  the  cylinder  placed  over  the 
pneumatic  trough  becomes  filled  with  a  colourless  gas  which 
can  be  easily  shown  to  be  oxygen  (Hofmann). 


Fm.  145. 


Pickering  has  succeeded  in  isolating  two  hydrates  of  nitric 
acid,  having  the  composition  HN03,  H2O  and  HNO3,  3H2O 
respectively.  The  former  separates  in  large  fairly  transparent 
crystals,  and  the  latter  in  smaller,  more  opaque,  gritty 
crystals.1 

274  Commercial  Manufacture. — In  order  to  prepare  nitric  acid 
on  the  commercial  scale,  sodium  nitrate  is  substituted  for  nitre,  as 
it  is  much  cheaper,  and  the  salt  is  decomposed  with  sulphuric 
acid  as  before.  The  proportions  of  these  two  substances  em- 
ployed are  not  the  same  in  all  works.  If  one  molecule  of 

1  Journ.  Chem.  Soc.  1893    i.  441. 


MANUFACTURE  OF  NITRIC  ACID 


489- 


490  THE  NON-METALLIC  ELEMENTS 

sulphuric  acid  and  two  of  sodium  nitrate  be  taken,  the  follow- 
ing are  the  reactions.  In  the  first  place  we  have  : — 

H2S04  +  NaN03  =  NaHSO4  +  HNO3. 

When  the  heat  is  raised,  the  acid  sodium  sulphate  acts  upon  a 
second  molecule  of  sodium  nitrate ;  thus  : — 

NaHS04  +  NaN03  =  Na.2S04  +  HNO3. 

In  this  case,  however,  a  part  of  the  acid  is  decomposed  owing 
to  the  high  temperature,  and  nitrogen  peroxide  is  evolved  in  the 
form  of  red  fumes  which  dissolve  in  the  concentrated  acid,  giving 
it  the  red  appearance  usually  noticed  in  the  strong  commercial 
product.  When  a  large  excess  of  sulphuric  acid  is  employed, 
a  certain  quantity  of  acid  sodium  sulphate  is  formed,  which 
lowers  the  melting  point  of  the  residual  mass  so  that  it  can 
be  withdrawn  from  the  retorts  in  the  fused  state,  whereas  in 
the  other  case  the  residue  can  only  be  removed  in  the  solid 
state  after  the  cylinder  has  been  cooled.  The  ordinary  com- 
mercial acid  has  a  specific  gravity  of  from  1*38  to  T41,  and  is 
usually  prepared  by  means  of  chamber  (sulphuric)  acid  ;  but  if 
a  more  concentrated  acid  is  required  a  stronger  sulphuric  acid 
must  be  employed.  The  strongest  nitric  acid  occurring  in  com- 
merce has  a  specific  gravity  of  1'53,  and  this  is  obtained  by 
distilling  well-dried  Chili  saltpetre  with  sulphuric  acid  having  a 
specific  gravity  of  T85.  The  retorts  in  which  nitric  acid  is 
usually  prepared  on  the  large  scale  in  England  consist  of  cast- 
iron  cylinders  built  in  a  furnace  in  such  a  way  that  they  may 
be  heated  as  uniformly  as  possible,  as  shown  in  Fig.  146.  Some 
manufacturers  cover  the  upper  half  of  the  cylinder  with  fire- 
bricks in  order  to  protect  the  iron  from  the  action  of  the  nitric 
acid  vapours.  This,  however,  is  unnecessary,  if  the  retorts  are 
so  thoroughly  heated  that  no  nitric  acid  condenses  on  the  surface 
of  the  iron.  The  ends  of  the  cylinders  which  are  not  exposed 
to  the  action  of  the  flame  are  closed  by  plates  of  Yorkshire 
flag  cemented  on  to  the  iron  with  a  mixture  of  iron  filings, 
sulphur,  sal-ammoniac,  and  vinegar.  In  the  upper  part  of  one  of 
these  flags  is  a  hole,  through  which  the  sulphuric  acid  is  intro- 
duced after  the  charge  of  Chili  saltpetre.  The  hole  is  then  closed 
by  a  clay  plug.  A  similar  hole  in  the  other  flag  is  furnished 
with  a  bent  earthenware  tube  (c)  passing  into  a  series  of  large 
Woulffe's  bottles  (bfy,  one  placed  behind  the  other,  containing 


MANUFACTURE  OF  NITRIC  ACID  491 

small  quantities  of  water  in  which  the  nitric  acid  condenses, 
and  from  which  the  acid  is  withdrawn  by  leaden  syphons. 
The  last  of  these  Woulffe's  bottles  is  placed  in  connection  with  a 
tower  filled  with  coke,  down  which  a  current  of  water  runs. 
Any  uncondensed  nitrogen  peroxide  passes  up  this  tower,  and, 
coming  in  contact  with  the  water  and  the  oxygen  of  the  air,  is 


FIG.  147. 

oxidized  to  nitric  acid.  When  the  operation  is  complete,  one  of 
the  flags  is  removed  and  the  residual  fused  sulphate  of  soda 
scraped  out. 

A  usual  charge  for  one  retort  is  305  kilos  of  Chili  saltpetre 
and  240  kilos  of  strong  sulphuric  acid.  The  mixture  is 
heated  uniformly  for  about  eighteen  hours,  and  the  quantity 
of  water  placed  in  the  Woulffe's  bottles  is  such  that  a  yield  of 
363  kilos  of  nitric  acid,  of  specific  gravity  1*35,  is  obtained, 


492  THE  NON-METALLIC  ELEMENTS 

whilst  295  kilos  of  fused  sodium  sulphate  remain  behind  in  the 
cylinder. 

In  a  German  factory,  where  the  strongest  nitric  acid  is  made, 
a  cast-iron  vessel  is  employed  for  its  generation,  the  construction 
of  which  is  seen  in  Fig.  147.  The  charge,  in  this  case,  consists  of 
700  kilos  of  sulphuric  acid,  of  specific  gravity  1*84,  and  600  kilos 
of  Chili  saltpetre.  The  retort  is  placed  in  connection  with  two 
series  of  receivers,  twenty-five  in  number  According  to  this 
process  100  parts  of  Chili  saltpetre,  containing  96  parts  of  pure 
nitrate,  yield  68  parts  of  nitric  acid,  of  specific  gravity  1*5,  and 
17  parts  of  weaker  acid.  This  corresponds  to  about  96  per  cent, 
of  the  theoretical  yield. 

A  new  arrangement  has  lately  been  devised,  whereby  a  more 
rapid  generation  and  condensation  is  possible  than  by  the  older 
processes.  According  to  this  plan,  proposed  by  Guttmann,  a  charge 
of  610  kilos  can  be  worked  off  in  10 — 11  hours  with  a  yield  of 
strong  acid  only  3 — 7  per  cent,  below  the  theoretical  amount, 
whilst  if  the  fumes  are  condensed  by  water  and  the  weak  acid 
is  used,  there  is  absolutely  no  loss  in  the  process. 

The  charge  of  sodium  nitrate  and  sulphuric  acid  is  brought  into 
an  iron  retort  similar  to  that  shown  in  Fig.147,  bat  which  allows 
of  the  spent  sodium  bisulphate  being  drawn  off  at  the  bottom  ; 
in  order  to  reduce  the  quantity  of  nitrogen  peroxide  in  the 
condensed  acid  to  a  minimum,  a  current  of  air  is  injected  into 
the  gaseous  products,  the  mixed  gases  then  passing  into  a  very 
carefully  made  system  of  earthenware  pipes  arranged  in  a  some- 
what similar  manner  to  the  well-known  atmospheric  condensers 
of  the  gas-works,  which  are  cooled  by  water.  Each  set  of  upright 
pipes  is  connected  with  a  main,  into  which  the  acid  falls,  and 
flows  away  into  a  receiver  placed  below.  The  uncondensed 
gases  pass  away  into  a  tower  built  of  earthenware,  and  filled 
with  perforated  plates  of  special  construction,  down  which  water 
is  continuously  allowed  to  trickle.  The  arrangement  of  the 
perforated  plates  is  the  invention  of  Prof.  Lunge. 

The  wash  tower  is  of  great  value  even  if  the  weak  acid  be 
thrown  away,  inasmuch  as  without  it  the  draught  becomes  too 
great  and  serious  escape  of  nitrous  gas  takes  place.  This  process 
is  particularly  useful  where  a  very  strong  acid  is  needed,  as  for 
example  in  the  preparation  of  nitro-glycerine  and  gun-cotton.1 

Commercial  red   nitric   acid    always   contains   chlorine,   and 
sometimes  iodine  in  the  form  of  iodic  acid,  originating  from  the 
1  Journ.  Soc.  Chem.  2nd.  1893,  203. 


NITRIC  ACID  493 


•Chili  saltpetre.  In  addition  it  also  contains  nitrogen  peroxide, 
iron  oxide,  sulphuric  acid,  and  sodium  sulphate,  which  have 
been  mechanically  carried  over.  In  order  to  purify  the  acid,  it 
must  be  distilled  in  glass  retorts,  chlorine  and  nitrogen  peroxide 
coming  over  in  the  first  portion.  As  soon  as  the  acid  distillate 
is  free  from  chlorine  the  receiver  is  changed,  and  the  liquid  may 
be  distilled  until  only  a  small  residue  is  left,  containing  the 
whole  of  the  iodic  acid,  sulphuric  acid,  and  sodium  sulphate. 

Concentrated,  as  well  as  dilute  nitric  acid  is  largely  used  in  the 
arts  and  manufactures.  Large  quantities  are  employed  in  the 
manufacture  of  the  various  coal-tar  colours,  of  nitro-glycerine, 
of  gun-cotton,  of  sulphuric  acid,  and  of  nitrate  of  silver,  which 
is  largely  used  for  photographic  purposes.  The  acid  is 
also  used  in  large  quantities  for  the  preparation  of  certain 
nitrates,  especially  lead  nitrate,  iron  nitrate,  and  aluminium 
nitrate,  all  of  which  are  employed  in  the  processes  of  dyeing 
and  calico-printing,  whilst  the  nitrates  of  barium  and  strontium 
are  used  for  pyrotechnic  purposes.  In  the  laboratory  it  is  an 
indispensable  reagent,  and  is  used  in  the  preparation  of  a 
large  number  of  inorganic  and  organic  substances. 

Nitric  acid  is  a  monobasic  acid  forming  a  series  of  salts 
which  are  termed  the  nitrates.  These  are  almost  all  easily  soluble 
in  water,  and  as  a  rule  crystallize  well.  They  may  be  obtained 
by  neutralizing  the  acid  with  an  oxide  or  a  carbonate,  and 
;are  almost  all  formed  by  dissolving  the  metal  in  nitric  acid.  In 
this  case  the  metal  is  oxidized  at  the  expense  of  a  portion  of  the 
acid  which,  according  to  the  concentration  or  the  temperature,  is 
reduced  to  N02,  N2O3,  NO,  N2O,  and  even  to  nitrogen  and 
.ammonia. 

Several  other  bodies,  such  as  sulphur,  phosphorus,  carbon, 
.and  many  organic  substances,  are  easily  oxidized,  especially 
by  the  concentrated  acid.  In  order  to  exhibit  this  action, 
some  nitric  acid  may  be  poured  upon  granulated  tin,  which  is 
then  oxidized  with  the  evolution  of  dense  red  fumes,  whilst 
a  white  powder  of  tin  oxide  is  deposited.  Turpentine  when 
poured  into  the  concentrated  acid  is  likewise  oxidized  with 
.almost  explosive  violence,  light  and  heat  being  evolved.  In 
like  manner  ignition  may  take  place  when  straw  or  sawdust 
becomes  impregnated  with  the  strong  acid. 

Other  organic  bodies  treated  with  nitric  acid  undergo  no 
.apparent  alteration.  Thus,  for  instance,  with  cotton-wool  no 
ignition  or  evolution  of  red  fumes  occurs.  If,  however,  the 


494  THE  NON-METALLIC  ELEMENTS 

cotton-wool  after  having  been  thus  soaked  in  strong  nitric 
acid  is  washed  and  dried,  it  is  found  to  possess  very  different 
properties  from  ordinary  cotton,  although  in  appearance  it  can 
hardly  be  distinguished  from  it.  Cotton-  wool,  or  cellulose,  has 
the  formula  C12H20O10,  whilst  after  treatment  with  nitric  acid 
it  consists  of  nitro-cellulose,  gun-cotton  or  collodion-cotton. 
This  consists  of  cellulose  in  which  some  of  the  hydroxyl  groups 
varying  in  number  from  four  to  six  according  to  the  strength  of 
nitric  acid  employed,  are  replaced  by  NO3,  the  changes  which 
have  occurred  being  represented  by  the  equation  :  — 

C12H20010  +  4HN03  =  C12H1606(N03)4  +  4H2O, 
or  CJ2H20010  +  6HN03  =  C12HU04(NO3)6  +  6H20. 

Nitric  acid  acts  in  a  similar  way  on  many  other  organic  bodies. 
275  Action  of  Nitric  Acid  on  Metals.  —  As  already  mentioned, 
nitric  acid  dissolves  a  large  number  of  metals  with  formation  of 
nitrates.  Hydrogen  is  not  evolved  at  the  same  time  as  is  the 
case  with  sulphuric  and  hydrochloric  acids,  but  in  its  place  lower 
oxides  of  nitrogen  and  even  nitrogen  itself  and  ammonia  are 
formed.  The  explanation  usually  given  of  this  change  is  that 
hydrogen  is  first  evolved,  but  that  it  at  once  acts  on  the  excess  of 
nitric  acid  present,  forming  water  and  the  lower  oxides  of 
nitrogen.  Thus,  for  example,  the  formation  of  nitric  oxide  by 
the  action  of  copper  on  nitric  acid  is  supposed  to  take  place 
in  the  two  following  stages  :  — 

Cu  +  2HN03  =  Cu(NO3)2  +  2H, 


According  to  Veley,  however,  this  explanation  is  not  correct, 
inasmuch  as  pure  copper,  mercury  and  bismuth  do  not  dissolve 
in  pure  dilute  nitric  acid,  but  dissolve  readily  when  nitrous  acid 
is  present,  the  change  at  any  moment  being  directly  proportional 
to  the  mass  of  nitrous  acid  in  the  solution,  and  the  more  rapid 
the  greater  the  proportion  of  the  former  to  the  latter.  He  there- 
fore believes  that  the  reaction  is  started  either  by  traces  of  nitrous 
acid  already  present,  or  by  impurities  in  the  metal  inducing  alocal 
galvanic  current  ;  the  first  product  of  the  reduction  of  the  nitric 
acid  is  nitrous  acid,  and  the  production  of  lower  oxides  of  nitro- 
gen he  regards  as  due  to  the  subsequent  changes  occurring  be- 
tween nitrous  acid  and  cuprous  and  cupric  nitrate  or  nitrite  in 


DETECTION  OF  NITEIC  ACID  495 


presence  of  an  excess  of  nitric  acid,  the  nitrous  acid  being 
decomposed  as  fast  as  it  is  formed.1 

Detection  and  Estimation. — The  presence  of  nitric  acid  or 
of  its  salts  is  easily  ascertained.  Thus  for  instance,  if  we  heat  a 
few  drops  of  not  too  dilute  nitric  acid  with  copper  turnings, 
brownish  red  vapours  of  nitrogen  peroxide  are  emitted.  The 
nitrates  give  the  same  reaction  if  they,  or  their  concentrated 
aqueous  solutions,  are  treated  with  sulphuric  acid  and  copper. 
In  order  to  detect  nitric  acid,  or  a  nitrate  in  a  very  dilute 
solution,  a  cold  solution  of  ferrous  sulphate  is  added  to  the 
liquid,  and  about  an  equal  volume  of  strong  sulphuric  acid  is 
then  allowed  to  flow  slowly  down  to  the  bottom  of  the  inclined 
test-tube,  care  being  taken  that  the  two  liquids  do  not  mix. 
If  nitric  acid  be  present  it  is  liberated  by  the  action  of  the 
sulphuric  acid,  and  at  once  oxidises  the  ferrous  sulphate  where 
it  comes  in  contact  with  it,  nitric  oxide  being  simultaneously 
given  off.  The  latter  at  once  unites  with  the  excess  of  ferrous 
sulphate  forming  a  dark  brown  compound,  the  solution  of  which 
is  seen  as  a  dark-coloured  ring  at  the  point  where  the  dense 
sulphuric  acid  joins  the  lighter  aqueous  solution. 

Another  very  delicate  test  for  the  presence  of  nitric  acid  is 
aniline.  In  order  to  apply  this  test,  ten  drops  of  aniline  are 
brought  into  50  cc.  of  a  dilute  sulphuric  acid  containing  15 
per  cent,  of  pure  acid,  and  0'5  cc.  of  this  solution  is  poured  on 
to  a  watch-glass  together  with  1  cc.  of  concentrated  sulphuric 
acid.  If  a  glass  rod  moistened  with  the  solution  under  exami- 
nation be  now  brought  in  contact  with  the  edge  of  the 
liquid  in  the  watch-glass,  a  red  streak  will  be  produced  if  a 
nitrate  be  present,  and  the  colour  will  increase  in  intensity  until 
the  whole  liquid  becomes  red.  If  a  larger  quantity  of  nitric  acid 
be  present,  the  whole  mass  will  assume  more  or  less  of  a  brown 
tint  (C.  D.  Braun). 

The  organic  base  called  brucine,  bearing  a  close  resemblance 
to  strychnine,  serves  as  a  test  for  nitric  acid  even  more  delicate 
than  aniline.  If  to  half  a  drop  of  a  solution  of  one  part  of  nitric 
acid  to  100,000  parts  of  water,  one  or  two  drops  of  a  solution 
of  brucine  be  added,  and  then  a  few  drops  of  concentrated 
sulphuric  acid,  a  distinct  pink  coloration  will  be  observed  if 
the  solution  be  viewed  against  a  white  ground.2 

In  order  quantitatively  to  determine  the  amount  of  nitric  acid 

1  Proc.  Roy.  Soc.  46,  216  ;  52,  27  ;  Phil.  Trans.  1891  (A),  312  ;  Journ.  Soc. 
Chem.  Ind.  1891,  204.  2  Reichardt,  Jahresbcricht,  1871,  893. 


496  THE  NON-METALLIC  ELEMENTS 

contained  in  potassium-  or  sodium-saltpetre,  the  well-dried  sub- 
stance is  heated  to  dull  redness  for  half  an  hour  with  freshly- 
ignited  and  finely-powdered  quartz  or  silica,  SiO2.  The  nitrates 
are  thus  completely  decomposed,  whilst  any  sulphates  or 
chlorides  which  may  be  present  undergo  no  change.  The 
decomposition  which  here  takes  place  may  be  represented  as 
follows : — 

2KN03  =  K20  -t  2N02  +  O. 

-Oxygen  and  nitrogen  peroxide  are  evolved,  whilst  the  potash 
combines  with  the  silica  to  form  silicate  of  potash.  From  the 
loss  of  weight  thus  ensuing  the  amount  of  nitre  present  can 
easily  be  calculated. 

Another  good  method,  which  is  particularly  useful  in  the 
determination  of  the  nitrates  contained  in  drinking  water, 
depends  upon  the  fact  that  a  thin  zinc  plate,  which  has  been 
covered  with  a  deposit  of  spongy  metallic  copper  by  dipping  it 
into  a  solution  of  copper  sulphate,  on  being  heated  with  water 
containing  nitrates  reduces  them  to  ammonia,  zinc  hydroxide 
and  free  hydrogen  being  at  the  same  time  formed  (Gladstone 
and  Tribe)  ;  thus  : — 

KN03  +  8H  =  NH3  +  KOH  +  2H2O. 

The  ammonia  thus  obtained  is  distilled  over  into  an  excess  of 
hydrochloric  acid  and  determined  in  the  usual  way,  or  if  present 
in  only  small  quantities  by  nesslerisation  (see  p.  300). 

A  further  method,  proposed  by  Lunge,  consists  in  mixing  the 
nitrate  with  sulphuric  acid  and  shaking  in  a  graduated  tube 
with  mercury;  the  whole  of  the  nitrogen  is  liberated  in  the 
form  of  nitric  oxide,  the  volume  of  which  is  measured.  The 
reaction  which  takes  place  is  represented  by  the  following 
equation : — 

3Hg  +  3H2S04  +  2HN03  =  3HgS04  +  4H20  +  2NO. 

A  special  apparatus  known  as  the  nitrometer  has  been 
designed  by  Lunge  for  carrying  out  this  reaction l  which  is 
especially  useful  in  the  determination  of  the  nitrogen  contents 
of  organic  nitrates  such  as  the  nitro-celluloses. 

1  Ber.  11,  434. 


NITROGEN  PENTOXIDE  497 


AQUA  REGIA. 

276  This  name  is  given  to  a  mixture  of  nitric  and  hydro- 
chloric acids  which  is  frequently  employed  for  dissolving  the 
noble  metals,  such  as  gold  and  platinum,  as  well  as  many 
metallic  ores  and  other  bodies.  A  method  of  preparing  this 
substance  was  described  by  Geber  in  his  work  De  Inventions 
Veritatis,  by  dissolving  sal-ammoniac  in  nitric  acid ;  and  he 
states  that  the  liquid  thus  obtained  has  the  power  of  dissolving 
gold  and  sulphur.  The  name,  aqua  regia,  is  first  found  in  the 
writings  of  Basil  Valentine.  He,  like  Geber,  prepared  it  by 
dissolving  four  ounces  of  sal-ammoniac  in  1  Ib.  of  aqua-fortis; 
he  also  states  that  strong  aqua  regia  can  be  obtained  by  mixing 
hydrochloric  and  nitric  acids.  The  solvent  power  of  aqua  regia 
depends  upon  the  fact  that,  on  heating,  this  mixture  of  acids 
evolves  chlorine  ;  thus : — 

HN03  +  3HC1  =  2H20  +  NOC1  +  CL, 

The  compound  nitrosyl  chloride,  NOC1,  which  is  liberated  at  the 
same  time  is  described  on  page  510. 


NITROGEN  PENTOXIDE,  N2O5=  107-28. 

277  This  substance,  which  is  commonly  called  nitric  anhydride, 
was  discovered  in  1849  by  Deville,1  who  obtained  it  by  leading 
perfectly  dry  chlorine  gas  over  dry  silver  nitrate  contained  in  a 
U-'tube  placed  in  a  water-bath.  The  reaction  begins  at  95°, 
and  when  cooled  to  60°,  the  decomposition  of  the  nitrate  goes 
on  regularly.  The  pentoxide  is  collected  in  a  bulb  tube 
surrounded  by  a  freezing  mixture.  The  following  equation 
represents  the  reaction  which  takes  place  : — 

4AgN03  +  2C12  =  4AgCl  +  2N2O5  +  O2. 

In  the  preparation  of  the  substance  all  joints  of  cork  and  caout- 
chouc must  be  avoided,  and  the  parts  of  the  glass  apparatus 
must  be  connected  either  by  fusion,  or  by  placing  the  end  of 
one  tube  inside  the  other  and  closing  the  space  between  the 
tubes  with  asbestos,  the  pores  of  which  are  filled  up  with  melted 
paraffin. 

Nitrogen  pentoxide  can  be  also  prepared  still  more  simply 

1  Ann.  Chim.  Phys   [3]  28,  241. 
88 


498  THE  NON-METALLIC  ELEMENTS 

from  pure  perfectly  anhydrous  nitric  acid  by  withdrawal  from 
this  substance  of  the  elements  of  water.  For  this  purpose 
nitric  acid  is  distilled  two  or  three  times  with  concentrated 
sulphuric  acid  to  remove  all  water,  and  is  then  intro- 
duced into  a  retort  to  the  neck  of  which  a  long  glass  tube 
is  fused  to  serve  as  a  condenser.  Phosphorus  pentoxide  is  next 
gradually  added  until  the  mixture  has  a  syrupy  consistency, 
the  retort  being  cooled  with  ice-water  during  the  addition  ;  if  the 
nitric  acid  has  been  properly  dehydrated  no  hissing  noise  is 
heard  on  addition  of  the  anhydride.  The  reaction  which  here 
takes  place  may  be  thus  represented  :  — 

2HN03  +  P205  =  N206  +  2HP03. 

By  gently  heating  the  retort,  a  deep  orange-coloured  distillate 
is  obtained,  which  on  standing  separates  out  into  two  layers. 
The  upper,  or  lighter  layer  is  then  poured  into  a  thin  stoppered 
tube  and  cooled  down  by  plunging  the  tube  into  ice-cold  water. 
Crystals  of  the  pentoxide  soon  separate  out,  and  these  may  be 
purified  by  pouring  off  the  orange-coloured  liquid  from  which 
they  are  deposited,  melting  the  crystals  at  a  moderate  heat, 
again  allowing  them  to  deposit,  and  finally  pouring  off  the 
mother  liquor. 

Properties.  —  Nitrogen  pentoxide  is  a  white  colourless  solid, 
crystallizing  in  bright  rhombic  crystals,  or  in  six-sided  prisms 
derived  from  these.  When  heat°d  to  15°  —  20°  the  crystals 
become  of  a  yellowish  colour,  and  molt  at  about  30°  to  a  dark 
yellow  liquid,  which  decomposes  between  45°  and  50°  with  the 
evolution  of  dense  brown  fumes.  When  suddenly  heated,  the 
pentoxide  decomposes  with  explosive  violence  into  nitrogen 
peroxide  and  oxygen,  and  this  sudden  decomposition  occurs 
sometimes  even  at  ordinary  temperatures,  if  the  crystals  have 
been  kept  for  some  time.  The  lower  the  temperature  is  kept 
the  longer  does  the  substance  remain  unaltered,  and  below  30° 
it  may  be  sublimed  in  a  closed  vessel,  depositing  in  crystals  in 
the  cool  part  of  the  tube.  In  dry  air  the  pentoxide  volatilizes 
very  quickly,  whilst  in  moist  air  it  deliquesces  with  formation 
of  nitric  acid.  Thrown  into  water,  it  dissolves  with  evolution 
of  heat,  forming  nitric  acid  ;  thus  :  — 


The   pentoxide   possesses   very   powerful   oxidizing  properties. 


NITROUS  OXIDE  499 


Thus,  if  brought  in  contact  with  sulphur  it  forms  white  vapours, 
which  condense  to  a  white  sublimate  of  nitrosulphonic  anhy- 
dride, S2O5(NO2)2.  Phosphorus  and  potassium  burn  with 
brilliancy  in  the  slightly  warmed  anhydride.  Charcoal  does 
not  decompose  even  the  boiling  anhydride,  but  when  ignited 
and  brought  into  the  vapour  it  burns  with  a  brilliant  light. 
When  brought  in  contact  with  nitric  acid,  the  anhydride  com- 
bines to  form  the  compound  N2O5  +  2HN03.  This  substance, 
which  is  a  liquid  at  the  ordinary  temperature,  possesses  at  18° 
a  specific  gravity  of  1'642,  and  solidifies  at  5°  to  a  crystalline 
mass.  It  forms  the  heavy  layer  obtained  in  the  preparation  of 
the  pentoxide,  and  decomposes  with  explosion  when  heated. 
Its  formation  is  perfectly  analogous  to  that  of  disulphuric  acid, 
and  the  constitution  is  probably  represented  by  the  formula  — 

N02—  0—  NO—  OH. 


—  O—  NO—  OH. 


NITROGEN  MONOXIDE  OR  NITROUS  OXIDE,  N2O  =  43*76. 

278  This  gas  is  formed  by  the  action  of  easily  oxidizable  sub- 
stances, such  as  potassium  sulphide,  moist  iron  filings,  the  sul- 
phites, and  other  bodies  upon  nitric  oxide,  and  according  to  these 
methods  it  was  first  prepared  by  Priestley  in  the  year  1772.  It 
is  moreover  formed  when  zinc  and  other  metals  are  dissolved  in 
very  dilute  nitric  acid,  and  by  the  action  of  sulphur  dioxide 
on  nitric  oxide  in  presence  of  moisture1 

Preparation. — In  order  to  prepare  the  gas  we  do  not,  how- 
ever, usually  employ  any  of  these  methods,  but  we  have  re- 
course to  the  decomposition  which  ammonium  nitrate  undergoes 
on  heating.  This  salt  splits  up  into  water  and  nitrous  oxide 
gas  ;  thus  : — 

NH4N03  =  N2O  +  2H2O. 

It  is  best,  before  the  experiment,  to  melt  the  nitrate,  in  order  to 
free  it  from  moisture  ;  and  the  powdered  dry  substance  is  then 
introduced  into  a  flask  furnished  with  a  cork  and  delivery  tube. 
The  flask  must  be  heated  gently  until  a  regular  evolution  of 
gas  begins,  and  then  the  flame  moderated,  as  sometimes,  if  the 

1  Lunge,  Ber.  14,  2196. 


500 


THE  NON-METALLIC  ELEMENTS 


heat  applied  be  too  great,  the  decomposition  takes  place  so 
violently,  with  evolution  at  the  same  time  of  nitric  oxide,  that  an 
explosion  may  occur.  In  order  to  free  the  gas  from  traces  of 
nitric  oxide  it  can  be  shaken  up  with  a  solution  of  ferrous 
sulphate,  which  combines  with  the  latter  gas ;  whilst  in  order  to 
remove  traces  of  chlorine  derived  from  the  chloride  of  ammo- 
nium, which  the  commercial  nitrate  often  contains,  it  must  be 
allowed  to  stand  over  a  solution  of  caustic  potash  or  soda. 
These  precautions  are  especially  needed  when  the  gas  is  used 
for  inhaling.  A  regular  stream  of  the  gas  may  also  be  obtained 
by  heating  sodium  nitrate  with  a  slight  excess  of  ammonium 
sulphate  at  2400.1 


FIG.  148. 


As  nitrous  oxide  is  somewhat  soluble  in  cold,  but  not  nearly 
so  soluble  in  hot  water,  it  is  best  to  fill  the  pneumatic  trough 
with  warm  water  before  collecting  the  gas.  The  arrangement 
used  for  this  purpose  is  seen  in  Fig.  148,  and  requires  no  further 
explanation. 

279  Properties. — Under  ordinary  circumstances  nitrogen  mon- 
oxide is  a  colourless  gas  possessing  a  pleasant  smell  and  sweet, 
agreeable  taste,  and  having  a  specific  gravity  of  1*52  (Colin).  Its 
solubility  in  water  between  0°  and  25°  is  represented  by  the 
formula 

c  =  1-30521  -  0-045620£  +  0-0006843*2 ; 

or  its  coefficients  of  absorption  are  as  follows  : — 

1  W.  Smith,  Jaurn.  Soc.  Chem.  Ind.,  U,  867,  12,  10. 

2  Carius,  Annalen,  94,  140. 


NITROUS  OXIDE  501 


0°  5°  10°  15°  20°  25° 

1-3052     1-0954     0'9196     07778     0'6700     0*5962 

It  is  still  more  soluble  in  alcohol,  one  volume  of  this  liquid 
absorbing,  according  to  the  experiments  of  Carius,  a  quantity 
of  the  gas  found  by  the  formula 

c  =  4-17805  -  0-0698160^  +  0'0006090£2. 

Nitrous  oxide  was  first  liquefied  by  Faraday  in  1823  by  heating 
nitrate  of  ammonium  in  a  bent  tube  (Fig.  149).  The  liquid  is 
now  prepared  on  the  manufacturing  scale  by  compressing  the 
gas  into  strong  cylinders ;  it  is  a  colourless  very  mobile  liquid, 
which  has  a  specific  gravity  of  0'9369  1  at  0°  and  boils  at  -  87°'9 
under  767"3  mm.  pressure.  For  experimental  purposes  the  liquid 
may  be  transferred  to  open  vessels  by  placing  the  cylinder  with 
the  valve  downwards,  and  carefully  opening  the  latter,  the  liquid 
rushing  out  in  a  fine  stream  through  the  nozzle.  For,  although 


the  liquid  boils  at  a  temperature  more  than  80  degrees  below 
the  freezing-point  of  water,  it  may  be  kept  for  more  than  half 
an  hour  in  tubes  or  other  open  vessels. 

.Like  other  condensed  gases,  liquid  nitrous  oxide  has  a  very 
high  coefficient  of  expansion;  one  volume  of  the  liquid  at  0° 
becoming  1'1202  volumes  at  20°,  whereas  one  volume  of  the 
gas  at  0°  becomes  only  1*0732  volumes  when  raised  to  20°.  A 
drop  of  the  liquid  brought  on  to  the  skin  produces  a  blister,  and 
when  water  is  thrown  into  the  liquid  it  at  once  freezes  to  ice, 
at  the  same  time  producing  a  dangerously  explosive  evolution 
of  gas.  Phosphorus,  potassium,  and  charcoal  do  not  undergo 
any  change  when  thrown  into  liquid  nitrous  oxide,  but  if  a 
piece  of  burning  charcoal  be  thrown  on  the  liquid,  it  swims 
on  the  surface  and  continues  to  burn  with  great  brilliancy. 
On  pouring  a  little  mercury  into  a  tube  containing  the  liquid 
nitrous  oxide,  the  metal  solidifies,  whilst  at  the  same  moment  a 
piece  of  ignited  charcoal  may  be  seen  to  be  brilliantly  burning 
on  the  surface  of  the  liquid. 

1  Andreef,  Annalcn.  110.  H- 


502  THE  NON-METALLIC  ELEMENTS 

When  liquid  nitrous  oxide  is  poured  into  carbon  bisulphide 
the  two  liquids  mix,  and  if  the  mixture  be  brought  under  the 
receiver  of  an  air-pump  the  temperature  sinks  to  —  140°.  If  a 
tube  filled  with  liquid  nitrous  oxide  be  dipped  into  a  bath  of 
solid  carbon  dioxide  and  ether,  and  if  this  mixture  be  allowed 
to  evaporate  in  vacuo,  the  liquid  nitrous  oxide  freezes  to 
colourless  crystals,  whose  tension  is  less  than  one  atmosphere. 
Poured  into  an  open  vessel,  liquid  nitrous  oxide  cools  down  by 
evaporation  to  a  temperature  of  —  100°,  and  if  the  alcoholic 
thermometer  be  taken  out  of  the  liquid,  a  portion  of  the  ad- 
hering substance  solidifies  and  the  temperature  sinks  to  —  115°. 

Solid  nitrous  oxide  in  the  form  of  snow  has  also  been  prepared 
by  Wills,1  who  obtained  it  by  a  modification  of  Thilorier's  method 
of  obtaining  solid  carbon  dioxide. 

Gaseous  nitrous  oxide,  like  oxygen,  supports  combustion 
vigorously.  A  red-hot  splinter  of  wood  rekindles  when  brought 
into  the  gas,  a  watch  spring  burns  with  bright  scintillations,  and 
a  bright  flame  of  sulphur  continues  to  burn  with  a  brighter  flame. 
If,  however,  the  sulphur  be  only  just  kindled,  the  flame  is  ex- 
tinguished on  bringing  it  into  the  gas,  as  then  the  temperature 
of  the  flame  is  not  sufficiently  high  to  decompose  the  gas  into 
its  constituents.  All  combustions  in  this  gas  are  simply 
combustions  in  oxygen,  the  burning  body  not  uniting  with  the 
nitrous  oxide,  but  with  its  oxygen,  the  nitrogen  being  liberated. 

Potassium  and  sodium  also  burn  brightly  in  the  gas  when 
slightly  heated,  with  formation  of  the  peroxides  of  these  metals, 
and  these  again  when  more  strongly  ignited  in  the  gas  yield  the 
nitrates  of  the  metals. 

The  very  remarkable  effects  on  the  organism  produced  by 
the  inhalation  of  nitrous  oxide,  first  observed  by  Davy,  have 
been  further  investigated  by  Hermann.2  The  first  effects 
noticed  are  singing  in  the  ears,  then  insensibility,  and,  if  the 
inhalation  be  continued,  death  through  suffocation.  In  the 
case  of  small  animals,  such  as  birds,  fatal  effects  are  observed 
in  30  seconds,  and  in  rabbits  after  the  expiration  of  a  few 
minutes.  If,  however,  air  be  again  allowed  to  enter  the  lungs 
as  soon  as  insensibility  has  set  in,  the  effects  quickly  pass  away 
and  no  serious  results  follow.  When  a  mixture  of  four  volumes 
of  this  gas  and  one  volume  of  oxygen  is  breathed  for  from 
one-and-a-half  to  two  minutes,  a  curious  kind  of  nervous  ex- 
citement or  transient  intoxication  is  produced,  without  loss  of 
1  Journ.  Ohem.  Soc.  1874,  p.  21.  Jahresberieht,  1865,  662. 


NITROUS  OXIDE  503 


consciousness,  and  this  soon  passes  off  without  leaving  any  evil 
consequences.  Hence  this  substance  received  the  name  of 
laughing-gas.  Nitrous  oxide  is  now  largely  employed  as  an 
anaesthetic  agent  instead  of  chloroform  in  cases  of  slight  surgical 
operations,  especially  in  dentistry,  where  only  a  short  period  of 
unconsciousness  is  needed.  Care  must,  however,  be  taken  that 
for  these  purposes  the  gas  is  free  from  chlorine  and  nitric  oxide 
The  composition  of  nitrous  oxide  may  be  ascertained  in 
various  ways  ;  thus,  a  given  volume  of  the  gas  is  brought  into  a 
bent  glass  tube  over  mercury  in  the  upper  part  of  which  (Fig. 
150)  a  small  piece  of  sodium  is  placed.  The  lower  and  open 
end  of  the  tube  is  then  closed  under  the  mercury  by  the  finger, 
and  the  part  of  the  tube  containing  the  sodium  heated  with 
a  lamp.  After  the  combustion,  the  tube  is  allowed  to  cool,  and 
the  volume  of  the  residual  gas  measured.  This  is  found  to  be 
the  same  as  the  original  volume  taken,  and  to  consist  entirely 


FIG.  150. 

of  nitrogen.  Now,  as  22 '3  litres  of  nitrous  oxide  are  found  by 
experiment  to  weigh  43*76,  and  22'3  litres  of  nitrogen  are  known 
to  weigh  27'88,  it  is  clear  that  the  difference,  or  15'88,  is  due 
to  the  oxygen.  Hence  we  see  that  the  molecule  of  nitrous  oxide 
consists  of  two  atoms  of  nitrogen  combined  with  one  of  oxygen. 

The  same  result  is  attained  when  a  spiral  of  steel  wire  is 
placed  in  a  given  volume  of  the  gag  and  heated  to  redness  by  a 
galvanic  current.  The  iron  then  burns  in  the  gas,  and  a 
volume  of  nitrogen  remains  equal  to  that  of  the  nitrous  oxide 
employed. 

By  means  of  eudiometric  analysis  we  may  likewise  determine 
the  composition  of  the  gas,  and  for  this  purpose  the  nitrous 
oxide  must  be  mixed  with  hydrogen  and  the  mixture  exploded 
by  an  electric  spark,  when  water  is  formed  and  nitrogen  gas  is 
left  behind  ;  thus  : — 

N0O  -f  H,  -  N0  -f-  H,0. 


504  THE  NON-METALLIC  ELEMENTS 


HYPONITROUS  ACID,  H2N2O2. 

280  The  salts  of  this  acid  were  first  obtained  by  Divers  by  the 
reduction  of  an  aqueous  solution  of  potassium  nitrate  with 
sodium  amalgam ;  the  first  product  of  the  reaction  is  potassium 
nitrite,  which  on  further  reduction  yields,  in  addition  to  some 
hydroxylamine,  potassium  hyponitrite,  which  was  found  to  have 
the  empirical  composition  KNO.1  It  is  also  obtained  by  the 
electrolysis  of  a  solution  of  potassium  nitrite,2  and  by  the 
action  of  nitric  oxide  on  alkaline  ferrous  or  stannous  hydroxides,3 
and  is  always  isolated  by  the  addition  of  silver  nitrate  which 
yields  silver  hyponitrite  as  a  yellow  precipitate.  The  best 
method  of  preparation  of  hyponitrites  is  however  by  the 
action  of  alkalis  on  the  salts  of  hydroxylaminesulphonic  acid, 
HO.NH.SO3H  (p.  321).  It  has  already  been  stated  that 
dilute  acids  convert  this  substance  into  hydroxylamine  sulphate, 
but  with  alkalis  no  trace  of  hydroxylamine  is  formed,  the 
sole  products  being  a  sulphite,  a  hyponitrite,  and  nitrous 
oxide,  the  latter  being  a  decomposition  product  of  the  hyponi- 
trites.4 

2HO.NH.S03K  +  4KOH  =  K2N2O2  +  2K2S03  +  2H2O. 

The  formation  of  hyponitrites  from  derivatives  of  hydmxylamine 
shows  that  in  these  salts  the  oxygen  atom  must  be  beTween  the 
nitrogen  atom  and  that  of  the  metal  N.O.K;  nitrogen  is, 
however,  never  known  to  act  as  a  monad  element,  and  it  is, 
therefore,  probable  that  the  hyponitrites  have  the  double 
formula 

N.O.K 

.O.K 

This  is  further  confirmed  by  the  fact  that  the  vapour  density 
of  ethyl  hyponitrite  corresponds  to  the  formula  (C9H5)9N202.5 

Hyponitrous  acid  is  further  obtained  by  adding  a  solution  of 
sodium  nitrite  to  one  of  hydroxylamine  sulphate  and  rapidly 
heating  to  60°,  the  following  reaction  taking  place : — 

HO.NH2  +  ON.OH  =  HO.N  :  N.OH  +  H2O. 

1  Divers,  Proc.  Roy.  Soc.  19,  425.  2  Zorn,  Ber.  12,  1509. 

3  Divers  and  Haga,  Journ.  CKem.   Soc.   1885,  i.  561  ;  Dunstan  and  Dymond, 
Journ.  Chem.  Soc.  1887,  i.  646. 

4  Divers  and  Haga,  Journ.  Chem.  Soc.  1889,  i.  760. 

5  Zorn,  Her.  H,  1630. 


NITRIC  OXIDE  505 


The  acid  quickly  splits  up  into  nitrous  oxide  and  water,  but  if 
silver  nitrate  solution  be  at  once  added  a  considerable  quantity 
of  silver  hyponitrite  is  thrown  down.1 

Free  hyponitrous  acid  has  not  been  prepared,  as  when  it  i& 
liberated  from  its  salts  by  addition  of  acids  it  very  rapidly  splits 
up  into  its  anhydride  (nitrous  oxide)  and  water.  Even  the 
salts  split  up  readily  in  a  similar  manner  yielding  nitrous  oxide 
and  alkali.  According  to  Berthelot,  the  reason  that  nitrous 
oxide  does  not  combine  with  bases  to  form  hyponitrites  is  that 
the  difference  between  the  heat  of  formation  of  hyponitrous 
acid  in  solution  (-57*4  cal.)  and  that  of  nitrous  oxide 
(— 20'6  cal.)  is  greater  than  the  heat  of  neutralization  of  the 
acid. 

An  aqueous  solution  of  hyponitrous  acid  may  be  obtained  by 
the  action  of  the  calculated  amount  of  dilute  hydrochloric  acid 
on  the  silver  salt;  it  is  colourless  and  strongly  acid,  and  is 
stable  towards  acids  and  alkalis  even  on  boiling.  On  titration 
with  potash  in  presence  of  either  phenolphthalei'n  or  litmus  it 
remains  acid  until  the  acid  salt  is  formed  and  then  becomes 
alkaline,  but  it  does  not  expel  carbon  dioxide  from  the  alkaline 
carbonates.  In  acid  solution  it  is  converted  by  potassium  per- 
manganate quantitatively  into  nitric  acid,  but  in  alkaline  solu- 
tion yields  nitrous  acid.  It  neither  decolorises  iodine  nor 
liberates  the  latter  from  solutions  of  potassium  iodide.2 

With  lead  acetate  solution  the  salts  give  a  white  precipitate 
which  becomes  dense  and  yellow  after  a  time,  and  with  silver 
nitrate  a  yellow  precipitate,  which  dissolves  in  acids,  but  is 
reprecipitated  by  ammonia.  The  acid  solution  does  not  colour 
ferrous  sulphate,  but  on  addition  of  strong  sulphuric  acid  the 
black  coloration  characteristic  of  nitric  oxide  is  observed. 


NITROGEN  DIOXIDE,  OR  NITRIC  OXIDE,  NO  =  29-82. 

281  This  gas  was  first  observed  by  Van  Helmont,  who 
included  it  under  the  term  gas  sylvestre.  It  was  afterwards 
more  fully  investigated  by  Priestley,  who  named  it  nitrous  air 
(see  "  Historical  Introduction  "). 

Preparation. — The  gas  is  formed  when  nitric  acid  acts  on  certain 
metals  such  as  copper,  silver,  mercury,  zinc.  &c.,  as  also  upon 

1  Wislicenus,  Ber.  26,  772  ;  Thum,  Monatsh.  14,  294. 

2  Thum,  Monatsh   14,  294, 


506  THE  NON-METALLIC  ELEMENTS 


phosphorus  and  some  other  easily  oxidizable  substances.  It  is 
usually  prepared  by  dissolving  copper  foil  or  copper  turnings  in 
nitric  acid  of  specific  gravity  1'2,  washing  the  gas  by  passing  it 
through  water  and  caustic  soda,  and  collecting  it  over  cold 
water  in  the  pneumatic  trough.  The  reaction  occurring  in  this 
case  is  expressed  as  follows  : — 

3Cu  +  8HNO3  =  2NO  +  3Cu(N08)2  +  4H2O. 

The  gas  thus  obtained  is,  however,  not  pure,  as  it  invariably 
contains  free  nitrogen  and  nitrous  oxide,  the  quantity  of  the 
latter  gas  increasing  with  the  amount  of  copper  nitrate  which 
is  formed.1  In  order  to  obtain  a  much  purer  gas  the  nitric  oxide 
must  be  passed  into  a  cold  concentrated  solution  of  ferrous  sulphate 
or  chloride,  with  which  it  forms  a  singular  compound,  to  be 
described  hereafter,  and  which  dissolves  in  water  with  formation 
of  a  deep  blackish-brown  solution.  On  heating  this  solution, 
nitric  oxide  is  given  off,  but  this  gas  still  contains  one-500th  of 
its  volume  which  is  not  absorbable  by  ferrous  salts.2  The  almost 
pure  gas  can  also  be  obtained  by  heating  ferrous  sulphate  with 
nitric  acid,  or  by  heating  a  mixture  of  ferrous  sulphate  and  sodium 
nitrate  with  dilute  sulphuric  acid  ;  or  by  allowing  a  concentrated 
solution  of  sodium  nitrite  to  drop  into  a  hydrochloric  acid 
solution  of  ferrous  sulphate  or  chloride.3  The  pure  gas  may  be 
prepared  by  acting  on  mercury  with  a  mixture  of  sulphuric  and 
nitric  acids,4  a  reaction  which  has  been  made  use  of  by  Lunge 
in  his  nitrometer  for  the  estimation  of  the  oxides  of  nitrogen 
in  sulphuric  acid. 

Properties. — Nitric  oxide  is  a  colourless  gas  having  a 
specific  gravity  of  1*039  (Berard).  It  was  first  liquefied  by 
Cailletet  in  November,  1877,  by  exposing  the  gas  to  a  pressure  of 
104  atmospheres  at  a  temperature  of  —  11°.  It  forms  a  colourless 
liquid  which  boils  under  7T2  atmospheres  pressure  at  —  93°'5 
(critical  temperature),  and  at  —  153°'6  at  atmospheric  pressure.5 
When  mixed  with  an  excess  of  oxygen  it  yields  almost  entirely 
nitrogen  peroxide,  but,  according  to  Lunge,  when  nitric  oxide 
is  in  excess,  nitrogen  trioxide  is  also  formed  ;  with  an  excess 
of  oxygen  and  moisture  it  is  chiefly  converted  into  nitric  acid, 
whilst  with  even  an  excess  of  oxygen  in  presence  of  concentrated 

1  Ackworth,  Journ.  Ohem.  Soc.  1875,  828. 

2  Leduc,  Compt.  Rend.,  116,  323.  3  Thiele,  Annalen,  253,  246. 
4  Emich,  Monatsh.  13,  73.                     5  Olszewski,  Compt.  Rend.  100,  940. 


NITRIC  OXIDE  507 


sulphuric  acid  it  yields  solely  nitrosylsulphuric  acid  (p.  516), 
and  no  nitrogen  peroxide  or  nitric  acid.1  On  exposure  to  air, 
this  gas  at  once  combines  with  the  atmospheric  oxygen,  with 
evolution  of  heat  and  formation  of  red  fumes  of  higher  oxides. 
No  combination  occurs  when  the  perfectly  dry  gas  is  mixed 
with  dry  oxygen.2 

Nitric  oxide  is  extremely  stable  when  heated,  not  being  com- 
pletely decomposed  until  the  temperature  of  melted  platinum  is 
reached.3  Its  formation  from  its  elements  is  attended  by  absorption 
of  heat  (p.  232),  and  when  exposed  to  the  shock  from  an  ex- 
plosion of  mercuric  fulminate,4  it  is  resolved  into  its  elements. 
This  property  is  also  shown  by  other  substances  which  are 
formed  with  absorption  of  heat  such  as  carbon  bisulphide  and 
acetylene. 

If  a  spiral  of  iron  wire  be  heated  to  redness  in  this  gas  by 
means  of  a  galvanic  current,  the  iron  burns  brilliantly  so  long  as 
any  nitric  oxide  remains  undecom posed,  and  after  the  combustion 
the  residual  gas  is  found  to  consist  of  nitrogen  exactly  equal  in 
volume  to  one-half  of  the  gas  employed. 

When  a  stream  of  the  gas  is  passed  over  heated  potassium  this 
metal  takes  fire  and  burns  brilliantly ;  whereas  metallic  sodium, 
even  wiien  heated  with  a  spirit-lamp,  remains  unaltered  in  the 
gas.  Phosphorus  also  burns  with  a  dazzling  brilliancy  in  nitric 
oxide,  but  only  when  it  is  brought  into  the  gas  already  brightly 
burning.  The  flame  of  feebly  burning  phosphorus,  as  well  as  that 
of  sulphur  and  of  a  candle,  are  on  the  other  hand  extinguished 
on  plunging  them  into  nitric  oxide,  because  the  temperature  of 
these  flames  is  not  sufficiently  high  to  decompose  this  gas  into 
its  elementary  constituents.  If  a  few  drops  of  carbon  bisul- 
phide be  poured  into  a  long  glass  cylinder  filled  with  nitric 
oxide  vapour,  and  the  cylinder  well  shaken  so  that  the*  vapour 
of  the  bisulphide  is  well  mixed  with  the  gas,  the  mixture 
burns  with  a  splendid  blue  and  intensely  luminous  flame,  which 
is  characterised  by  its  richness  in  the  violet  or  chemically  active 
rays.  So  intense  is  this  light  for  the  violet  and  ultra-violet 
rays,  that  a  lamp  in  which  the  two  gases  are  burnt  has  been 
constructed  for  the  use  of  photographers.5 

The  name  nitrogen  dioxide  has  been  given  to  this  gas  because 
for  the  same  quantity  of  nitrogen  it  contains  twice  as  much 

1  Ber.  18,  1384.  2  Baker,  Proc.  Chem.  Soc.  1893,  129. 

3  Enrich,  Monatsh.  13,  78.  Berthelot,  Compt.  Rend.  93,  613. 

5  Sell,  Ber.  7,  1522. 


508  THE  NON-METALLIC  ELEMENTS 

oxygen  as  nitrous  oxide  or  nitrogen  monoxide.  Its  density, 
however,  which  remains  constant  down  to  -  700,1  is  about  14,  and 
the  gas  has  therefore  the  formula  NO,  and  consequently  possesses 
a  simpler  constitution  than  nitrous  oxide.  The  chemical  and 
physical  properties  of  nitric  oxide  bear  out  this  view.  Thus,  it 
does  not  condense  under  circumstances  which  effect  the  lique- 
faction of  nitrous  oxide  ;  it  is  also  much  more  stable  than  this 
latter  gas,  so  that  it  follows  a  law  which  we  find  to  hold  good 
with  regard  to  analogous  gaseous  bodies,  viz.  that  those  possess- 
ing the  simpler  constitution  are  much  less  easily  condensible, 
and  much  less  easily  decomposable,  than  those  of  more  compli- 
cated constitution. 


NITROGEN  TRIOXIDE  OR  NITROUS  ANHYDRIDE,  N2O3.  =  75*52, 

282  When  starch,  sugar,  arsenious  anhydride  and  other  easily 
oxidisable  bodies  are  heated  with  nitric  acid,  red  fumes  are 
given  off,  which  consist  of  a  varying  mixture  of  nitric  oxide, 
nitrogen  trioxide  and  nitrogen  peroxide,  the  relative  propor- 
tions varying  with  the  strength  of  the  acid  employed.  The 
largest  proportion  of  trioxide  is  obtained  by  acting  on  arsenious 
anhydride  with  nitric  acid  of  specific  gravity  1*35,  or  on  starch 
with  an  acid  of  specific  gravity  I'.S.S.1  A  red  gas  is  thus  formed, 
which  on  passing  through  a  freezing  mixture  condenses  to  a 
deep  blue  mobile  liquid,  and  does  not  solidify  at  -  90°. 

During  the  past  few  years  doubts  have  arisen  as  to  whether 
nitrogen  trioxide  exists  in  the  gaseous  condition.  Witt,2 
Ramsay,  and  others  3  maintain  that  the  red  fumes  having  the 
composition  N2O3  consist  in  reality  of  a  mixture  of  equal 
volumes  of  NO  and  N%02  (or  N2O4,  see  p.  513).  Ramsay  and 
Cundall  have  shown  that  when  nitrogen  peroxide  and  nitric 
oxide  are  mixed  over  mercury  no  contraction  of  volume  takes 
place,  as  would  be  the  case  if  any  nitrogen  trioxide  were  formed, 
and  they  conclude  from  the  density  of  the  gas  itself  that  it  does 
not  contain  any  N?O3.  Lunge  on  the  other  hand  has  urged 
that  whereas  nitric  oxide  when  mixed  with  an  excess  of  oxygen 
yields  nitrogen  peroxide  almost  entirely,  the  red  fumes  having 
the  composition  N2O3  even  when  mixed  with  a  large  excess  of 

1  Dacconio  and  V.  Meyer,  Annalen,  240,  326. 

2  Lunge,  Ber.  H,  1641. 

3  Witt,  Ber.   H,  756  ;  12,  2188  ;  Ramsay  and  Cundall,   Journ.   Chem.   Soc. 
1885,  i.  187,  672  ;  Geuther,  Annalen,  245,  96. 


NITROUS  ACID  509 


oxygen  at  temperatures  varying  from  4°  to  150°  are  only  con- 
verted to  a  comparatively  small  extent  into  the  peroxide  ;  hence. 
he  concludes  that  the  dissociation  of  the  trioxide  into  dioxide 
and  peroxide  is  only  a  partial  one.  He  further  makes  the 
objection  to  Ramsay  and  Cundall's  experiment  that  nitrogen 
peroxide  is  not  unacted  upon  by  mercury,  and  that  no  valid 
conclusions  can  be  drawn  from  experiments  made  with  gases 
confined  by  that  liquid.1 

Ramsay  concludes  that  even  liquid  N2O3  dissociates  partially 
at  as  low  a  temperature  as  —  90°,  and  can  only  exist  dissolved  in 
<tn  excess  of  nitrogen  peroxide.2 

NITROUS  ACID,  HNO2. 

283  Nitrogen  trioxide  dissolves  in  ice-cold  water,  giving  rise 
to  a  beautiful  blue  liquid,  which  contains  nitrous  acid,  as  shown 
in  the  following  equation  :  — 


0 
NOj          Hj       -  H     P 

Nitrous  acid  is  not  known  in  the  pure  state,  it  being  a  very 
unstable  substance,  which  even  in  aqueous  solution  rapidly 
undergoes  decomposition  when  warmed,  giving  rise  to  nitric 
,acid  and  nitric  oxide  gas  ;  thus  :  — 

3HNO,  =  HNO3  +  2NO  +  H2O. 

This  reaction  is,  however,  according  to  Veley,  a  reversible  one, 
nitric  oxide  yielding  with  nitric  acid  and  water  a  small  quantity 
of  nitrous  acid.3  The  salts  of  this  acid,  or  the  nitrites,  are,  on 
the  contrary,  very  stable  bodies.  They  are  not  only  formed  by 
the  action  of  the  acid  upon  oxides,  but  also  by  the  reduction  of 
nitrates  and  by  the  oxidation  of  ammonia.  Thus,  for  instance, 
potassium  nitrite,  KNO2,  is  formed  either  by  fusing  saltpetre, 
•or,  more  easily,  by  heating  this  salt  with  lead  or  copper  ; 
thus  :  — 

2KN03  =  2KN02  +  O2. 

It  is  also  formed  to  some  extent  by  the  action  of  sunlight  on  a 
sterilized  solution  of  potassium  nitrate.4  Nitrites  also  occur  in 

1  Lunge,  Ber.  U,  1229,  1641  ;  12,  357  ;  15,  495  ;  18,  1376. 

3  Journ.  Chem.  Soc.  1890,  i.  597.  3  Proc.  Roy.  Soc.  52,  23. 

4  Laurent,  Bull.  acad.  Belg.  [3],  21,  337. 


510  THE  NON-METALLIC  ELEMENTS 

nature.  Thus  the  atmosphere  contains  small  quantities  of 
ammonium  nitrite,  and  traces  of  nitrites  have  been  detected  in 
the  juices  of  certain  plants  (Schonbein).  All  the  normal  nitrites 
are  soluble  in  water,  and  most  of  them  soluble  in  alcohol.  The 
silver  salt  is  the  nitrite  which  is  most  difficultly  soluble  in  cold 
water,  crystallizing  out  in  long  glittering  needle-shaped  crystals 
when  the  hot  aqueous  solution  is  cooled.  The  nitrites  deflagrate 
when  thrown  on  to  glowing  carbon,  as  do  the  nitrates.  They 
can,  however,  be  distinguished  from  the  latter  salts  by  the 
action  of  dilute  acids,  which  produce  an  evolution  of  red  fumes 
from  the  nitrites  but  not  from  the  nitrates. 

In  a  similar  way  aqueous  solutions  of  the  neutral  nitrites 
become  of  a  light  brown  colour  when  mixed  with  a  solution  of 
ferrous  sulphate,  and  this  colour  deepens  to  a  dark  brown  on  the 
addition  of  acetic  acid.  In  order  to  detect  the  presence  of  a 
nitrite  in  dilute  solution,  iodide  of  potassium,  starch  paste,  and 
dilute  nitric  acid  are  added.  The  latter  acid  sets  free  the  nitrous 
acid,  and  this  instantly  decomposes  the  iodide  with  liberation  of 
iodine.  As,  however,  other  oxidising  agents  act  in  a  similar 
way,  a  small  quantity  of  potassium  permanganate  solution  is 
added  to  another  portion  of  the  liquid ;  if  a  nitrite  be  really 
present,  the  colour  of  the  permanganate  solution  will  be  at 
once  destroyed. 

In  the  case  of  the  presence  of  nitrites  in  very  small  quantities, 
as  in  certain  waters,  Fresenius  recommends  the  distillation  of 
the  water  previously  acidified  with  acetic  acid,  the  first  few  drops 
of  the  distillate  being  allowed  to  fall  into  a  solution  of  iodide 
of  potassium  and  starch,  to  which  a  small  quantity  of  sulphuric 
acid  has  been  added.  Another  very  delicate  reagent  employed 
for  the  detection  of  nitrites  in  water  is  meta-diamido-benzene, 
which  gives  in  sulphuric  acid  solution  a  brown  coloration  with 
nitrites  (Griess). 


NITROSYL  CHLORIDE,  NOC1.  =  65'01. 

284  This  chloride  of  nitrous  acid  is  formed  by  the  direct 
union  of  nitric  oxide  and  chlorine,  as  well  as  by  the  action  of 
phosphorus  pentachloride  upon  potassium  nitrite ;  thus  : — 

PC15  +  NOOK  =  NOC1  +  KC1  +  POC13. 
It  is  likewise  formed  together  with  free  chlorine  when  a  mixture 


NITROSYL  BROMIDE  511 


of  hydrochloric  and  nitric  acids,  the  so-called  aqua  regia,  is 
slowly  heated  ;  thus  :  — 

HNO3  +  3HC1=  NOC1  +  C12  +  2H2O. 

In  order  to  obtain  the  chloride  in  the  pure  state,  a  mixture  of  one 
volume  of  nitric  acid  of  specific  gravity  1*42  and  four  volumes 
of  hydrochloric  acid  of  specific  gravity  1*16  is  gently  warmed, 
the  gases  which  are  evolved  being  first  dried  by  passing  through 
a  chloride  of  calcium  tube,  and  then  led  into  strong  sulphuric 
acid.  The  chlorine  and  hydrochloric  acid  gases  thus  -escape, 
whilst  nitrosyl  sulphate,  SO4H(NO),  a  body  to  be  described 
later  on,  is  formed.  As  soon  as  the  sulphuric  acid  is  saturated, 
the  liquid  is  heated  with  an  excess  of  perfectly  dry  sodium 
chloride,  when  nitrosyl  chloride  is  evolved  ;  l  thus  :  — 


S04  +  NaCl  =  S04  +  NOC1. 

Nitrosyl  chloride  is  an  orange-yellow  gas,  the  colour  of  which 
is  quite  different  from  that  of  chlorine.  It  liquefies  readily 
when  passed  through  a  tube  surrounded  by  a  freezing  mixture, 
forming  a  deep  orange  limpid  liquid  which  boils  about  —  8°. 
This  substance  combines  with  many  metallic  chlorides,  forming 
peculiar  compounds,  whilst,  brought  into  contact  with  basic 
oxides,  it  is  decomposed  with  formation  of  a  nitrite  and 
chloride  ;  thus  :  — 

NOC1  +  2KOH  =  KNO2  +  KC1  +  H2O. 

NITROSYL  BROMIDE,  NOBr.  =  109.18. 

285  In  order  to  prepare  this  compound,  nitric  oxide  is  led 
into  bromine  at  a  temperature  of  —7°  to  —15°  as  long  as  it  is 
absorbed.  In  this  way  a  blackish-brown  liquid  is  obtained, 
which  begins  to  decompose  at  the  temperature  of  —  2°,  nitric 
oxide  being  evolved.  If  the  temperature  is  allowed  to  rise 
to  +  20°,  a  dark-brownish  red  liquid  remains  behind,  which 
has  the  composition  NOBr3  ;  and  this  is  also  formed  when 
bromine  is  saturated  with  nitric  oxide  at  the  ordinary 
atmospheric  temperature.  Nitrosyl  tribromide,  NOBr3,  is 
volatilized  when  quickly  heated,  almost  without  decomposition, 
but  if  it  is  slowly  distilled  it  decomposes  into  its  constituents 
(Landolt). 

1  Tilden,  Journ.  Ckem.  Soc.  1860,  630. 


512 


THE  NON-METALLIC  ELEMENTS 


NITROGEN  TETROXIDE,  OR  NITROGEN  PEROXIDE,  NO2,  OR  N2O4. 

286  The  red  fumes  which  are  formed  when  nitric  oxide  comes 
into  contact  with  oxygen  or  air,  consist  chiefly  of  nitrogen 
peroxide.  If  one  volume  of  dry  oxygen  be  mixed  with  two 
volumes  of  dry  nitric  oxide  and  the  red  fumes  produced  led 
into  a  tube  surrounded  by  a  freezing  mixture,  the  peroxide 
condenses  in  the  tube  either  as  a  liquid  or  in  the  form  of 
-crystals. 

Nitrogen  peroxide  is  also  formed  by  the  decomposition 
which  many  nitrates  undergo  when  heated,  and  is  usually 
prepared  by  strongly  heating  lead  nitrate  in  a  retort  of  hard 


FIG.  151. 

glass,  as  shown  in  Fig.  151,  when  the  following  decomposition 
occurs : — 

2Pb(N08)2  =  2PbO  +  4N0.2  +  O. 

This  mode  of  preparation  is,  however,  not  very  convenient, 
and  a  considerable  loss  of  material  occurs,  as  the  oxygen  which 
is  evolved  carries  away  some  quantity  of  the  peroxide  even 
when  the  tube  into  which  the  fumes  are  led  is  plunged  into 
a  freezing  mixture. 

The  following  method  is  free  from  the  above  objections; 
arsenious  oxide  (white  arsenic)  in  the  form  of  small  lumps  is 
placed  in  a  flask  and  covered  with  ordinary  nitric  acid,  or 


UNIVERSITY 


NITROGEN  PEROXIDE  513 


according  to  Cundall,1  with  a  mixture  of  nitric  acid  of  specific 
gravity  1*5  and  half  its  weight  of  sulphuric  acid  ;  the  red  fumes, 
which  are  given  off  in  quantity  on  gently  heating,  are  led  into 
a  receiver  surrounded  by  a  freezing  mixture,  where  a  mixture 
of  trioxide  and  tetroxide  of  nitrogen  collects.  The  mixture  is 
freed  from  the  trioxide  by  the  addition  of  strong  nitric  acid 
and  a  large  quantity  of  phosphorus  pentoxide,  the  tetroxide  is 
then  poured  off  from  the  syrupy  layer,  and  distilled. 

287  Properties. — Nitrogen  tetroxide  is  a  liquid  at  the  ordinary 
atmospheric  temperatures;  at  —  10°'l  it  solidifies  to  a  mass  or 
colourless  crystals.  Slightly  above  this  temperature  the  liquid 
compound  is  also  colourless,  but  when  warmed  above  this,  it 
first  becomes  of  a  pale  greenish  yellow,  then  at  +10°  it  attains 
a  decided  yellow  colour,  whilst  at  15°  it  becomes  orange-coloured, 
and  at  higher  temperatures  it  assumes  a  still  darker  tint.  The 
absorption  spectrum  of  gaseous  nitrogen  tetroxide  is  a  cha- 
racteristic band  spectrum  which  has  been  mapped  by  Brewster 
and  Gladstone. 

Liquid  nitrogen  tetroxide  has  a  specific  gravity  of  T49  at  0° 
and  boils  at  22°,  forming  a  reddish  brown  vapour,  possessing  a 
very  strong  and  unpleasant  smell.  When  the  temperature  of  the 
gas  is  raised,  the  colour  becomes  darker  arid  darker,  until  at 
last  it  appears  almost  black  and  opaque.  This  is  well  shown 
by  sealing  some  of  the  gaseous  tetroxide  in  two  wide  glass 
tubes,  and  heating  one  for  some  little  time  in  the  flame  of  a 
lamp  whilst  the  other  remains  at  the  ordinary  temperature. 

These  remarkable  changes  in  appearance  cannot  be  recognized 
by  any  equally  striking  changes  in  the  absorption  spectrum  of 
the  gas,  although  it  is  probable  that  the  peroxide  exists  in  two 
distinct  forms,  as  indeed  has  been  shown  by  the  variations 
which  its  density  exhibits.  Thus,  at  low  temperatures  the 
density  corresponds  to  the  formula  N2O4,  and  at  higher  ones 
to  N02.  The  density  of  the  vapour  at  different  temperatures 
was  found  by  Playfair  and  Wanklyn  2  to  be  as  follows  : — 

Temperature.  Density.  Corresponding  mole- 

Air  =  1 .  cular  weight. 

97°-5 1783 51-5 

24°-5 2-520 — 

ll°-3 2-645 

4°-2 2-588 74-8 

1  Journ.  Chem.  Soc.  1891,  i.  1076.        a  Journ.  Chem.  Soc.  1863,  156. 
34 


514  THE  NON-METALLIC  ELEMENTS 

The  density  required  for  the  compound  NO2  is  1*585,  that 
for  the  compound  N2O4,  the  double  of  this,  or  3'17.  It  will 
be  seen  that  the  numbers  obtained  all  lie  between  these  two, 
the  density  at  the  .highest  temperature  not  lying  far  from 
the  lower  number,  whilst  those  found  for  the  lower  tempera- 
tures correspond  more  nearly  to  the  density  of  the  substance 
N<,O4.  From  these  facts  we  draw  the  conclusion  that  at  low 
temperatures  the  molecule  of  the  compound  is  represented  by 
the  formula  N2O4,  and  the  density  46,  but  that  as  the  tempera- 
ture rises  a  gradual  change  in  the  density  of  the  gas  takes 
place,  one  molecule  of  N2O4  splitting  up,  or  becoming  dissociated, 
into  two  of  NO2,  possessing  a  density  of  23.  From  Deville  and 
Troost's  l  experiments  the  following  percentage  composition  of 
the  gas  at  various  temperatures  has  been  calculated  : — 

X02  N204 

at    26°7     .     .     .     20-00  .     .     .  80*00 

„     60°2     .     .     .     50-04  .     .     .  49-96 

„  100°-1     .     .     .     79-23  .     .     .  20-77 

„  135°        .     .     .     98-96  ...  1-04 

„  140°        .     .     .  100-00  .     .     .  0-00 

We  thus  see  that  at  140°  the  black  vapour  consists  entirely 
of  the  simpler  molecule  NO2. 

Liquid  nitrogen  peroxide  when  diluted  with  chloroform  under- 
goes a  similar  dissociation,  the  amount  of  dissociation  increasing 
with  dilution  and  rise  of  temperature.2 

Nitrogen  tetroxide  is  decomposed  by  cold  water  with  pro- 
duction of  nitric  and  nitrous  acids  ;  thus  : — 

2N02+  H20  .  HN03  +  HN02. 

This  decomposition,  however,  only  occurs  at  low  temperatures 
and  with  small  quantities  of  water.  When  nitrogen  peroxide 
is  added  to  an  excess  of  water  at  the  ordinary  temperature,  the 
nitrous  acid  is  at  once  decomposed  into  nitric  oxide  and  nitric 
acid  ;  thus  : — 

3N02  +  H20  =  2HN03  +  NO. 

If  oxygen  be  present  at  the  same  time,  this  of  course  com- 
bines with  the  nitric  oxide,  forming  nitrogen  peroxide,  which  is 
again  decomposed  by  water,  and  in  this  way  the  peroxide  may 

1  Jahresb.  1867,  p.  177.  2  Cundall,  Journ.  Chem.  Soc.  1891,  i.  1076. 


NITROGEN  PEROXIDE 


515 


be  completely  transformed  into  nitric  acid.  In  order  to  exhibit 
this  decomposition,  as  well  as  to  show  the  formation  of  the 
peroxide  and  nitric  acid,  the  following  apparatus  may  be  used 
(Fig.  152) :  The  upper  vessel,  containing  a  little  water,  is  filled 
with  nitric  oxide,  and  is  connected  with  the  lower  vessel  by  the 
tube  (A),  which  is  drawn  out  to  a  point.  This  lower  vessel 
contains  water,  coloured  with  blue  litmus.  If  oxygen  be  now 


FIG.  152. 


led  slowly  into  the  upper  vessel  by  means  of  the  tube  (B),  red 
fumes  are  formed,  which  are  absorbed  by  the  water.  A  vacuum 
is  thus  produced,  and  the  coloured  water  rises  in  the  form  of  a 
fountain  into  the  upper  vessel,  and  becomes  coloured  red.  If 
the  nitric  oxide  be  perfectly  pure,  and  if  care  be  taken  that  the 
oxygen  is  allowed  to  enter  but  slowly  towards  the  end  of  the 
operation,  the  whole  of  the  upper  vessel  may  be  filled  with 
water. 


516  THE  NON-METALLIC  ELEMENTS 


COMPOUNDS  OF  NITROGEN  WITH  SULPHUR 
AND  SELENIUM. 

NITROGEN  SULPHIDE,  N2S2. 

288  This  body  is  obtained,  together  with  other  compounds,  by 
the  action  of  dry  ammonia  upon  chloride  of  sulphur,  or  upon 
thionyl  chloride.  It  is  a  yellow  powder,  crystallizing  from 
solution  in  bisulphide  of  carbon  in  yellowish-red  rhombic 
prisms.  When  heated  to  120°  it  becomes  darker  coloured,  and 
emits  vapours  which  attack  the  mucous  membrane  violently. 
Heated  to  about  135°,  it  sublimes  in  the  form  of  fine  yellowish- 
red  crystals,  and  at  158°  begins  to  melt  with  evolution  of  gas. 
At  160°  it  decomposes  rapidly  with  the  evolution  of  light  and 
heat,  and  on  percussion  detonates  very  violently. 

When  sulphur  dichloride  is  added  to  a  solution  of  nitrogen 
sulphide  in  bisulphide  of  carbon,  several  different  compounds 
are  produced,  according  to  the  quantity  of  the  chloride  of 
sulphur  which  is  present.  If  this  last  body  be  present  in  excess, 
a  yellow  crystalline  precipitate  is  formed,  having  the  com- 
position (N9S.2)SC19,  which,  on  heating,  sublimes  in  needles.  If 
after  this  compound  has  separated  out,  more  nitrogen  sulphide 
solution  be  added,  the  yellow  powder  is  changed  into  a  red 
substance,  (N2S2)2SC12,  and  this  again,  on  addition  of  more 
nitrogen  sulphide,  or  on  heating,  yields  a  compound  (N2S2)3SC12, 
which  forms  a  beautiful  yellow  powder,  unalterable  in  contact 
with  the  air  (Fordos  and  Gelis). 

Nitrogen  Selenide,  N2Se2.  —  This  compound  is  formed  by  the 
action  of  ammonia  on  selenium  tetrachloride.  It  is  an  orange  - 
yellow  mass,  which,  on  heating  to  200°  as  well  as  by  slight 
pressure,  detonates  strongly. 

NlTROSULPHONIC   ACID,  S0  - 


289  This  compound,  which  is  frequently  termed  nitrosyl- 
sulphuric  acid,  is  commonly  known  as  the  crystals  of  the  leaden 
chambers  or  nitrosyl  sulphate,  produced  during  the  process  of 
the  manufacture  of  sulphuric  acid  whenever  the  supply  of 
steam  is  insufficient  to  produce  sulphuric  acid.  Nitrosulphonic 


.       NITROSULPHONIC  CHLORIDE  517 

acid  is,  however,  best  prepared  by  acting  upon  sulphur  dioxide 
with  concentrated  nitric  acid  ;  thus  :  — 


For  this  purpose,  dry  sulphur  dioxide  is  led  into  cold  fuming 
nitric  acid  until  the  mass  becomes  syrupy.  The  semi-solid 
mass  is  then  drained  on  a  dry  porous  plate  over  sulphuric  acid. 

Another  mode  of  preparing  the  substance  is  to  pass  the 
vapour  of  nitrosyl  chloride  into  sulphuric  acid  :  — 

NOC1  +  SO,  =  S02  °  +  HCL 


It   is   likewise   formed   when   nitrogen   trioxide   is  brought 
together  with  sulphuric  acid. 


N203  +  2S02 

Nitrogen  peroxide  also  yields  the  same  product  with  sulphuric 
acid  together  with  nitric  acid. 

The  substance  obtained  by  these  reactions  crystallizes  in 
four-sided  rhombic  prisms,  or  sometimes  in  tabular  or  nodular 
crystalline  masses,  which  begin  to  melt  at  30°,  with  evolution 
of  vapour. 

The  crystals  dissolve  in  small  quantities  of  cold  water  without 
any  evolution  of  gas,  forming  a  blue  liquid,  which  contains 
sulphuric  and  nitrous  acids  : 

qo  J  O.NO     H  \  0      QO  J  OH  a  TTO 

2  1  OH     +  H  I  (    =  b°2  i  OH  4 

Nitrosulphonic  acid  dissolves  in  concentrated  sulphuric  acid 
without  decomposition,  and  this  solution  can  be  distilled. 


NITROSULPHONIC  CHLORIDE,  SO2 1 


290  This  chloride  is  formed  by  the  direct  union  of  sulphur 
trioxide  and  nitrosyl  chloride,  and  by  the  action  of  thionyl 
chloride  on  silver  nitrate.1  It  forms  a  white  crystalline  mass, 
which  melts  on  heating,  with  separation  of  nitrosyl  chloride.  It 

1  Thorpe,  Journ.  Cham.  Soc.  1882,  i.  297. 


518  THE  NON-METALLIC  ELEMENTS 

is  decomposed  in  contact  with  water  into  sulphuric,  hydrochloric, 
and  nitrous  acids  (Weber). 

S02  j  O.NO. 

NITROSULPHONIC    ANHYDRIDE,  £  O 

SOJO.NO. 

291  When  nitrosulphonic  acid  is  heated,  it  decomposes  into 
water  and  this  anhydride,  but  the  latter  substance  cannot  be 
obtained  in  a  pure  state  in  this  way,  as  sulphuric  acid  is  formed 
by  the  further  decomposition  of  nitrosulphonic  acid,  and  these 
bodies  cannot  be  separated,  since  they  both  boil  nearly  at  the 
same  temperature  (Michaelis).1  It  is,  however,  easy  to  obtain 
this  anhydride  by  passing  dry  nitric  oxide  into  sulphur  trioxide, 
so  long  as  it  is  absorbed,  and  warming  the  solution  until  the 
boiling  point  of  the  liquid  is  nearly  reached  ;  thus  :  — 

S02JONO 

3S020  +  2NO  =  O         +  S02. 

SCUONO. 

The  same  compound  is  formed  when  sulphur  dioxide  acts 
upon  nitrogen  peroxide,  as  well  as  when  the  next  compound  to 
be  described  is  heated.  Nitrosulphonic  anhydride  crystallizes 
in  hard  colourless  quadratic  prisms,  melts  at  217°  to  form  a 
yellow  liquid,  becomes  darker  on  further  heating,  and  distils 
over  unchanged  at  about  360°.  The  compound  dissolves  readily 
in  strong  sulphuric  acid,  forming  nitrosulphonic  acid. 


SO2     ONO 

OXYNITROSULPHONIC   ANHYDRIDE,  J:  O 

S02  1  ON02. 

292  When  sulphur  trioxide  and  nitrogen  peroxide  are  brought 
together  in  the  cold,  the  above  compound  separates  out  as  a 
white  crystalline  mass,  which  on  heating  gives  off  oxygen,  and 
forms  the  anhydride  last  described  (Weber). 


S00-[O.N02. 
>O 


NlTROXYPYROSULPHURIC  ACID, 

S02JON02. 

When  sulphur  trioxide  and   nitric  acid  are  mixed  together 
in  the  cold,  a  thick  oily  liquid  is  formed,  from  which  the  above 

1  Ber.  7,  1075. 


NITBILOSULPHONIC  ACID  519 

•compound  crystallizes  out  under  certain  conditions  of  concen- 
tration. It  is  soluble  without  decomposition  in  warm  dilute 
nitric  acid,  and  on  cooling  the  liquid,  crystals  of  this  substance 
separate  out,  containing  one  molecule  of  water  of  crystallization. 

SULPHONIC  ACIDS  OF  AMMONIA  AND  HYDROXYLAMINE. 

293  In  the  year  1845  a  number  of  crystalline  salts  derived  from 
acids  containing  both  nitrogen  and  sulphur  were  obtained  by 
Fremy1  by  the  interaction  of  nitrites  and  sulphites.  He  only 
determined  their  empirical  formula?,  but  they  were  further  in- 
vestigated by  Glaus,2  who  showed  that  they  are  sulphonic  acids, 
that  is,  compounds  containing  the  group  SO2OH.  He,  however, 
wrongly  regarded  a  number  of  them  as  sulphonic  acids  of  the 
hypothetical  substance  NH5,  whereas  the  later  investigations  of 
Berglund  3  and  especially  of  Raschig  4  have  shown  that  all  these 
compounds  are  sulphonic  acid  derivatives  of  ammonia,  hydroxyl- 
amine  and  the  hypothetical  dihydroxylamine,  NH(OH)2. 

Potassium  nitrilosulphonate ,  N(SO3K)3  +  2H2O. — This  com- 
pound, which  was  termed  by  Claus  potassium  trisulphammonate, 
is  obtained  by  adding  an  excess  of  a  concentrated  solution  of 
potassium  sulphite  to  a  similar  solution  of  potassium  nitrite, 
and  separates  out  as  a  crystalline  precipitate  which  may  be 
recrystallized  from  alkaline  solution.  The  reaction  which  takes 
place  may  be  represented  as  follows : — 

KN02  +  3K2SO3  +  2H2O  =  N(S08K)S  4  4KOH. 

The  reaction  is  remarkable  inasmuch  as  one  of  the  most 
powerful  alkalis  is  produced  by  the  interaction  of  two  neutral 
salts. 

Potassium  nitrilosulphonate  crystallizes  with  2  mols.  H20, 
and  forms  thin  silky  rhombic  needles,  which  are  decomposed 
in  the  following  manner  by  boiling  water : — 

N(S03K)3  +  2H2O  =  NH2SO3H  +  K2S04  +  KHSO4. 

Hence  only  two-thirds  of  the  sulphur  is  precipitated  as  barium 
sulphate  on  adding  barium  chloride  to  the  hot  solution,  amido- 
sulphonic  acid,  which  will  be  subsequently  described,  not  being 
affected  by  barium  chloride. 

1  Annalen,  56,  315.  2  Annalen,  152,  336,  351  ;  158,  52,  194. 

3  Lunds  Universitas  Arskrift,  12  and  13.  4  Annalen,  241,  161 . 


520  THE  NON-METALLIC  ELEMENTS 

Potassium  imidodisulphonate,  NH(SO3K)2. — When  the  nitrilo- 
sulphonate  is  boiled  for  only  a  short  time  with  water,  the 
reaction  does  not  proceed  so  far  as  shown  in  the  above  equation, 
the  chief  product  being  potassium  imidodisulphonate  : — 

N(S03K)3  +  H2O  =  NH(SO3K)2  +  KHS04. 

This  is  more  readily  obtained  by  moistening  the  nitrilo- 
sulphonate  with  dilute  sulphuric  acid,  allowing  to  stand  for  a 
day,  and  recrystallizing  from  dilute  ammonia.  It  forms  mono- 
symmetric  crystals,  and  is  decomposed  by  boiling  water  into 
amidosulphonic  acid  and  potassium  sulphate. 

A  large  number  of  other  imidodisulphonates  have  been  pre- 
pared by  Divers  and  Haga ; l  the  most  interesting  of  these  are 
ammonium  imidodisulphonate,  NH(S03NH4)2,  and  basic  am- 
monium  imidodisulphonate,  NH4.N(SO3NH4)2,  which  were 
first  prepared  by  Rose  in  1834  by  the  action  of  ammonia  on 
sulphur  trioxide  and  termed  parasulphatammon  and  sulphatam- 
mon  respectively. 

Free  imidodisulphonic  acid  has  only  been  obtained  in  aqueous 
solution. 

Amidosulphonic  actW,NH2.S03H. — This  acid  is,  as  already  men- 
tioned, the  final  product  of  the  action  of  boiling  water  on  the 
salts  just  described.  It  is  a  very  stable  compound  sparingly 
soluble  in  water,  and  forms  colourless  rhombic  prisms.  Its 
potassium  salt,  NH2.SO3K,  crystallizes  in  very  similar  forms. 

Hydroxylaminedisulphonic  acid,  HO.N.(S03H)2. — This  acid  is 
not  known  in  the  free  state,  but  its  potassium  salt  is  formed  by 
the  action  of  potassium  hydrogen  sulphite  on  potassium  nitrite 
in  aqueous  solution  at  0°.  The  reaction  proceeds  more  readily 
with  the  sodium  salts,  which  are  mixed  in  the  proportions 
required  by  the  equation  : — 

NaN02  +  2NaHS03  =  HO.N(S03Na)2  +  NaOH. 

The  sodium  salt  is  however  readily  soluble  in  water,  and  it 
is  therefore  best  converted  into  the  potassium  salt  by  adding 
an  equivalent  quantity  of  a  concentrated  potassium  chloride  so- 
lution. Potassium  hydroxylaminedisulphonate,  HO.N(SO3K)2, 
2H2O,  separates  out  on  standing  in  compact  crystals,  which  may 
be  readily  separated  from  the  small  quantity  of  slender  needles 
of  potassium  nitrilosulphonate  simultaneously  produced.  It 

1  Journ   Ghem.  Soc.  1892,  i.  943. 


DIHYDROXYLAMINESULPHONIC  ACID  521 

forms  monosymmetric  crystals,  which  are  very  slightly  soluble 
in  water ;  when  boiled  with  the  latter  or  with  acids  it  is  first 
converted  into  hydroxylaminesulphonic  acid  and  then  into 
hydroxylamine,  which  combines  with  the  sulphuric  acid  simul- 
taneously produced  to  form  the  sulphate.  This  method  is  now 
employed  for  the  preparation  of  hydroxylamine  (p.  475). 

Hydroxylaminesulphonic  acid,  HO.NH.SO3H,  like  amido- 
sulphonic  acid,  is  stable  in  the  free  state  and  forms  a  syrupy 
liquid.  The  potassium  salt,  HO.NH.S03K,  is  best  obtained  by 
the  partial  hydrolysis  of  the  disulphonate  with  hot  water,  and 
may  be  purified  by  quick  recrystallization  from  that  liquid. 
When  treated  with  alkalis  it  yields  salts  of  hyponitrous  acid 
(p.  504). 

NitrosoliydroxylaminesidpTionic  acid,  or  Dinitrososulplionic  acid, 
H2SN2O5. — The  alkali  salts  of  this  acid,  which  are  colourless 
crystalline  substances,  are  formed  when  nitric  oxide  is  passed 
into  an  alkaline  solution  of  a  sulphite.  The  free  acid  is 
unknown,  as  all  acids,  even  carbonic  acid,  decompose  the  salts 
into  sulphates  and  nitrous  oxide.  When  reduced  with  sodium 
amalgam  the  salts  are  converted  into  a  hyponitrite  and  sulphite, 
from  which  it  appears  that  they  contain  the  group  N2O2  com- 
bined with  both  potassium  and  the  sulphonic  acid  group. 
These  reactions  are  satisfactorily  explained  by  the  constitutional 
formula  suggested  by  Raschig,  viz.  ON.N(OK)SO3K,  according 
to  which  the  potassium  salt  is  a  basic  salt  of  hydroxylamine- 
sulphonic acid,  in  which  one  of  the  hydrogen  atoms  has  been 
replaced  by  the  group  NO. 

Dihydroxylamincsulphonic  acid,  (HO)2N.S03H,is  unknown,but 
its  basic  potassium  salt  is  sometimes  formed  in  the  preparation 
of  hydroxylaminedisulphonates  from  potassium  nitrite  and 
hydrogen  sulphite.  It  is  a  crystalline  unstable  substance  which 
is  decomposed  by  acids  with  evolution  of  nitrous  oxide.  It  is 
probably  identical  with  Freiny's  potassium  sulphazinite. 

Fremy  also  described  a  salt  under  the  name  potassium 
sulphazinate,  which  according  to  Raschig  is  identical  with 
another  derivative  of  dihydroxylamine,  obtained  by  gradually 
adding  a  solution  of  potassium  hydrogen  sulphite  with  con- 
stant shaking  to  a  solution  of  potassium  nitrite,  till  the 
solution  forms  a  magma  of  crystals.  It  has  the  composition 
KHN2O3(SO3K)2,  decomposes  on  heating  with  evolution  of  red 
fumes,  gives  off  nitrous  oxide  on  addition  of  acids,  and  decom- 
poses in  aqueous  solution  into  hydroxylaminedisulphonic  acid  and 


THE  NON-METALLIC  ELEMENTS 


potassium  nitrite.  It  is  probably  formed  from  two  potassium 
salts  of  dihydroxylaminesulphonic  acid  with  elimination  of 
water  in  the  following  manner  :  — 


S03K.N  +  gQ>N.S03K  =  S03K.N.O.N.S03K  +  H20 

OK  OH. 

In  addition  to  the  foregoing  Raschig1  has  investigated  a 
series  of  salts  known  as  the  sidphazotinates,  which  appear  to  be 
derived  from  an  acid  having  the  constitution 

(S03H)2:N/°\N:(S03H)2. 


NATURE  OF  THE  REACTION  BETWEEN  NITRITES  AND 
SULPHITES. 

294  Nitrous  acid  is  usually  supposed  to  have  the  constitutional 
formula  O  :  N.OH,  but  it  is  not  unlikely  that  in  aqueous  solution 
it  may  further  combine  with  another  molecule  of  water,  having 
then  the  constitution  N(OH)3.     The  reactions  which  occur  with 
sulphites  may  then  be  readily   explained  as  consisting  in  the 
substitution  step  by  step  of  the  group  S03H  for  the  hydroxyl 
group  with  elimination  of  water  and  the  successive  formation  of 
dihydroxylaminesulphonic  acid,  hydroxylaminedisulphonic  acid, 
and  nitrilotrisulphonic  acids,  or  rather  their  potassium  salts,  as 
shown  in  the  following  equations : — 

N(OH)3  +  H.S03K  =  (HO),N.S08K  +  H2O. 
(HO)2N.S03K  +  HS03K  =  HO.N(SO,K),  +  H2O. 

HO.N(S03K)2  +  HS03K  =  N(S03K)3  +  H2O. 

AMIDE  AND  IMIDE  OF  SULPHURIC  ACID. 

295  When  ammonia  is  allowed  to  act  on  a  solution  of  sul- 
phuryl  chloride  in  chloroform,  it  yields  a  mixture  of  the  amide 
and  imide  of  sulphuric  acid,  S02(NH2)2  and  SO2:  NH,  together 
with  another  substance  which  is  probably  imidosulphurylamide 
NH(S02NH2)2. 

S09C12  +  4NH3  =  2NH4C1  +  SO2(NH2)2 
S02C12  +  3NH3  =  2NH4C1  +  SO2NH. 

1  Annalen,  241,  232. 


THE  ATMOSPHERE  523 


The  three  compounds  are  separated  from  one  another  and 
from  the  ammonium  chloride  simultaneously  formed  by  taking 
advantage  of  the  different  properties  of  their  silver  salts,  which 
after  separation  are  converted  into  the  free  amides  by  the  action 
of  hydrochloric  acid. 

Sulphaniide,  S02(NH2)2,  forms  large  colourless  crystals,  which 
are  extremely  soluble  in  water,  and  on  heating  soften  at  75° 
and  melt  at  81°.  It  is  converted  by  alkalis  into  salts  of 
amidosulphonic  acid  (sulphaminic  acid),  NH2.S(X,OH  (p.  520), 
and  yields  a  silver  compound,  SO2(NHAg)2,  which  is  a  white 
amorphous  powder. 

Sulphimide,  S00NH,  is  formed  together  with  sulphamide  in 
the  manner  stated  above,  and  is  also  obtained  when  the  latter  is 
heated  above  its  melting  point.  It  has  not  been  obtained  pure  ; 
its  aqueous  solution  is  fairly  stable  at  the  ordinary  tempera- 
ture, but  on  boiling  is  converted  into  ammonium  hydrogen 
sulphate.  Its  silver  compound,  SO2NAg,  crystallizes  from  hot 
water  in  long  needles,  which  require  500 — 600  parts  of  cold 
water  for  their  solution.1 


THE   ATMOSPHERE. 

296  The  term  air  was  used  by  the  older  chemists  in  the 
general  sense  in  which  we  now  employ  the  word  gas,  to  signify 
the  various  kinds  of  aeriform  bodies  with  which  chemistry  has 
made  us  familiar.  At  the  present  day,  however,  we  confine  the 
signification  of  the  word  air  to  the  ocean  of  aeriform  fluid  or 
the  atmosphere  (CLT/JLOS,  vapour,  and  crfalpa,  a  sphere)  at  the 
bottom  of  which  we  live  and  move. 

Of  the  existence  of  an  invisible  gaseous  envelope  lying 
above  the  solid  mass  of  the  earth's  crust,  we  become  aware 
by  the  resistance  offered  to  our  bodies  when  we  pass  rapidly 
from  place  to  place,  as  well  as  by  the  effects  produced  by  the 
motion  of  the  particles  of  the  atmosphere  which  we  term  wind. 
The  most  convincing  proof  of  the  existence  of  the  air  is  how- 
ever given  by  showing  that  air  has  weight.  This  can  readily 
be  done  by  hanging  a  large  thick-walled  glass  globe,  closed 
with  a  cork  through  which  a  tube  passes,  furnished  with  a 
stopcock,  on  to  one  end  of  the  beam  of  a  balance,  and  placing 

1  Traube,  Ber.  26,  607. 


524 


THE  NON-METALLIC  ELEMENTS 


weights  in  the  opposite  pan  until  the  arrange- 
ment is  in  equilibrium.  On  exhausting  the 
globe  by  placing  it  in  communication  with  an 
air-pump,  and  again  weighing  the  globe  par- 
tially freed  from  air,  the  weight  will  be  seen  to 
be  considerably  less  than  that  which  it  possessed 
before  the  evacuation. 

A  knowledge  of  the  composition  of  the  at- 
mosphere forms  the  beginning  of  the  present 
epoch  of  chemical  science,  experiment  having 
shown,  in  opposition  to  the  older  views,  that 
the  air  is  not  a  simple  body,  but  consists  mainly 
of  two  different  kinds  of  air  or  gases,  oxygen 
and  nitrogen. 

Although  some  of  the  ancients,  especially 
Vitruvius,  appear  to  have  held  the  view  that 
the  air  possesses  weight,  yet  it  is  to  Torricelli 
that  we  owe  the  first  distinct  proof  that  this  is 
the  case.  In  the  year  1640  a  Florentine  pump- 
maker  observed  that  his  lift-pumps  would  not 
raise  water  to  a  height  greater  than  thirty-two 
feet,  and  consulted  his  great  townsman  Galileo 
as  to  the  cause  of  this  phenomenon.  Galileo 
does  not  appear  to  have  given  the  correct  solu- 
tion, as  he  compared  the  water  column  to  an 
iron  rod  hung  up  by  one  end,  which,  when  long 
enough,  will  at  last  break  with  it  own  weight. 
Torricelli,  however,  in  1643,  made  an  experi- 
ment which  gave  the  true  explanation  of  the 
pump-maker's  difficulty.  Filling  with  mercury 
a  glass  tube  three  feet  in  length,  and  closed 
at  one  end,  but  open  at  the  other,  he  closed 
the  open  end  with  his  finger  and  inverted  the 
tube  in  a  basin  filled  with  mercury.  The  mer- 
cury then  sank  in  the  tube  to  a  certain  level, 
whilst  above  this  level  there  was  an  empty 
space,  which  is  still  called  the  Torricellian 
vacuum.  Above  the  mercury  in  the  basin 
was  water,  and  Torricelli  then  raised  the  tube 
FIG.  153.  so  that  the  open  end  came  into  the  water. 

The  mercury  then  flowed  out  and  the  water 
rushed  up,  completely  filling  the  tube.  Fig.  153  represents 


THE  ATMOSPHERE  525 


the  actual  tubes  employed  by  Torricelli,  photographed  from 
the  original  instruments  placed  in  the  Science  Loan  Exhibition 
at  South  Kensington.  The  rise  of  mercury  or  water  in  a  vacuous 
tube  is  caused  by  the  pressure  of  the  atmosphere ;  the  water 
is,  however,  13*5  times  lighter  than  the  mercury,  and  hence  the 
column  of  the  former  liquid  which  is  supported  by  the  atmo- 
spheric pressure  is  13 '5  times  as  high  as  that  of  the  latter 
liquid.  Thus  was  the  barometer  discovered,  though  this  name 
was  first  made  use  of  by  Boyle.1 

Hearing  of  Torricelli's  discovery  Blaise  Pascal  resolved  to  put 
this  theory  to  a  further  test.  If,  argued  he,  the  suspension  of  the 
mercury  in  the  barometric  tube  is  due  to  the  pressure  or  weight 
of  the  air,  the  mercurial  column  must  sink  when  the  barometer 
is  taken  to  the  top  of  a  mountain,  owing  to  the  pressure  on  the 
mercury  being  lessened.  Unable  to  try  this  experiment  himself 
Pascal  instructed  his  brother-in-law,  Perier,  to  ascertain  whether 
this  is  so  or  not,  and  on  September  19,  1648,  Perier  took  a 
barometer  to  the  summit  of  the  Puy-de-D6me,  and  showed 
that  the  mercury  sank  as  he  ascended,  proving  conclusively  the 
correctness  of  Torricelli's  explanation. 

A  simple  arrangement  enables  us  to  reproduce  this  experiment 
in  the  lecture-room ;  a  barometer  tube  filled  with  mercury  is 
inverted  over  mercury  contained  in  a  trough,  when  the  mercury 
will  be  seen  to  sink  to  a  certain  level,  the  space  above  this  being 
vacuous.  A  tubulated  receiver  furnished  with  a  tight-fitting 
caoutchouc  stopper  is  then  brought  over  the  tube,  and  the  air 
pumped  out  from  the  interior  of  the  receiver.  As  the  pressure 
of  the  air  is  by  degrees  removed,  the  level  of  mercury  in  the 
tube  will  gradually  become  lower,  until  at  last  it  will  nearly, 
but  not  quite,  reach  the  level  of  the  mercury  in  the  trough.  On 
opening  the  stopcock  the  air  will  rush  in,  and  the  level  of  the 
mercury  in  the  tube  will  rise  until  it  has  attained  its  former 
elevation. 

297  Following  then  the  laws  of  gravitation,  the  air  forms 
part  of  the  earth's  body,  and  accompanies  the  solid  and  liquid 
portions  in  their  axial  and  orbital  motions.  The  absolute  height 
to  which  the  atmosphere  extends  above  the  earth's  surface  has 
not  been  ascertained  with  accuracy.  As  its  density  is  not 

1  See  New  Experiments  on  Cold,  published  1664-5,  Boyle's  Works  (Edn.  1772), 
vol.  ii.  p.  487-  "The  barometer,  if  to  avoid  circumlocutions  I  may  so  call 
the  whole  instrument,  wherein  a  mercurial  cylinder  of  29  or  30  inches  is  kept 
.suspended,  after  the  manner  of  the  Torricellian  experiment." 


526  THE  NON-METALLIC  ELEMENTS 

uniform,  but  diminishes  as  the  distance  from  the  earth's  surface 
increases,  the  exact  point  at  which  the  atmosphere  terminates 
is  difficult  to  determine.  The  height  is  certainly  not  uniform, 
inasmuch  as  owing  to  the  variation  of  gravitation  at  the  poles 
and  at  the  equator,  and  owing  also  to  the  different  velocities  of 
rotation  as  well  as  to  changes  of  temperature,  a  column  of  polar 
air  is  considerably  shorter  than  a  column  of  equatorial  air. 

The  atmospheric  pressure  at  the  sea's  surface  would  naturally 
be  constant  if  it  were  not  that  owing  to  the  variations  in  the 
solar  radiation,  the  temperature,  and,  therefore,  the  pressure  of 
the  air,  undergoes  frequent  alterations.  These  irregular  varia- 
tions necessitate  our  reading  off  the  height  of  the  barometer 
whenever  volumes  of  gas  have  to  be  measured.  There  is  no 
doubt  that  the  atmosphere  has  a  definite  limit,  and,  from 
observations  of  the  time  during  which  the  twilight  extends  to 
the  zenith,  it  appears  that  the  atmosphere  reaches  in  a  state  of 
sensible  density  to  a  height  of  from  forty  to  forty-five  miles 
above  the  earth's  surface.  The  relation  between  this  height 
and  the  diameter  of  the  earth  may  be  illustrated  by  the  state- 
ment that  if  a  globe  of  one  foot  in  diameter  represents  the 
earth,  a  film  of  air  TV  °f  an  mc^  m  diameter  will  represent  the 
atmosphere. 

If  the  air  were  an  incompressible  fluid,  instead  of  being  an 
elastic  one,  and  if  it  had  throughout  the  density  which  it 
possesses  at  the  sea's  level,  the  height  of  the  atmosphere 
would  be 

10513  X  0760  =  8360  meters,  or  5'204  English  miles. 

As,  however,  the  air  is  elastic,  it  diminishes  in  density  as  the 
distance  from  the  earth's  level  increases  ;  thus  at  a  height  of 
5528  metres,  the  air  expands  to  twice  its  volume ;  whilst  at  a 
height  of  twice  5528  metres  the  density  of  the  air  is  only  J  of 
that  which  it  possesses  at  the  sea's  level.  At  greater  elevations 
the  volume  increases  in  the  following  ratios: — 

Geographical  Miles.  Volume. 

o-ooo  i 

0-587  2 

1-174  4 

1-761  8 

2-348  16 

2-935  32 

3-522  64 


THE  ATMOSPHEKE  527 


According  to  the  corrected  determinations  of  Regnault  (p.  105),. 
one  litre  of  dry  pure  air  at  0°,  and  under  the  pressure  of  760  mm. 
at  the  latitude  of  Paris  weighs  1*29349  gram,  whilst  according  to 
Lasch  l  the  weight  at  Berlin  is  1*293635  gram,  or  almost  exactly 
T^  of  the  weight  of  water.  It  must  be  remembered  that 
since  the  composition  of  air  varies  within  certain  narrow 
limits,  the  weight  of  a  given  volume  of  it  must  also  vary 
slightly. 

The  weight  of  the  air  at  the  level  of  the  sea  in  our  latitude  is 
equal  to  that  of  a  column  of  mercury  at  0°  of  a  height  of 
760  millimetres,  and  this  is  taken  as  the  normal  barometric 
pressure.  Hence,  as  one  cc.  of  mercury  weighs  13*596  grams, 
the  pressure  exerted  by  the  air  on  one  square  centimetre  of 
surface  at  the  sea's  level  will  be  13*596  X  76  =  1033*3  grams 
(or  nearly  15  Ibs.  on  every  square  inch). 

In  common  with  all  bodies  at  the  earth's  surface,  the 
human  frame  has  to  support  this  weight,  but,  under  ordinary 
circumstances  the  pressure  is  exerted  in  all  directions,  and  it  is 
not  felt.  If,  however,  the  pressure  in  one  direction  be  removed, 
as  when  the  hand  is  placed  over  the  open  end  of  a  cylinder  from 
which  the  air  is  being  pumped  out,  the  weight  of  the  air  is  at 
once  perceived.  As  the  air  follows  Boyle's  law,  its  density 
being  directly  proportional  to  the  pressure  to  which  it  is  sub- 
jected, it  follows  that  when  the  height  above  the  sea  level 
increases  by  equal  intervals,  the  density  of  the  air  decreases  in 
a  geometric  ratio.  The  difference  in  height  of  two  stations  in 
the  same  vertical  line  is  therefore  in  the  ratio  of  the  difference 
between  the  logarithms  of  the  barometric  readings  at  the  two 
stations,  and,  if  the  temperature  of  the  two  stations  be  the  same, 
we  need  only  multiply  the  difference  of  the  logarithms  of  the 
two  readings  (reduced  to  0°  for  the  expansion  of  mercury)  by 
the  number  18363  to  obtain  the  elevation  in  meters. 

The  average  or  mean  annual  temperature  of  the  air,  like  its 
density,  is  not  the  same  throughout  the  mass.  It  diminishes 
as  the  elevation  above  the  earth's  surface  increases,  so  that  at 
a  certain  height,  differing  for  different  latitudes,  a  line  is  reached 
at  which  the  mean  temperature  of  the  air  does  not  rise  above 
the  freezing  point.  This  is  called  the  line  of  perpetual  snow.  At 
latitude  75°  it  reaches  the  sea  level,  in  the  latitude  of  60°  it 
is  found  at  a  height  of  3,818  feet,  whilst  under  the  equator  the 
snow-line  exists  at  a  height  of  15,207  feet  above  the  sea. 

1  Pogg.  Ann.  Erganzungsbd.  3,  321. 


528  THE  NON-METALLIC  ELEMENTS 

The  height  of  the   snow-line  is  also   affected  by  local  causes 
and  is  found  to  vary  considerably  even  in  the  same  latitude. 

Owing  to  the  unequal  heating  effect  produced  by  the  sun  on 
different  portions  of  the  earth's  surface,  great  variations  are 
observed  in  the  temperature  of  the  atmosphere  in  different 
places,  and  these  give  rise  to  those  motions  of  the  atmosphere 
which  are  termed  winds.  Winds  may  either  be  caused  by 
local  alterations  of  temperature  confined  to  narrow  limits,  as 
with  the  land  and  sea-breezes  of  our  coasts,  or  they  may  be 
produced  by  a  general  unequal  diffusion  of  heat  over  the  sur- 
face of  the  globe,  as  with  the  so-called  trade-winds,  which  are 
caused  by  the  temperature  of  the  air  in  the  equatorial  zones 
being  higher  than  that  of  the  air  in  the  polar  regions. 

298  Air  was  first  liquefied  by  Cailletet,1  and  liquid  air  has 
since  been  more  closely  investigated  by  Wroblewski 2  and  Dewar.3 
It  has  not  a  constant  boiling  point,  as  the  nitrogen  boils  off  more 
rapidly  than  the  oxygen,  the  ebullition   proceeding  somewhat 
roughly.     Dewar  has  also  succeeded  in  obtaining  air  in  the  solid 
state,  but  it  is  as  yet  doubtful  whether  this  is  solid  throughout, 
or  consists  of  a  mixture   of  solid   nitrogen  and  liquid  oxygen, 
the  latter  gas  not  having  yet  been  solidified  when  in  the  pure 
condition 

THE  COMPOSITION  OF  THE  ATMOSPHERE. 

299  Atmospheric  air,  in  addition    to   oxygen  and   nitrogen, 
contains  as  normal  constituents,  aqueous  vapour,  carbon  dioxide, 
ammonia,  and   perhaps   ozone.     Other   gases   and   vapours  do 
indeed  occur  in  different   places,  under  a  variety  of  circum- 
stances, and  in  varying  quantities.     Furthermore  certain  sub- 
stances, such  as  common    salt,  ammonium  nitrate,  and  some 
other  salts,  occur  as  finely-divided  solid  particles,  together  with 
other  minute  floating  particles  of  animal,  vegetable,  and  mineral 
origin. 

The  discovery  of  the  composition  of  the  atmosphere  has 
been  described  in  the  Historical  Introduction.  We  saw  there 
that  we  owe  to  Cavendish  the  first  exact  determination  of  the 
relation  existing  between  the  two  important  constituents 
oxygen  and  nitrogen.4  "  During  the  last  half  of  the  year 

1  Compt.  Rend   85,  1270. 

2  Monatsh.  6,  204  ;  Compt.  Rend.  102,  1010. 

3  Proc.  Royal  Institution. 

4  Phil.  Trans.  1783,  p.  106.      "An  Account  of  a  new  Eudiometer." 


COMPOSITION  OF  THE  ATMOSPHERE  529 

1781,"  he  writes,  "  I  tried  the  air  of  near  sixty  different  days 
in  order  to  find  whether  it  was  more  phlogisticated  at  one 
time  than  another,  but  found  no  difference  that  I  could  be  sure 
of,  though  the  wind  and  weather  on  these  days  was  very 
various,  some  of  them  being  very  fair  and  clear,  others  very 
wet,  and  others  very  foggy."  1  This  result  was  founded  on  a 
long  series  of  experiments,  for  seven  or  eight  analyses  of  air 
collected  on  the  same  day  were  made  by  different  processes,  so 
that  altogether  Cavendish  cannot  have  made  fewer  than  400 
determinations  of  the  composition  of  atmospheric  air.  Ex- 
periments were  likewise  made  to  see  whether  London  air 
differed  from  that  of  the  country,  and  slight  differences  were 
sometimes  found  in  favour  of  London  air,  in  Marlborough 
Street,  sometimes  in  favour  of  country  air,  in  Kensington.  On 
taking  a  mean  of  the  numbers  no  difference  whatever  was 
perceptible,  the  result  of  all  his  experiments  being  that  100 
volumes  of  air  contain  20'83  parts  by  volume  of  dephlogisticated 
air  or  oxygen. 

The  constant  results  thus  obtained  by  Cavendish  led  several 
chemists,  such  as  Prout,  Dobereiner,  and  Thomson,  to  maintain 
that  the  air  is  a  chemical  compound  of  one  volume  of  oxygen 
with  four  volumes  of  nitrogen.  Against  this  assumption  John 
Dalton  protested,2  insisting  that  the  air  is  merely  a  mechanical 
mixture  of  constant  composition,  and  contending  that,  because 
nitrogen  is  lighter  than  oxygen,  the  relative  amounts  of  the  two 
gases  must  vary  at  different  heights  above  the  earth's  surface, 
the  oxygen  diminishing  and  the  nitrogen  increasing  as  we 
ascend.  This  view  was,  however,  shown  to  be  erroneous  by 
Gay-Lussac  and  Thenard,  who  collected  air  in  a  balloon  at  an 
elevation  of  7,000  metres,  and  found  it  to  contain  exactly  the 
same  proportional  quantity  of  oxygen  as  that  collected  at  the 
same  time  in  Paris  and  analysed  in  the  same  way.  Their 
results  have  since  been  corroborated  by  the  more  exact  in- 
vestigations of  Brunner,  who  analysed  the  air  collected  at  the 
top  and  at  the  bottom  of  the  Faulhorn  and  found  in  each  case 
exactly  the  same  proportion  between  the  oxygen  and  the 
nitrogen. 

The  very  important  question  whether  the  composition  of  the 
air  undergoes  variation,  under  varying  conditions  of  time  and 
place  and  what  is  the  percentage  of  oxygen  it  contains  was 

1  Phil.  Trans.  1783,  p.  126. 

2  Manchester  Memoirs,  2nd  series,  vol  i.  p.  244. 
35 


530  THE  NON-METALLIC  ELEMENTS 

further  investigated  by  many  chemists.  Gay-Lussac  and 
Humboldt  in  Paris  found  that  the  air  contained  from  20'9  to 
21 '1  per  cent,  of  oxygen;  Davy  in  London  obtained  from  20'8 
to  21-1  per  cent.;  Thomson  in  Glasgow  211,  and  Kuppfer  in 
Kasan  21 '1  per  cent,  of  oxygen.  It  was,  however,  necessary 
that  more  accurate  methods  of  analysis  should  be  employed. 

300  Two  processes  are  now  used,  viz.  (1)  measurement  of 
the  volumes  of  the  component  gases,  (2)  determination  of  their 
weight.  The  first  of  these  processes,  or  the  eudiometric  method, 
has  been  practised  by  Regnault,  Bunsen,  Lewy,  and  Angus 
Smith,  whilst  the  latter  method  has  chiefly  been  used  by 
Dumas  and  Boussingault  in  their  celebrated  research  carried 
out  in  the  year  1841.1  In  their  analyses  of  air  by  the  latter 


FIG.  154. 

method,  the  two  French  chemists  employed  an  apparatus 
shown  in  Fig.  154.  The  large  balloon  (v)  was  rendered  as 
perfectly  vacuous  as  possible,  and  brought  in  connection  with 
the  vacuous  tube  (a  b),  containing  metallic  copper  reduced 
by  means  of  hydrogen,  and  placed  in  a  furnace  in  which  it 
could  be  heated  to  redness  by  means  of  charcoal  or  gas.  At 
the  other  end  this  tube  was  connected  with  the  tubes  c  and 
B,  and  with  the  bulbs  A  ;  these  last  contained  caustic  potash,  and 
the  others  pumice-stone  moistened  with  strong  sulphuric  acid 
for  the  purpose  of  taking  up  all  moisture  from  the  air,  as  well 
as  of  abstracting  from  it  the  whole  of  its  ammonia  and  carbon 
dioxide.  As  soon  as  the  tube  a  b  had  been  heated  to  dull  red- 

1  Ann.  Chim.  Phys.  [3],  3,  257. 


COMPOSITION  OF  THE  ATMOSPHERE  531 

ness  the  stopcocks  r  were  opened  so  as  to  let  the  air  pass  slowly 
into  the  apparatus.  Entering  the  tube  a  I,  and  coming  in 
contact  with  the  glowing  copper,  the  whole  of  the  oxygen  was 
absorbed,  only  the  nitrogen  going  over  into  the  vacuous  globe. 
When  the  experiment  was  complete,  the  stopcocks  were  closed, 
the  tube  and  balloon  detached  from  the  apparatus  and  each 
accurately  weighed.  Both  tube  and  balloon  were  then  again 
rendered  vacuous,  and  weighed  a  third  time.  The  tube  contain- 
ing the  copper  oxide,  which  had  been  weighed  empty  to  begin 
with,  is  thus  weighed  full  of  nitrogen,  after  which  the  nitrogen 
is  withdrawn  by  the  air-pump  and  its  weight  again  determined. 
The  following  details  of  an  actual  experiment  may  illustrate  the 

method : — 

Grams. 
Vacuous   tube    containing   copper   before   the  ) 

experiment j 

Tube  filled  with  nitrogen  and  copper  after  the  j     651-415 

experiment ) 

Vacuous  tube  after  experiment 651*346 

Balloon  containing  nitrogen  at  19°  and  under  (   1403-838 

pressure  7627  mm ) 

Balloon,  vacuous,  at  19°'4  and  7627  mm.    .     .      1391-554 

Hence  the  weight  of  oxygen  is  found  to  be  3*680  grams,  whilst 
the  weight  of  nitrogen  in  the  balloon  was  12*304  grams,  and  the 
weight  of  nitrogen  in  the  tube  0'069,  giving  a  total  of  12*373 
grams. 

Or  the  percentage  composition  is  : — 

Oxygen 22'92 

Nitrogen 77'08 

100-00 


Dumas  and  Boussingault  used  two  balloons,  with  which  they 
obtained  the  following  results  : — 

Percentage  by  weight  of  oxygen. 
With  small  With  large 

1841.  balloon.  balloon. 

April  27 .     22-92     .     .     .     22'92 

,,28 23-03     .     .     .     23-09 

29.  23-03  .     23-04 


Mean  .  22*993  23'016 


532  THE  NON-METALLIC  ELEMENTS 

The  mean  of  these  determinations  is  23*005  parts  by  weight  of 
oxygen  and  76' 99 5  of  nitrogen. 

These   experiments   were   subsequently    repeated    by   other 

chemists,  with  the  following  results : — 

Mean  Percentage  by 
weight  of  oxygen. 

Lewy,  in  Copenhagen,  1841 22'998 

Stas,  in  Brussels,  1842 23100 

Marignac,  in  Geneva,  1842 22'990 

According  to  these  experiments,  therefore,  the  air  contains 
about  23  parts  by  weight  of  oxygen  and  77  parts  by  weight  of 
nitrogen. 

If  the  composition  of  air  by  weight  be  calculated  from  its 
volumetric  composition  by  the  aid  of  the  known  specific 
gravities  of  nitrogen  and  oxygen,  the  numbers  obtained  are : — 

Oxygen 23'2 

Nitrogen 76'8 


100-00 

and  these  numbers  have  also  been  obtained  experimentally  by 
a  gravimetric  method  by  Leduc.1 

301  This  method,  although  capable  of  giving  exact  results, 
requires  large  apparatus  and  good  air-pumps  and  balances.  It  can 
only  be  carried  on  in  a  laboratory,  and  it  necessitates  the  employ- 
ment of  large  volumes  of  air.  The  eudiometric,  or  volumetric 
method  is  less  difficult  and  tedious,  so  that  the  determinations 
may  be  repeated  by  thousands  and  require  a  much  smaller  volume 
of  air.  This  method  is  liable  to  so  small  an  experimental 
error  that,  with  an  observer  well  skilled  in  its  use,  it  never 
reaches  the  T^^TF^  Part>  and  may  sometimes  not  exceed  the 
woW  Part  of  the  whole. 

The  process  depends  on  the  well-known  fact  that  when 
oxygen  and  hydrogen  gases  are  mixed  and  fired  by  an  electric 
spark,  they  unite  to  form  water  in  the  proportion  of  one 
volume  of  the  former  to  two  volumes  of  the  latter.  The  volume 
of  the  liquid  water  formed  (p.  249)  is  so  small  in  proportion 
to  that  of  the  constituent  gases  that,  except  in  cases  of  very 
exact  estimations,  it  may  be  altogether  neglected.  Hence  if 

1  Compt.  Rend.  113,  129. 


COMPOSITION  OF  THE  ATMOSPHERE  533 

we  bring  together  a  given  volume  of  air  and  more  hydrogen 
than  is  needed  to  combine  with  the  oxygen  in  it,  and  if  an 
electric  spark  be  passed  through  the  mixture,  one-third  of  the 
observed  contraction  will  be  due  to  the  oxygen.  In  order  to 
obtain  by  this  method  exact  results l  a  very  carefully  calibrated 
eudiometer  is  employed  1  meter  long  and  about  O025  wide, 
and  the  observations  are  conducted  in  a  space  within  which 
the  changes  of  temperature  are  as  small  and  as  gradual  as 
possible. 

The  air  for  these  determinations  is  collected  either  in  small 
flasks  of  about  half  a  liter  in  capacity,  the  necks  of  which  have 
been  previously  elongated  before  the  blowpipe,  or  in  long  tubes, 
the  ends  of  which  have  been  drawn  out.  Inside  the  flask  or 
tube  a  small  piece  of  fused  chloride  of  calcium  is  placed  for 
the  purpose  of  absorbing  the  ammonia,  and  a  similar  piece  of 
fused  caustic  potash  to  absorb  the  carbonic  acid,  and  both 
substances  are  allowed  to  crystallize  on  the  sides  of  the  glass 
by  the  addition  of  a  drop  of  water.  It  is  quite  necessary  to 
remove  the  carbonic  acid  of  the  air  previous  to  analysis.  Even 
if  the  quantity  present  were  only  0'05  per  cent,  of  the  total 
volume,  it  would  produce  an  appreciable  error  in  the  oxygen 
determination,  carbonic  acid  when  exploded  with  an  excess  of 
hydrogen  in  presence  of  the  detonating  mixture  of  oxygen 
and  hydrogen  being  decomposed  into  an  equal  volume  of 
carbonic  oxide,  while  an  equal  volume  of  hydrogen  disappears, 
so  that  the  volume  of  combined  gas  would  be  0'05  per  cent, 
too  large. 

In  carrying  out  this  method  with  exactitude  the  same  mani- 
pulatory precautions  have  to  be  attended  to  which  have  already 
been  described  in  connection  with  the  determination  of  the 
composition  of  water  (p.  246). 

The  following  numbers'2  show  the  approximation  obtained 
by  Bunsen  in  two  analyses  of  the  same  sample  of  air  collected 
in  Marburg  on  9th  January,  1 846  : — 

Volume.        Pressure       Temp.     Vol.  at  0° 
at  0°.  C.         and  1  m. 

pressure. 

Air  employed  ....  841 -8  O'olOl  0'3  428'93 
After  addition  of  hydrogen  10517  07137  0*3  74977 
After  the  explosion  .  .  878'8  0'5460  0'3  480'09 

1  For  details  on  these  points  Bunsen's  Gasometry  may  be  consulted. 

2  Bunsen's  Gasometry ,  71. 


534 


THE  NON-METALLIC  ELEMENTS 


Composition  of  Air  in  100  parts  by  volume. 

Nitrogen 79'030 

Oxygen 20'970 


100-000 


Air  employed 859*3 

After  addition  of  hydrogen  1051 '9 
After  the  explosion    .    .    .   870*3 


0-5225 
0-7079 
0-5317 


0-6  448-00 
0-6  743-01 
0-6  461-72 


Composition  of  Air  in  100  parts  ly  volume. 

Nitrogen 79*037 

Oxygen 20'963 

100-000 


Bunsen,  who  has  brought  all  the  processes  of  gas  analysis  to 
a  marvellous  degree  of  perfection,  points  out 1  that  in  normal 
determinations  of  the  composition  of  the  air  still  greater 
precision  may  be  attained  by  repeating,  several  times,  and  at 
regular  intervals,  the  observation  of  the  height  of  the  mercury 
in  the  eudiometer.  From  the  agreement  between  the  reduced 
volumes  which  are  read  off,  the  point  in  the  series  of  observa- 
tions is  found  at  which  the  temperature  has  been  most  constant. 
As  an  example  of  such  an  accurate  analysis  Bunsen  gives  the 
following : — 


- 

Vol. 

Press. 

Temp. 

Vol  at  0°  C.  and  1  meter 
pressure. 

(6fc  0' 
7    0 
8     0 

754-9 
755-0 
755-2 

05045 
0-5046 
0-5047 

15-4 
15-4 
15-5 

360-52  ) 
360-63  >  360  -62 
36070  } 

After  addition    j    Jl     jj 
of  Hydrogen.    )  .  "    J 

904-0 
904-6 
904-9 

0-6520 
0-6521 
0-6518 

15-8 
16-0 
16-0 

557-20  ) 
557-24  \  557  '20 
557-17  ) 

After  the  Ex-    ( 
plosion.             )      5 

732-3 
732-5 
732-7 

0-4781 
0-4777 
0-4777 

16-1 
161 
16-1 

330-64^ 
330-45  V  330  -54 
330  -54  J 

Hence  Nitrogen 79*036  volumes. 

Oxygen 20'964 

100-000 
1  Loc.  cit.  77. 


COMPOSITION  OF  THE  ATMOSPHERE  535 

As  the  result  of  twenty-eight  analyses  Bunsen  found  the 
.average  percentage  of  oxygen  to  be  20 '9  24  volumes,  whilst  the 
lowest  percentage  found  was  20*840. 

Regnault,1  using  a  different  volumetric  method,  has 
analysed  a  very  large  number  of  samples  of  air  collected  in  a 
uniform  manner  in  various  quarters  of  the  globe,  according  to 
instructions  which  he  had  given.  The  error  of  two  analyses 
made  on  the  same  sample  rarely  reached  0*02  per  cent.  In 
more  than  100  samples  of  air  collected  in  or  near  Paris,  Reg- 
nault found  a  maximum  amount  of  20*999,  a  minimum  of 
20*913,  and  a  mean  of  20*96  percentage  of  oxygen.  The  differ- 
ence of  0*086  per  cent,  is  too  large,  according  to  Regnault,  to  be 
due  to  errors  of  experiment,  and  it  must,  therefore,  be  ascribed 
to  variations  in  the  composition  of  the  air  occurring  from  day 
to  day. 

Air  collected  from  other  localities  gave  Regnault  the  following 

results : — 

Percentage  of  oxygen. 

9  samples  from  Lyons  gave 20*918  to  20*966 

30  samples  from  Berlin 20*908    „    20'998 

10  „          „      Madrid 20'916   „   20*982 

23        „          „      Geneva  and  Chamounix  .    .  20*909    „    20*993 
17        „          „      Toulon  Roads  and  Mediter-  ) 

5  „  „  Atlantic  Ocean 20*918  „  20*965 

2  „  „  Ecuador 20*96 

2  „  „  Summit  of  Pichincha    .    .    .  20*949  „  20*988 

2  „  „  Antarctic  Seas 20*86  „  20'94 

The  conclusion  which  Regnault  draws  from  these  determi- 
nations, and  which  all  subsequent  observers  have  confirmed,2 
is,  that  the  atmosphere  shows  perceptible,  though  very  small, 
-alterations  in  the  amount  of  oxygen  at  different  times  and 
in  different  localities.  This  variation  ranges  from  20*9  to  21*0 
per  cent.,  but  from  special  unknown  causes  the  amount  of  oxygen 
seems  sometimes  to  sink  in  tropical  countries  as  low  as  20*4 
per  cent.,  as  was  seen  in  the  Bay  of  Bengal  on  March  8,  1849. 

302  Dr.  R.  Angus  Smith3  has  extended  our  knowledge 
of  the  variations  which  the  percentage  of  oxygen  undergoes 
in  the  air  of  towns  and  that  of  closed  inhabited  spaces.  He 

1  Ann.  Chem.  Phys.  [3],  36,  385. 

2  Morley,  Amer.  Journ.  of  Science,  22,  429  ;  Chem.  News,  45,  283  ;  Hempel, 
Ber.  18,  267,  1800  ;  20,  1864.  8  On  Air  and  Rain,  Longmans,  1872. 


• 20'912  -  20'982 


536  THE  NON-METALLIC  ELEMENTS 

finds  that  the  percentage  of  oxygen  in  air  from  the  sea  shore, 
and  from  Scotch  moors  and  mountains,  is  as  high  as  20*999. 
In  the  free  air  of  towns,  and  especially  during  foggy  weather,  it 
may  sink  to  2082.  In  inhabited  rooms  and  crowded  theatres, 
the  percentage  of  oxygen  may  sink  sometimes  to  20*28  (Lewy), 
whilst  according  to  the  very  numerous  (339)  analyses  of  Angus 
Smith,1  the  percentage  of  oxygen  in  mines  does  not  average 
more  than  20*26.  The  exact  composition  of  the  air  at  high 
elevations  has  been  investigated  by  Frankland,2  who  has  shown 
that,  as  far  as  nitrogen  and  oxygen  are  concerned,  the  composi- 
tion of  the  air  up  to  an  elevation  of  14,000  feet  is  constant, 
and  that  the  variations  exhibited  in  air  collected  at  the  above 
height  fall  within  the  limits  noticed  by  former  experimenters. 

303  That  the  air  is  a  mechanical  mixture  and  not  a  chemical 
combination  of  oxygen  and  nitrogen  is  seen  from  the  following 
facts  : — 

(1)  The  quantities  of  nitrogen  and  oxygen  in  the  air  do  not 
present  any  simple  relation  to  the    atomic    weights   of   these 
elements,  and,  indeed,  the  proportions  in  which  they  are  mixed 
are  variable. 

(2)  On  mixing  oxygen  and   nitrogen  gases   mechanically  in 
the  proportion  in  which  they  occur  in  air,  no  contraction  or 
evolution  of  heat  is  observed,  and  the  mixture  behaves  in  every 
way  like  air. 

(3)  When  air  is  dissolved  in  water,  the  proportion  between 
the  oxygen  and  nitrogen  in  the  dissolved  air  is  quite  different 
from  that  of  the  undissolved  air,  the  difference  being  in  strict 
accordance    with   the   laws  of  gas-absorption  on  the  assump- 
tion that  the  air  is  a  mixture.     When  water  is  saturated  with 
air  at  any  temperature  below  30°,  the  following  is  the  proportion 
of  oxygen  and  nitrogen  contained  in  the    dissolved   and   the 
original  air : — 

Air  dissolved  Air  undissolved 

in  water.  in  water. 

Oxygen      .    .    .  35*1 20*96 

Nitrogen    .    .    .  64*9 79*04 

100-0  100-00 


If  the   air   were   a   chemical    combination   of    oxygen   and 
nitrogen,  such  a  separation  by  solution  would  be  impossible. 

1  On  Air  and  Rain,  p.  106. 

2  Journ.  Chcm.  Soc.  13,  22. 


COMPOSITION  OF  THE  ATMOSPHERE 


537 


(4)  When  liquefied  air  is  allowed  to  boil,  the  nitrogen  passes 
off  much  more  rapidly  than  the  oxygen,  which  could  not  take 
place  if  the  two  gases  were  chemically  combined. 

In  order  to  show  the  composition  of  the  air,  the  apparatus 
Fig.  155  is  often  used.  This  consists  of  a  calibrated  and  divided 
glass  tube  filled  to  a  given  point  with  air  over  mercury.  Into 
this  is  introduced  a  small  piece  of  phosphorus  supported  upon 
a  copper  wire.  Gradually  all  the  oxygen  is  absorbed  and  the 
mercury  rises  in  the  tube.  After  a  while  the  volume  of  residual 
nitrogen  is  read  off,  and,  corrections  having  been  made  for  tem- 


FIG.  155. 


FIG.   156. 


perature  and  pressure,  it  is  found  that  100  volumes  of  the  air 
contain  about  21  volumes  of  oxygen. 

Another  less  exact  but  more  rapid  method  of  exhibiting  the 
same  fact  is  carried  out  by  help  of  the  arrangement  shown  in 
Fig.  156.  In  the  beaker  glass  (c)  is  placed  the  iron  stand  (d) 
carrying  the  iron  cup  (e),  containing  a  small  piece  of  phosphorus. 
Over  this  stand  is  placed  the  tubulated  cylinder  (a).  The  upper 
part  of  this  cylinder  is  graduated  into  five  equal  divisions, 
and  water  is  poured  into  the  beaker  glass  until  the  level  reaches 
the  first  division.  The  phosphorus  in  the  cup  is  ignited  by 
dropping  down  on  it  a  chain  which  has  been  heated  in  a 


538  THE  NON-METALLIC  ELEMENTS 

flame.  The  phosphorus  then  burns,  the  fumes  of  phosphorus 
pentoxide  are  absorbed  by  the  water,  and,  when  the  gas  has 
cooled,  and  the  pressure  been  equalized  by  bringing  the  level  of 
the  water  outside  up  to  that  inside  the  cylinder,  four-fifths  of 
the  original  volume  of  the  air  remain  unabsorbed. 

SUBSTANCES  PRESENT  IN  SMALLER  QUANTITY  IN  THE 
ATMOSPHERE. 

The  carbonic  acid,  aqueous  vapour,  organic  matter,  and  the  other 
constituents  of  the  atmosphere  vary  in  amount  in  different  places 
and  at  different  times  much  more  than  the  oxygen  and  nitrogen. 

304  Atmospheric  Carbonic  Acid. — Reference  has  already  been 
made  to  the  part  played  by  atmospheric  carbon  dioxide  (carbonic 
acid).  The  whole  vegetable  world  depends  for  its  existence  on 
the  presence  of  this  gas,  which  serves,  in  the  sunlight,  as  the 
chief  food  of  plants. 

The  normal  amount  of  carbonic  acid  existing  in  the  air  was 
formerly  supposed  to  be  4  vols.  in  10,000,  but  the  investigations 
of  the  last  25  years  have  shown  that  the  methods  formerly  in 
use  gave  too  high  results  and  that  the  amount  is  nearly  3  vols. 
in  1 0,000.!  This  number  is  the  average  of  a  large  number  of 
analyses  of  country  air,  but  it  appears  that  the  amount  is  slightly 
greater  in  the  night  than  in  the  day,  probably  owing  to  the  in- 
fluence of  vegetation ;  over  the  sea  the  amount  is  also  3  vols.  in 
10,000,  but  no  difference  is  observed  between  the  day  and  night 
values.2  In  dull  cloudy  weather  the  amount  is  somewhat  larger 
than  in  bright  fine  weather,  and  variations  are  also  observed 
according  to  the  direction  of  the  wind,  which  when  it  has  come 
over  a  large  extent  of  land  causes  an  increase  in  the  quantity 
of  carbonic  acid. 

In  large  towns  where  much  coal  is  used  the  amount  of  car- 
bonic acid  may  rise  as  high  at  6'0  and  even  7'0  vols.  per  10,000, 
and  at  high  elevations  the  proportion  of  carbonic  acid  is 
generally,  but  not  invariably  greater  than  at  lower  levels.  In 
closed  inhabited  spaces  the  volume  of  carbonic  acid  proceeding 
from  respiration  and  from  the  combustion  of  illuminating 
materials  is  usually  much  higher  than  in  the  open  air,3  and  the 
proportion  is  much  higher  in  mines  than  in  the  air  above  ground. 

1  For  a  full  discussion  of  the  results  of  the  various  investigators  see  Blochmann, 
Annalen,  237,  39.  2  Thorpe,  Jnurn.  Chem.  Soc.  1867,  189. 

3  Roscoe,  Journ.  Chem.  Soc.  1867,  189. 


ATMOSPHERIC  CARBONIC  ACID  539 

Air  containing  more  than  7'0  vols.  of  carbonic  acid  per 
10,000  is  as  a  rule  harmful  for  respiration;  this  is,  however, 
probably  not  due  to  the  carbonic  acid  itself,  but  to  the  other 
organic  impurities  which  are  formed  along  with  it  especially 
during  respiration,  for  the  deleterious  properties  of  such  air 
remain  after  the  removal  of  the  former  gas. 

As  the  quantity  of  carbonic  acid  serves  as  the  readiest  and 
most  reliable  test  of  the  healthiness  or  otherwise  of  an  atmo- 
sphere, it  becomes  a  matter  of  importance  to  ascertain  its 
amount  with  accuracy.  The  methods  in  use  for  this  purpose  are 
(1)  the  gravimetric  and  (2)  the  volumetric  method.  In  the  first 


FIG.  157. 

of  these  l  the  carbonic  acid  is  absorbed  from  a  known  volume 
of  air,  freed  from  ammonia  and  aqueous  vapour,  drawn  through 
weighed  tubes  containing  caustic  potash.  This  necessitates  the 
passage  of  not  less  than  forty  litres  of  air  drawn  by  means  of 
the  aspirator  (v,  Fig.  157),  filled  with  water,  through  the  tubes,  in 
order  that  a  sufficient  weight  of  carbonic  acid  for  an  exact 
weighing  may  be  obtained.  The  tubes  A  and  B  are  filled  with 
pumice-stone  moistened  with  sulphuric  acid.  The  air  is  thus 
dried  and  freed  from  ammonia,  c  and  D  contain  moist  but  solid 
caustic  potash  for  the  absorption  of  the  carbon-dioxide.  E  and 
F  are  also  drying- tubes,  prepared  like  the  tubes  A  and  B,  and 
1  Saussure,  Pogg.  Ann.  19,  391. 


540  THE  NON-METALLIC  ELEMENTS 

serving  to  hold  back  any  moisture  which  the  dry  gas  might  have 
taken  up  from  the  tubes  c  and  D.  The  following  example  of  a 
determination  made  by  this  plan  illustrates  the  process : — 

Gravimetric  Determination  of  Carbonic  Acid  in  London  Air, 
Feb.  27, 1857. 

Volume  of  aspirated  air  at  8°  and  under  )  __4o.o  r* 

772'5  mm.  of  mercury ) 

Weight  of  absorbed  carbonic  acid  .     .     .     =0'0308  grm. 

Hence  10,000  volumes  of  air  contain  3'7  volumes  of  carbonic 
acid. 

305  A  much  more  convenient  process  is  the  volumetric  one 
proposed  by  Pettenkofer.1  For  this  a  volume  of  about  10  litres 
of  air  only  is  needed,  a  glass  cylinder  closed  by  a  caoutchouc  cap 
being  employed,  and  no  balance  being  required.  The  method 
depends  upon  the  fact  that  a  solution  of  hydrate  of  baryta  of 
known  strength  when  shaken  up  with  a  closed  volume  of  air 
containing  carbonic  acid,  abstracts  the  whole  of  that  carbonic 
acid  from  the  air  with  formation  of  insoluble  carbonate  of 
barium ;  thus : 

Ba(OH)2  +  C02  =  BaC03  +  H20. 

The  quantity  of  baryta  in  excess  which  remains  in  solution,  after 
shaking  up  with  the  air,  is  ascertained  by  adding  to  an  aliquot 
portion  of  the  milky  fluid  a  standard  solution  of  oxalic  acid  or 
sulphuric  acid  until  the  alkaline  character  of  the  baryta  water 
disappears.  When  this  point  is  reached,  the  whole  of  the 
residual  soluble  baryta  has  been  neutralised  by  the  oxalic  acid. 
The  baryta  and  oxalic  acid  solutions  are  made  of  such  a  strength 
that  equal  volumes  exactly  neutralise  each  other,  and  so  that 
1  cc.  of  baryta  solution  will  precipitate  exactly  1  mgrm.  of  car- 
bonic acid.  The  following  example  will  serve  to  explain  this 
process : — 

Volumetric  Determination  of  Carbonic  Acid  in  Manchester  Air, 
Nov.  10,  1873. 

Volume  of  air  employed,  10*80  litres  at  7°  and  under  a  pressure 
of  765  mm.  of  mercury. 

50  cc.  of  standard  baryta  solution  (1  cc.  =  1  mgm.  C02)  was 
1  Journ.  Chem.  Soc.  1858,  292. 


ATMOSPHERIC  MOISTURE  541 

shaken  up  with  this  air.  Of  this,  after  the  experiment,  25  cc. 
needed  22  cc.  of  standard  oxalic  acid  (1  cc.  =  1  mgm.  CO2)  for 
complete  neutralization.  Hence  6  cc.  of  baryta  solution  were 
neutralized  by  the  carbonic  acid  in  10*8  litres  of  air;  or  10,000 
volumes  of  air  contain  2*85  volumes  of  carbonic  acid. 

To  obtain  very  accurate  results  by  Pettenkofer's  method, 
which  is  in  any  case  more  reliable  than  the  gravimetric  method, 
a  number  of  precautions  must  be  taken.  For  these  reference 
may  be  made  to  Blochmann's  paper.1 

306  The  moisture  contained  in  the  air  is  liable  to  much  more 
extensive  changes  than  even  the  carbonic  acid.  Amongst  the 
circumstances  which  affect  the  atmospheric  moisture,  distance 
from  masses  of  water,  and  the  configuration  of  the  land,  seem 
the  most  important.  A  given  volume  of  air  cannot  take  up 
more  than  a  certain  quantity  of  aqueous  vapour  at  a  given 
temperature,  and  then  the  air  is  said  to  be  saturated  with 
moisture.  The  weight  in  grams  of  water  capable  of  being  taken 
up  by  1  cubic  meter  of  air,  at  different  temperatures,  is  given  in 
the  following  table  : — 


C°.       Grams. 
—10°     2-284 
0°    4-871 
5°    6795 
10°     9-362 

C°.        Grams. 

15°     12-746 
20°     17-157 
25°     22-843 
30°     30-095 

C°.         Grams. 

35°      39-252 
40°     50-700 
100°     588-73 

This  quantity  is  wholly  dependent  on  the  temperature  of  the  air, 
being  represented  by  the  tension  of  the  vapour  of  water  at  that 
temperature.  Thus  the  weight  of  aqueous  vapour  which  can  be 
taken  up  by  1  cubic  metre  of  air  at  10°,  at  which  temperature 
the  tension  of  its  vapour  is  9163  mm.,  is  obtained  as  follows : — 
1  cubic  metre  of  aqueous  vapour  at  0°  and  760  mm.  weighs 
304*75  grams.  Hence  one  cubic  metre  of  air  needs  for  satura- 
tion the  following  quantity  of  aqueous  vapour : — 

804-75  x  273  x  9-163 

283  X  760        —  9*82  grm. 

When  the  temperature  of  saturated  air  is  lowered,  the  aqueous 
vapour  is  precipitated  in  the  form  of  rain,  snow,  or  hail,  accord- 
ing to  the  temperature  of  the  air  during,  or  after,  the  deposition. 
If  one  cubic  mile  of  air  saturated  with  water  at  35°  be  cooled  to 

1  Loc.  cit. 


542  THE  NON-METALLIC  ELEMENTS 

0°,  it  will  deposit  upwards  of  140,000  tons  of  water  as  rain,  for 
one  cubic  metre  of  air  at  35°  is  saturated  when  it  contains  39'25 
grams  of  aqueous  vapour,  whereas  as  0°  it  can  hold  only  4*87 
grams  in  the  state  of  vapour.  It  seldom  happens  that  the  air  is 
completely  saturated  with  moisture,  and  as  seldom  that  the 
amount  of  moisture  sinks  below  y1^  of  the  saturating  quantity. 
Even  over  the  sea  the  air  is  never  completely  saturated  with 
moisture.  Large  amounts  of  watery  vapour  are,  however,  driven 
by  the  winds  into  the  interior  of  the  continents,  and  more  in 
summer  than  in  winter,  when  the  land  is  colder  than  the  sea, 
and  when,  therefore,  the  aqueous  vapour  is  more  easily  con- 
densed, and  does  not  so  readily  penetrate  for  great  distances. 
The  following  mean  tensions  of  aqueous  vapour  in  the  air  at 
different  places  exhibit  this  fact  clearly  : — 

Mean  Tension  of  Aqueous  Vapour  in  mm. 
Mean  yearly.  January.  July. 

London      ....  8'6  ...  5'5  ...  12'2 

Utrecht     ....  7'6  ...  4'8  ...  10'2 

Halle 7'5  ...  4-5  ...  11-6 

Berlin 7'3  .     .     .  4'3  .     .     .  1M 

Warsaw      ....  6'9  ...  3'4  ...  117 

St.  Petersburg    .     .  5'7  .     .     .  27  ...  10'5 

Kasan 5'0  .     .     .  To  .     .     .  9'8 

Barnaul     ....  4'8  ...  1'4  ...  11'3 

Urtschusk       .     .     .  4'0  .     .     .  0'4  .     .     .  11 -3 

In  order  to  determine  the  amount  of  moisture  in  the  air,  we 
may  employ  either  a  chemical  or  a  physical  method.  According 
to  the  first,  a  known  volume  of  air  is  drawn  through  weighed 
tubes  containing  hygroscopic  substances,  the  increase  in  weight  of 
these  tubes  giving  the  weight  of  the  aqueous  vapour.  Thus 
43'2  litres  of  London-  air  at  8°  and  772'5  mm.,  when  passed 
through  drying  tubes,  deposited  0*241  grm.  of  water. 

In  the  second  method  Hygrometers  are  employed ;  of  these 
Regnault's  dew-point  hygrometer  is  the  best.1  For  the  physical 
determinations  of  atmospheric  moisture,  works  on  Hygrometry 
must  be  consulted. 

307  Ammonia  is  another  important  constituent  of  the  air, 
originating  in  the  decomposition  of  nitrogenous  organic  matter. 
The  relative  proportion  in  which  this  substance  is  contained  in 

1  Ann.  Chem.  Phys.  [3]  15,  129. 


ATMOSPHERIC  AMMONIA  543 

the  atmosphere  is  extremely  small,  and  probably  very  varying, 
inasmuch  as  it  is  not  present  as  free  ammonia,  but  combined 
with  atmospheric  carbon  dioxide  and  other  acids,  and  these 
ammoniacal  salts  are  washed  down  by  the  rain  or  absorbed 
by  the  earth.  This  constituent  plays  an  important  part  in  vege- 
tation, for  it  is  from  it  that  unmanured  crops  derive  the  chief 
portion  of  the  nitrogen  which  they  require  for  the  formation  of 
seed  and  other  portions  of  their  structure. 

According  to  the  experiments  of  Goppelsroder,1  the  rain  water 
collected  at  Basel  contained  the  following  amounts  of  ammonia  in 
parts  per  million  : — 

1871.  Minimum.  *   Maximum. 

January  .     ...  4'6  ....     7'8 

February     ...  3'2  ....     6'5 

March     ....  3'8  ....  18'2 

April 3'2  .     e     .     .     6'8 

May 3'2  ....  14*8 

June 3-2  ....     9-1 

Small  quantities  of  nitrous  and  nitric  acids  are  also  found  in 
the  atmosphere,  in  combination  with  ammonia ;  their  formation 
is  probably  due  partly  at  any  rate  to  the  passage  of  electric 
discharges  through  the  air,  as  the  quantity  of  these  compounds 
found  in  rain  water  falling  during  thunderstorms  is  greater  than 
that  occurring  in  ordinary  rain.  Ozone  also  is  supposed  to  occur 
frequently  in  small  quantity,  but  as  already  stated,  this  cannot 
yet  be  regarded  as  definitely  proved  (p.  240). 

The  following  observations  have  been  made  by  Bechi 2  on  the 
amount  of  ammonia  and  nitric  acid  contained  in  the  rain  water 
falling  in  Florence  and  at  Vallambrosa  in  the  Apennines  957 
metres  above  the  sea-level  for  one  square  hectometer  of  surface. 

Florence.  Vallambrosa. 

1870.         1871.         1872. 

Rain  in  cubic  metres  .  9284  10789  12909  .  20278 
Ammonia  in  grams  .  .  13236  10572  12917  .  10433 
Nitric  acid  in  grams .  .15728  9153  13057  .  11726 

Tuxen  has  also  published  tables  showing  the  amount  of  nitric 
acid  and  ammonia  in  the  rain  falling  in  Denmark  during  the 
years  1880  to  1885.3 

1  J.  Pr.  Chem.  [2]  4,  139  and  383.         2  Ser.  8,  1203. 
3  Journ.  Chem.  Soc.  1892,  ii.  234. 


544  THE  NON-METALLIC  ELEMENTS 

308  Atmospheric  Organic  Matter. — The  atmosphere  also,  of 
course,  contains  gases  arising  from  the  putrefactive  decomposition 
of  organic  substances.  These  gases  do  not  remain  in  the  air 
any  length  of  time,  but  undergo  pretty  rapid  oxidation.  The 
particles  of  dust  which  we  see  dancing  in  the  air  as  motes  in  the 
sunbeam  are  partly  organic  and  partly  inorganic.  Bechi  found 
that  a  thousand  litres  of  rain  water  which  fell  in  November 
1870  in  a  garden  in  Florence  contained  4' 123  grams  of  total 
solid  residue,  of  which  one-half  consisted  of  organic  bodies  and 
ammoniacal  salts  and  one  quarter  of  gypsum  and  common  salt. 
Amongst  the  organic  substances  the  germs  of  plants  and  animals 
always  occur,  as  has  been  proved  by  the  classical  labours  of 
Pasteur.1  These  bodies  are  the  propagators  of  fermentation  and 
putrefaction,  and  air  which  has  been  freed  from  these  particles 
either  by  nitration  through  asbestos  or  cotton-wool,  or  by  igni- 
tion (Pasteur),  or  by  subsidence  (Tyndall),  may  be  left  in  contact 
for  any  length  of  time  with  liquids  such  as  urine,  milk,  or  the 
juice  of  meat,  without  these  organic  liquids  undergoing  the 
slightest  change.  Air  which  has  thus  been  filtered  is  termed  by 
Tyndall  optically  pure.  When  a  ray  of  light  is  allowed  to 
pass  through  air  thus  freed  from  solid  particles  no  reflection  is 
noticed,  and  the  space  appears  perfectly  empty,  the  motes  which 
in  ordinary  air  reflect  the  light  being  absent.  (See  also  p.  546.) 

The  organic  nitrogen  contained  in  the  air,  probably  chiefly 
contained  in  such  germinal  bodies,  has  been  quantitatively 
determined  by  Angus  Smith  2  in  the  form  of  ammonia.  He 
obtained  the  following  results  : — 

1  kilogram  of  air  contains  of  organic 
nitrogen  calculated  as  ammonia 
Grams. 

Innellan  (Frith  of  Clyde)    .     .     .  Oil 

London 0'12 

Glasgow 0-24 

Manchester 0*20 

Near  a  midden 0*31 

The  volatile  organic  products  arising  from  putrefaction,  which 
are  always  present  in  the  air,  appear  to  exist  in  larger  quantities 
in  marshy  districts  than  elsewhere,  and  in  all  probability  they  are 
the  cause  of  the  unhealthiness  of  such  situations.  The  unpleasant 
odour  invariably  noticed  on  entering  from  the  fresh  air  into  a 
1  Ann  Chim.  Phys.  [3]  64,  5.  2  Air  and  Earn,  438, 


VENTILATION  545 


closed  inhabited  space  is  also  due  to  the  presence  of  the  same 
organic  putrescent  bodies,  whilst  the  oppressive  feelings  which 
frequently  accompany  a  continued  habitation  of  such  spaces  do 
not  proceed  from  a  diminished  supply  of  oxygen,  or  an  increase 
in  the  atmospheric  carbonic  acid,  but  are  to  be  ascribed  to  the 
influence  of  these  organic  emanations. 

309  Hence  the  subject  of  ventilation  is  one  of  the  greatest 
consequence  to  well-being  as  well  as  to  comfort,  and  it  is  neces- 
sary to  provide  for  a  continual  renewal  of  the  deteriorated  air. 
Fortunately  this  renewal  takes  place  to  a  considerable  extent 
in  a  room,  even  when  doors  and  windows  are  shut,  by  what  may 
be  called  the  natural  means  of  ventilation,  by  the  chimney,  by 
cracks  and  crevices  in  doors  and  windows,  and  especially  through 
the  walls.  Almost  instinctively  man  appears  to  have  chosen 


FIG.  158. 


porous  building  materials,  thus  permitting  by  gaseous  diffusion 
an  exchange  of  fresh  for  deteriorated  air.  The  well-known 
unhealthiness  of  new  and  damp  houses,  as  well  as  of  those 
built  of  iron,  is  to  a  great  extent  to  be  attributed  to  the  fact 
that  the  walls  do  not  permit  a  free  diffusion  to  go  on. 

The  fact  that  gases  readily  pass  through  an  ordinary  dry 
brick-  or  sandstone-wall,  is  clearly  shown  by  the  following 
experiment  proposed  by  Pettenkofer.  (A)  Fig.  158  is  a  piece  o°f 
wall  built  of  ordinary  brick  or  sandstone  82  centimetres  in 
height,  40  cm.  broad,  and  13  cm.  thick.  On  each  side  of 
the  wall  two  rectangular  plates  of  iron  (C)  are  fixed,  and 
the  whole  of  the  outside  of  the  wall  is  then  covered  over  with 
a  coating  of  tar,  and  thus  made  air-tight.  A  tube  (c  c') 
is  soldered  into  a  hole  in  the  centre  of  each  iron  plate.  If 

36 


546  THE  NON-METALLIC  ELEMENTS 

a  candle-flame  be  held  in  front  of  the  opening  of  the  tube 
at  one  side  of  the  wall,  and  a  puff  of  air  be  blown  from  the 
lungs  through  the  open  end  of  the  tube  at  the  other  side 
of  the  wall,  the  candle  will  at  once  be  blown  out ;  whilst  if 
the  one  tube  be  connected  by  a  caoutchouc  tube  to  a  gas  jet, 
and  the  coal  gas  be  allowed  to  pass  through  the  tube,  a  flame 
of  gas  can,  in  a  few  seconds,  be  lighted  at  the  open  end  of  the 
opposite  tube.  If  the  bricks  or  stones  of  the  experimental 
wall  be  well  wetted,  it  will  be  found  very  difficult  to  blow  out 
the  candle  as  described. 


BACTERIOLOGY  OF  AIR. 

310  The  air  always  contains,  as  has  been  stated,  a  certain 
number  of  micro-organisms.  These  belong  to  the  groups  of 
moulds,  yeasts,  and  bacteria.  One  litre  of  air  contains  on  an 
average  from  four  to  five  microbes.  A  pure  unfiltered  river 
water,  on  the  other  hand,  contains  from  6,000  to  20,000  in  one 
cc.,  and  a  fresh  undisturbed  soil  about  100,000  microbes  in 
one  cc.1  The  atmosphere  is  therefore  relatively  poor  in  micro- 
organisms. Their  great  storehouse  in  Nature  is  the  soil,  and  those 
present  in  the  air  are  almost  entirely  derived  from  this  source. 

The  majority  of  the  air  microbes  consist  of  the  spores  of 
moulds  and  yeasts.  The  greater  number  of  the  bacteria  found 
belong  to  the  group  of  the  micrococci,  and  many  of  these  are 
characterised  by  the  production  of  pigment  when  grown  on 
culture  media.  Sarcina  forms  are  constantly  met  with.  The 
bacteria,  whether  bacilli  or  cocci,  are  almost  entirely  saprophytic 
organisms  i.e.  organisms  which  are  not  disease-producing.  It  is 
only  occasionally  that  pathogenic  or  disease  producing  forms  are 
found,  as  for  example  the  pus  cocci.  The  spores  of  moulds 
are  so  light  that  the  isolated  cells  can  float  freely  in  the 
atmosphere.  The  conditions  are  otherwise  with  regard  to 
bacteria.  These  are  not  found  isolated  in  the  air,  but  aggre- 
gated in  small  groups  and  adhering  to  particles  of  dust.  Dust 
is  the  vehicle  by  which  they  are  transmitted  to  the  air,  and  the 
bacteria  therefore  belong  to  the  more  ponderable  elements  of 
the  air-dust.  Thus  for  instance,  if  the  dust  in  a  room  be  stirred 
up,  large  numbers  of  bacteria  will  be  found  in  the  air ;  but  after 
the  dust  has  once  again  settled  to  the  ground,  the  bacteria 

1  Fliigge,  Grundriss  der  Hygiene.     Leipzig.     1889. 


BACTERIOLOGY  OF  THE  AIR  547 

disappear  in  great  part,  leaving  the  lighter  free  spores  of  the 
moulds  in  the  air. 

Bacteria  do  not  pass  into  the  air  from  a  moist  surface  or  from 
the  surface  of  water.  Indeed  the  air  of  sewers  is  found  to 
contain  fewer  organisms  than  the  outside  air,  the  damp  walls 
and  the  sewage  retaining  the  organisms  which  are  not  carried 
about  by  wind.1  It  is  only  when  the  surface  containing  them 
becomes  dry  that  the  wind  is  able  to  carry  up  dust  and  with 
it  bacteria.  The  factors  favouring  the  distribution  of  bacteria  in 
the  air  are  dryness  of  the  soil  and  wind  currents ;  the  factors 
hindering  their  presence  are  moistness  of  the  soil  and  a  still 
atmosphere.  The  number  of  microbes  present  in  the  air  will, 
accordingly,  vary  with  the  atmospheric  conditions.  The  air 
contains  few  microbes  after  a  prolonged  fall  of  rain,  and  many 
after  prolonged  heat  and  dry  weather.  The  spores  of  moulds, 
however,  form  an  exception  to  this  rule,  as  they  are  most 
abundant  in  the  air  during  damp  weather,  moisture  favouring 
their  production.  The  less  dust  there  is  in  the  air,  the  less 
the  number  of  bacteria,  and  vice  versa.  The  air  over  the  open 
sea  and  on  high  mountains  contains  few  or  no  bacteria ;  the  air 
of  a  town  more  than  the  air  of  the  country.  Drying  being 
essential  to  the  passage  of  microbes  into  the  air,  the  forms  most 
largely  present  will  be  those  least  affected  by  dryness,  e.g.  moulds 
and  their  spores.  In  the  open  air  from  10  to  20  times  as  many 
moulds  are  found  as  bacteria.  On  the  other  hand  drying  is 
fatal  to  a  large  number  of  bacteria,  and  especially  to  the 
pathogenic  organisms,  e.g.  Koch's  comma  bacillus,  glanders 
bacillus,  &c.  It  would  therefore  seem  that  the  dangers  of  air 
infection  have  been  over-estimated.  The  air  of  enclosed 
spaces  and  dwelling-houses  is  richer  in  microbes  than  the  open 
air. 

It  thus  appears  that  the  most  dangerous  factor,  hygienically, 
is  not  the  air  itself  nor  the  gases  and  sewage  emanations  that 
may  be  present  in  it,  but  the  dust  to  which  bacteria  cling. 
The  dust  of  dwellings  may,  therefore,  become  an  important 
factor"  in  carrying  infectious  microbes,  more  especially  by 
fragments  of  clothing,  linen,  &c.,  from  sick  people  and  their 
attendants.  Cornet  found  that  the  dust  of  dwellings  contains 
the  tubercle  bacillus,  and  that  such  dust  can  produce  an  infec- 
tion with  tubercle.'2  The  dust  in  the  air  of  dwellings  and 

1  Petri,  Zeitschrift  fur  Hygiene,  1887. 

2  Zeitschrift  fur  Hygiene,  Bd.  v.  1888. 


548  THE  NON-METALLIC  ELEMENTS 

hospitals  is  infectious  for  animals,  and  may  produce  tubercle, 
malignant  oedema,  tetanus,  or  septic  peritonitis. 

311  The  great  majority  of  air  microbes  are  however  sapro- 
phytic,  and  amongst  them  always  occur  the  organisms  to  which 
fermentation  and  putrefaction  are  invariably  due  (Pasteur, 
Tyndall.)  A  knowledge  of  their  nature  and  action  is  important 
for  industries  in  which  fermentation  is  an  essential  feature,  as 
the  presence  of  strange  forms  may  be  injurious  to  the  processes 
involved.  The  number  of  these  special  forms  present  in  the  air 
also  varies  according  to  the  locality,  weather,  and  season  of  the 
year.  There  are  fewer  in  the  open  air  than  in  dwellings,  in 
cold  weather  than  in  hot  weather,  and  in  winter  than  in  summer. 
Pasteur,  and  later  Hansen,  found  a  relatively  small  number  of 
saccharomyces  (yeast-plant)  in  the  open  air.  The  germs  of  the 
alcoholic  fermentation  are  however  already  present  on  the 
surface  of  fruits,  &c.,  and  do  not  require  to  be  introduced 
from  the  air.1 

The  methods  of  examining  the  air  for  microbes  fall  into  two 
groups  : — 

I.  The  methods  of  Miquel  and  their  modifications.     The   air 
is  filtered  through  glass-wool  or  soluble  filters,  and  the  filters 
are  then  distributed  in  flasks  of  nutrient  bouillon. 

II.  The  methods  in  which   Koch's  nutrient  gelatin  is  used 
directly  or  indirectly.2     The  air  may  be  aspirated  over  nutrient 
gelatin  in  Hesse's  tubes,3  or  through  soluble  or  insoluble  filters 
(sugar,  sand,  &c.).     In  the  latter  case  the  filters  and  the  microbes 
are  distributed  on  gelatin  culture  plates.     One  of  the  best  me- 
thods is  that  devised  by  Petri,4  in  which  the  air  is  aspirated 
through  previously  sterilised  sand  filters. 

By  means  of  the  above  methods  a  qualitative  and  quantita- 
tive examination  of  the  air  microbes  can  be  carried  out.  For 
other  points  concerning  the  hygienic  relations  of  the  air, 
Dr.  Renk's  book  should  be  consulted  ("  Die  Luft,"  ffandbuch 

1  Hansen's  important  investigations,  and  the  zymotechnical  methods  employed 
by   him  are   fully   described   in  Jorgensen's    book,    Die    Mikroorganismen  der 
Gahrung  Industrie  (Berlin,  1893).     The  book  also  contains  references  to  all  the 
most  important   papers.     The  micro-organisms  of  the  air  have  been  specially 
studied  by  Miquel,  and  a  full  account  of  his  results  is  contained  in  the  issues  of 
the  Annuaires  de  Montsouris  (1879,  1884,  1886,  &c.). 

2  Welz,    "  Bacteriologische  Unterschung  der  Luft,"  Zeitsehrift  fur  Hygiene, 
Bd.  xi.   1891.      Also  Uffelmaii,  "  Luft  Untersuchungen,"  Archiv  fur  Hygiene, 
Bd.  viii. 

3  Mittheil.  d.  k.  Gesundheitsamt.  1884. 

4  Zeitsehrift  filr  Hygiene,  Bd.  iii.  1887. 


PHOSPHORUS  549 


d.  Hygiene,  Leipzig,  1886),  and  also  the  monograph  on  "Air" 
in  A  Treatise  on  Hygiene  and  Public  Health,  vol.  i.  (London : 
J.  &  A.  Churchill,  1893). 


PHOSPHORUS,     P  =  30-8. 

312  A  considerable  amount  of  uncertainty  surrounds  the  dis- 
covery of  phosphorus,  inasmuch  as  several  chemists  have  claimed 
the  first  preparation  of  this  body,  while  each  has  contradicted  the 
other  in  a  variety  of  ways.  It  seems,  however,  tolerably  certain 
that  phosphorus  was  first  prepared  (1674)  by  the  alchemist  Brand, 
of  Hamburg,  who  obtained  it  from  urine  in  a  process  which  had 
been  previously  made  use  of  for  the  purpose  of  preparing  a 
liquid  supposed  to  have  the  power  of  turning  silver  into  gold. 
By  a  secret  process,  Brand  succeeded  in  preparing  phosphorus 
from  this  liquid,  and  he  is  said  to  have  sold  the  secret  of  the 
manufacture  to  Krafft,  from  whom  it  appears  that  Kunkel 
learnt  what  he  knew,  and  published  in  the  year  1678  a  pam- 
phlet on  this  remarkable  product.1 

In  these  early  days  phosphorus  was  a  very  costly  body,  being 
valued  as  one  of  the  most  remarkable  and  interesting  of 
chemical  substances.  Krafft  exhibited  it  as  one  of  the  wonders 
of  nature  to  various  crowned  heads,  amongst  others,  in  the  year 
1677,  to  King  Charles  II.  of  England.  Robert  Boyle  became 
acquainted  with  its  existence  without,  as  he  tells  us,  having 
been  informed  by  Krafft  of  the  mode  of  preparation  except 
so  far  as  that  it  was  obtained  from  an  animal  source,  and 
he  succeeded  in  the  year  1680  in  the  preparation  of  phosphorus, 
as  Kunkel  and  Brand  had  done  before  him,  by  strongly  heating 
a  mixture  of  evaporated  syrupy  urine  and  white  sand  in  an 
earthenware  retort.2  The  difficulty  of  thus  preparing  phosphorus 
was  considerable,  and  so  many  chemists  failed  in  the  attempt 
that  the  price,  as  late  as  the  year  1730,  was  extremely  high, 
ranging  from  ten  to  sixteen  ducats  the  ounce.  Gahn,  in  1769, 
discovered  the  existence  of  calcium  phosphate  in  bones,  but  it 
was  not  until  this  fact  was  published  by  Scheele  in  1771  that 

1  "  Oeffentliche  Zuschrift  vom  Phosphor  Mirabile  und  dessen  leuchtenden 
Wunderpilulen. " 

-  Boyle,  Phil.  Trans.,  1693  —  "  A  Paper  of  the  Hon.  Robert  Boyle,  deposited 
with  the  Secretary  of  the  Royal  Society  on  the  14th  of  October,  1680,  and 
opened  since  his  death." 


550  THE  NON-METALLIC  ELEMENTS 

phosphorus  was  obtained  from  bone-ash,  which  has  from  that 
time  invariably  served  for  its  preparation. 

The  name  phosphorus  (<£<w?,  light,  and,  (frepco  I  bear)  was 
originally  used  to  designate  any  substance  which  was  capable 
of  becoming  luminous  in  the  dark.  The  first  chemical  substance 
in  which  this  property  was  noticed  was  termed  Jlonnonian 
phosphorus  (see  barium  sulphide).  In  order  to  distinguish 
true  phosphorus  from  this  body,  the  name  of  phosphorus 
mirdbilis,  or  phosphorus  igneus,  was  given  to  it.  In  the 
eighteenth  century  it  was  usually  termed  Brand's,  Kunkel's, 
or  Boyle's  phosphorus,  or  sometimes  English  phosphorus, 
because  it  was  then  prepared  in  London  by  Hankwitz  in 
quantity  according  to  Boyle's  receipt. 

Up  to  the  time  of  Lavoisier,  phosphorus  was  considered  to  be 
a  compound  of  phlogiston  with  a  peculiar  acid  ;  but  in  1772 
the  great  French  chemist  showed  that  the  acid  body  formed  by 
the  combustion  of  phosphorus  weighed  more  than  the  phosphorus 
itself,  the  augmentation  in  weight  being  due  to  a  combination 
with  a  constituent  of  the  air.1  In  a  memoir  communicated  to  the 
Academy  in  1780,  he  represented  phosphoric  acid  as  a  compound 
of  phosphorus  and  oxygen,  and  investigated  its  salts. 

313  Phosphorus,  being  a  very  easily  oxidizable  body,  does  not 
of  course,  exist  in  the  free  state  in  nature.  It  is,  however,  very 
widely  distributed,  especially  in  combination  with  oxygen  and 
calcium  as  calcium  phosphate.  The  most  important  minerals 
containing  phosphorus  are,  estramadurite  or  phosphorite 
Ca3(PO4)9 ;  sombrerite,  an  impure  calcium  phosphate  ;  apatite 
SCyPOJ,  +  CaCl2;  wavellite  4A1(PO4)  +  2A1(OH)3  +  9H2O ; 
vivianite  Fe3(PO4)2  +  8H2O.  Calcium  phosphate  also  forms 
the  chief  constituent  of  coprolites,  and  occurs  in  small  quantity 
throughout  the  granitic  and  volcanic  rocks,  whence  it  passes 
into  the  sedimentary  strata,  and  thus  finds  its  way  into  the  soil. 

The  original  observation  of  Gahn,  that  phosphorus  forms  an 
essential  constituent  of  the  animal  body,  might,  it  may  be 
thought,  have  led  to  the  conclusion  that  this  element  is 
very  widely  distributed.  It  was,  however,  reserved  for  a  later 
time  to  show  that  almost  all  substances  found  on  the  earth's 
crust  contain  phosphorus,  that  it  is  always  present  in  sea-water, 
and  in  all  river  as  well  as  in  almost  every  spring-water. 
All  fruitful  soils  contain  phosphorus,  and  no  plants  will  grow  on 
a  soil  destitute  of  it,  as  it  is  required  to  build  up  certain 
1  Opuscules  Physiques  et  Chimiqucs,  1774. 


PREPARATION  OF  PHOSPHORUS  551 

essential  parts  of  the  vegetable  structure,  especially  the  fruit 
and  seeds.  From  the  plant  the  phosphorus  passes  into  the 
animal  body,  where  it  is  found  in  the  juices  of  the  tissues,  but 
especially  in  the  bones  of  vertebrate  animals,  the  ashes  of  which 
consist  almost  entirely  of  calcium  phosphate.  Phosphorus  is 
likewise  connected  with  the  higher  vital  functions  of  animals, 
the  substance  of  the  brain,  as  well  as  nervous  matter  in  general, 
always  containing  it.  When  the  animal  tissues,  whether 
muscular  or  nervous,  are  worn  out,  they  are  replaced  by  fresh 
material,  and  the  phosphorus  in  them  is  excreted  in  the  urine 
chiefly  as  sodium  ammonium  phosphate  or  microcosmic  salt 
Na(NH4)HP04  +  4H2O.  Phosphorus  is  likewise  found  in  small 
quantity  in  meteoric  stones,  a  fact  which  indicates  its  wide 
cosmical  distribution. 

314  Preparation. — The  preparation  of  phosphorus  from  bones 
was  first  described  by  Scheele  in  1775.  He  warmed  bone-ash  for 
many  days  with  dilute  nitric  acid,  precipitated  the  lime  with  sul- 
phuric acid,  evaporated  the  liquid  from  which  the  gypsum  had 
separated  out  to  a  thick  syrup,  and  distilled  the  residue  with 
charcoal.1  This  process  was  afterwards  simplified  by  Nicolas 
and  Pelletier,2  inasmuch  as  they  treated  the  bone-ash  at  once 
with  sulphuric  acid.  The  yield  of  phosphorus  by  this  process 
was,  however,  but  small,  until  Fourcroy  and  Vauquelin  3  deter- 
mined the  exact  proportion  of  sulphuric  acid  required  for  the 
complete  decomposition  of  the  bone-ash,  thus  preparing  the  way 
for  the  economical  production  of  the  element. 

In  order  to  obtain  calcium  phosphate  from  bones,  the  bones 
were  formerly  burnt  in  ovens.  At  present,  however,  the 
organic  material  contained  in  them  is  made  use  of  in  several 
ways.  Thus  the  bones  are  either  boiled  with  water,  or  treated 
with  superheated  steam,  to  extract  the  gelatine  which  they 
contain,  or  they  are  distilled  in  iron  retorts  to  obtain  the 
.ammonia  and  other  volatile  matters  which  they  yield.  In  the 
latter  case  bone-black  or  animal  charcoal,  which  consists  of 
a  mixture  of  charcoal  and  calcium  phosphate,  is  left  behind. 
This  bone-black  is  largely  used  by  sugar  refiners  for  clarifying 
the  syrup,  and  it  is  only  when  it  has  become  useless  for  this 
purpose  that  it  is  completely  burnt  in  an  open  fire,  and  obtained 
in  the  form  of  bone-ash.  To  prepare  phosphorus  from  bone-ash 
it  is  treated  with  sufficient  dilute  sulphuric  acid  to  convert  the 

1  Gazette  Salutaire  de  Bouillon,  1775.  2  Journ.  Phys.  H,  28. 

3  Journ.  Pharm.  1,  9. 


552 


THE  NON-METALLIC  ELEMENTS 


whole  of  the  calcium  present  into  sulphate,1  which  remains  as 
an  insoluble  precipitate,  whilst  the  liberated  phosphoric  acid 
dissolves  in  the  water.  The  sulphate  is  separated  by  filtering 
through  large  filter  beds,  washed  with  water  till  nearly  free 
from  phosphoric  acid,  and  the  filtrate  concentrated  to  a  syrup. 
The  liquor  is  then  mixed  with  about  a  quarter  of  its  weight  of 
coke  or  charcoal,  and  carefully  dried  in  cast-iron  pots  or  a  muffle 


FIG.  159. 


furnace,  the  orthophosphoric  acid,  H3PO4  being  thus  converted 
into  metaphosphoric  acid,  HP03.  The  dried  mixture  is  then 
heated  to  bright  redness  in  earthenware  retorts  of  the  shape 
shown  in  Fig.  159,  when  the  following  reaction  takes  place  :  — 

4HPO3  +  120  =  2H2  +  12CO  +  4P. 

The   mixture    of   phosphorus   vapour,    hydrogen   and   carbonic 
1  Readman,  Journ.  Soc.  Chem.  2nd.  9- 


PREPARATION  OF  PHOSPHORUS  553 

oxide  passes  through  the  bent  earthenware  pipe  (a),  which  dips 
under  the  surface  of  water  contained  in  the  vessel  (6),  whereby 
the  phosphorus  vapour  is  condensed. 

A  large  proportion  of  the  phosphorus  made  in  England  is  pre- 
pared from  sombrerite,  an  impure  calcium  phosphate  found  on 
the  Island  of  Sombrero  in  the  West  Indies.  It  appears  that 
almost  the  whole  of  the  phosphorus  made  in  the  world  is 
manufactured  in  two  works — namely,  that  of  Messrs.  Albright 
and  Wilson,  at  Oldbury,  near  Birmingham,  and  that  of 
MM.  Cognet  et  Fils,  in  Lyons.1  The  manufacture  of  phos- 
phorus is  somewhat  dangerous,  on  account  of  the  easy 
inflammability  of  the  product,  and  it  is  also  difficult,  inasmuch 
as  the  distillation  requires  forty-eight  hours  for  its  completion, 
and  necessitates,  during  the  whole  of  this  time,  constant 
watching. 

Many  other  processes  have  been  proposed  for  obtain- 
ing phosphorus,  but  as  a  rule  these  have  not  been  very 
successful.  Recently,  however,  a  process,  known  as  the  Head- 
man, Parker  and  Robinson  system,  has  been  successfully  carried 
out  on  the  large  scale,  in  which  the  phosphate  is  directly 
converted  into  phosphorus  without  previous  treatment  with 
sulphuric  acid.  In  this  process,  the  phosphate  is  mixed  with 
charcoal  and  suitable  fluxes,  and  after  heating  to  as  high  a 
temperature  as  possible,  is  introduced  into  an  electrical  furnace, 
consisting  of  an  iron  tank  lined  with  refractory  material, 
and  containing  large  carbon  electrodes  in  the  sides.  Through 
the  latter  a  powerful  electrical  current  is  passed,  and  at  the 
high  temperature  thus  attained  phosphorus  vapour  mixed  with 
other  gases  distils  over,  and  is  condensed  in  the  usual  manner ; 
the  residue  forms  a  liquid  slag  which  may  be  drawn  off  at 
intervals,  and  fresh  raw  material  introduced  into  the  furnace, 
thus  rendering  the  process  continuous.2 

The  crude  phosphorus  always  contains  small  particles  of 
carbon  mechanically  carried  over.  To  get  rid  of  this  and  other 
impurities,  the  phosphorus  is  either  melted  under  water,  and 
pressed  through  chamois  leather,  or,  more  frequently,  the  crude 
melted  material  is  mixed  with  sulphuric  acid  and  bichromate 
of  potash,  three -and-half  parts  of  each  being  used  for  every 
100  parts  of  phosphorus.  This  oxidizing  mixture  acts  upon 
the  impurities,  which  rise  as  a  scum  to  the  surface  of  the 

1  Hofmann,  Report  on  the  Vienna  Exhibition. 

2  Patents  No.  14,962  and  17,719  (1888) ;  Thorpe's  Diet.  3,  192. 


554  THE  NON-METALLIC  ELEMENTS 

liquid,  whilst  the  pure  phosphorus  remains  clear  and  colourless 
at  the  bottom.  It  was  formerly  cast  into  sticks  by  the  work- 
men sucking  the  melted  phosphorus  up  with  the  mouth  into 
glass  tubes.  Instead  of  this  dangerous  operation  an  apparatus 
is  now  employed  by  which  the  phosphorus  is  cast  in  brass  or 
copper  tubes  by  a  continuous  process,  proposed  by  Seubert.1 
The  apparatus  consists  of  a  copper  vessel,  in  which  the  phos- 
phorus is  melted  under  water ;  from  this  the  molten  phosphorus 
is  allowed  to  flow  into  a  tube  consisting  of  glass  or  copper. 
One-half  of  this  tube  is  surrounded  by  hot  and  the  other  by  cold 
water,  the  phosphorus  being  thus  obtained  in  the  form  of  solid 
sticks,  and  cut  under  water  into  pieces  of  a  convenient  length. 

The  quantity  of  phosphorus  manufactured  in  the  year  1874 
amounted  to  250  tons,  and  is  at  the  present  time  (1893)  prob- 
ably not  less  than  1,200  tons,  the  greater  portion  of  which  is 
used  for  the  manufacture  of  lucifer  matches,  a  certain  amount 
being  employed  as  a  vermin  poison,  and  a  small  quantity  being 
used  in  chemical  laboratories. 

The  distillation  of  phosphorus  can  easily  be  shown  in 
the  lecture-room,  by  placing  some  pieces  of  dry  phosphorus 
in  a  small  tubulated  retort  attached  to  a  tubulated  receiver 
containing  water,  which  communicates  with  the  air  by  means 
of  a  tube  one  metre  in  length  dipping  under  mercury.  A 
current  of  carbon  dioxide  gas  is  passed  through  the  tubulus  of 
the  retort  so  as  to  drive  out  the  air.  As  soon  as  all  the  air  has 
been  removed  the  phosphorus  can  be  heated  and  is  seen  to  boil, 
the  colourless  vapour  condensing  in  transparent  yellow  drops  on 
the  neck  of  the  retort  and  in  the  receiver.  The  barometer-tube 
prevents  the  entrance  of  atmospheric  oxygen. 

315  Properties. — Like  sulphur  and  other  elements,  phosphorus 
exists  in  different  allotropic  modifications.  Common  or  octo- 
hedral  phosphorus  is,  when  freshly  prepared  and  kept  in  the 
dark,  a  slight  yellow  or  almost  colourless  body,  which,  when 
slowly  solidified,  is  perfectly  transparent,  but  when  quickly  cooled 
is  translucent,  and  of  a  wax-like  character.  At  low  temperatures 
phosphorus  is  brittle,  but  at  15°  it  becomes  soft  like  wax,  so  that 
it  may  be  easily  cut  with  a  knife.  The  mass  of  the  substance  has, 
however,  a  crystalline  structure.  This  may  be  seen  by  leaving  the 
solid  for  some  time  in  contact  with  dilute  nitric  acid,  when  the 
surface  becomes  distinctly  crystalline.  According  to  v.  Schrotter 
its  specific  gravity  at  10°  is  1'83,  and  it  melts  at  44°*3,  forming 
1  Annalcn,  49,  346. 


PROPERTIES  OF  PHOSPHORUS  555 

a  colourless  or  slightly  yellow  strongly  refractive  liquid,  having  a 
specific  gravity  of  1*764.  Melted  phosphorus,  under  certain  cir- 
cumstances, remains  liquid  for  a  long  time  at  temperatures  much 
below  its  melting  point.  This  is  especially  the  case  when  it  is 
allowed  to  cool  slowly  under  a  layer  of  an  alkaline  liquid,  or 
when  the  solution  in  carbon  bisulphide  is  slowly  evaporated 
under  water.  Neither  solid  nor  melted  phosphorus  conducts 
electricity  (Faraday).  When  heated  in  an  atmosphere  free  from 
oxygen  to  a  temperature  of  290°,  phosphorus  boils,  yielding 
a  colourless  vapour  which,  according  to  the  experiments  of 
Mitscherlich,  has  a  specific  gravity  of  4*58  at  515°,  and  of  4*50 
at  1040°  according  to  those  of  Deville  and  Troost  (Air  =1*0). 
Hence  the  molecular  weight  of  phosphorus  between  these 
temperatures  is  123'84,  or  the  molecule  consists  of  four  atoms. 
At  a  temperature  of  1500 — 1700°,  the  vapour-density  decreases, 
the  numbers  obtained  falling  between  the  values  required  for 
the  formulae  P4  and  P2,  so  that  at  this  temperature  the  molecules 
P4  are  partially  dissociated  into  lighter  molecules.1  The 
determination  of  the  molecular  weight  of  phosphorus  in  solution 
has  led  to  different  results  with  different  investigators ;  thus 
Beckmann  2  from  the  boiling  point  of  solutions  of  phosphorus 
in  carbon  bisulphide  deduced  the  molecular  formula  P4,  which 
was  also  obtained  by  Hertz  3  from  the  freezing  point  of  its 
benzene  solutions.  On  the  other  hand  Paterno  and  Nasini 4 
obtained  by  the  latter  method  numbers  which  point  to  the 
existence  of  molecules  P4  and  P2  m  the  solution. 

•  Phosphorus  also  evaporates  at  temperatures  below  its  boiling 
point.  If  a  small  piece  of  phosphorus  be  placed  in  the  Torri- 
cellian vacuum,  it  gradually  sublimes  and  is  deposited  again  in 
the  form  of  bright  colourless  crystals.  Large  crystals  of 
phosphorus  are  obtained  by  placing  phosphorus  in  a  flask 
filled  with  carbon  dioxide,  then  hermetically  sealing  it  and 
allowing  the  bottom  of  the  flask  to  be  heated  on  a  water-bath 
for  some  days  to  40° ;  or  in  another  way  by  keeping  phosphorus 
in  vacuous  tubes  in  the  dark  for  some  time,  when  it  sublimes 
and  crystallizes  on  the  side  of  the  tube  in  colourless  transparent 
brightly  shining  crystals  (Hermann  and  Maskelyne). 

Phosphorus  is  nearly  insoluble  in  water  and  slightly  soluble  in 

1  Biltz  and  V.   Meyer,  Ber.  22,  726  ;  V.  Meyer  and  Mensching,   Annalen. 
240,  317. 

2  Zeit.  Phys.  Chem.  5,  76.  3  Zeit.  Phys.  Chem.    6,  358, 
4  Ber.  21,  2155. 


556  THE  NON-METALLIC  ELEMENTS 

ether,  oil  of  turpentine,  and  the  essential  oils.  It  is  readily  soluble 
in  chloride  of  sulphur,  phosphorus  trichloride,  sulphide  of  phos- 
phorus, and  carbon  bisulphide,  of  which  one  part  by  weight  will 
dissolve  from  seventeen  to  eighteen  parts  of  phosphorus.  From 
solution  in  carbon  bisulphide,  phosphorus  can  easily  be  obtained 
in  the  crystalline  state,  usually  in  the  form  of  rhombic  dodeca- 
hedra.  The  same  crystals  are  obtained,  according  to  Mitscherlich, 
by  heating  under  water  a  mixture  of  one  part  of  sulphur  with 
two  parts  of  phosphorus.  In  order  to  obtain  phosphorus  in  the 
state  of  fine  powder,  the  melted  substance  is  well  shaken  with 
cold  water  containing  a  little  urea,  the  small  drops  thus  formed 
congealing  into  solid  particles. 

Phosphorus  is  an  extremely  inflammable  substance,  and 
is  always  kept  under  water.  In  presence  of  air  and  light  it 
becomes  covered  under  water  with  a  white  crust,  which  gradually 
falls  off,  whilst  the  phosphorus  becomes  darker  coloured.  The 
crust  is  common  phosphorus,  which  falls  off  from  an  unequal 
oxidation  of  the  mass,  and  on  melting  it  under  water  it 
assumes  the  ordinary  appearance  of  the  element. 

Phosphorus  appears  luminous  in  the  dark,  when  in  contact 
with  moist  air,  and  it  evolves  fumes  possessing  a  strong  garlic- 
like  smell.  These  fumes  are  poisonous,  producing  phosphorus- 
necrosis,  a  disease  in  which  the  bones  of  the  jaw  are  destroyed,  and 
one  by  which  scrofulous  subjects  are  the  most  easily  affected.  The 
luminosity  of  phosphorus  in  the  air  depends  upon  its  slow  oxida- 
tion, with  formation  of  phosphorous  acid.  In  this  act  of  combina- 
tion so  much  heat  is  evolved,  that  if  a  large  piece  of  phosphorus  be 
allowed  to  lie  exposed  to  the  air  it  at  last  melts  and  then  takes 
fire.  The  luminosity  and  oxidation  of  phosphorus  are  best  seen 
by  pouring  a  few  drops  of  the  solution  of  this  body  in  carbon 
bisulphide  on  to  a  piece  of  filter  paper  and  allowing  the  solution 
to  evaporate.  In  the  dark  the  paper  soon  begins  to  exhibit  a 
bright  phosphorescence,  and  after  a  short  time  the  phosphorus 
takes  fire  and  burns.  It  was  formerly  believed  that  phosphorus 
becomes  luminous  in  gases  upon  which  it  can  exert  no  chemical 
action,  such  as  hydrogen  or  nitrogen.  This  is,  however,  not  so, 
the  luminosity  which  has  been  observed  in  these  cases  being  due 
to  the  presence  of  traces  of  oxygen.  From  these  facts  it  would 
naturally  be  inferred  that  phosphorus  must  be  more  luminous  in 
pure  oxygen  than  in  air.  Singularly  enough,  this  is  not  the 
case.1  At  temperatures  below  20°  phosphorus  is  not  luminous 
1  Quart.  Journ.  Science,  1829,  ii.  83. 


PROPERTIES  OF  PHOSPHORUS 


557 


in  pure  oxygen,  indeed  it  may  be  preserved  for  many  weeks 
in  this  gas  without  undergoing  the  slightest  oxidation.1  If, 
however,  the  gas  be  diluted  by  admixture  with  another 
indifferent  gas,  or  if  it  be  rarefied,  the  phosphorescence  is  at  once 
observed  (Graham).  The  phenomenon  can  be  very  beautifully 


FIG.  160, 

shown  by  placing  a  stick  of  phosphorus  in  a  long  tube  (a,  Fig. 
160),  closed  at  one  end  and  open  at  the  other,  and  partly  filled 
with  mercury,  into  which  some  pure  oxygen  is  brought.  The 
open  end  of  the  tube  is  connected  by  a  caoutchouc  tube  with 
the  vessel  (&)  containing  mercury,  so  that,  by  raising  or  lower- 
1  W.  Miiller,  Ber.  3,  84. 


558  THE  NON-METALLIC  ELEMENTS 

ing  the  vessel  the  pressure  on  the  gas  can  be  regulated.  If  the 
pressure  be  so  arranged  that  it  does  not  amount  to  more 
than  one-fifth  of  an  atmosphere,  the  phosphorus  will  be  seen 
to  be  brightly  luminous  in  the  dark.  If  the  pressure  be 
then  gradually  increased,  the  light  will  become  less  and 
less  distinct,  until,  when  the  level  of  the  mercury  is  the  same 
in  both  vessels,  the  luminosity  has  entirely  ceased.  The  phos- 
phorescence can,  however,  at  once  be  brought  back  again  by 
lessening  the  pressure.  The  luminosity  of  phosphorus  is  also 
stopped  when  certain  gases,  such  as  sulphuretted  hydrogen,  or 
the  vapours  of  certain  compounds,  such  as  ether  or  turpentine, 
are  present  even  in  minute  quantities. 

When  phosphorus  is  heated  slightly  above  its  melting-point 
in  moist  air  or  oxygen  it  takes  fire  and  burns  with  a  brightly 
luminous  flame  and  with  evolution  of  dense  white  fumes  of  phos- 
phorus pentoxide  P2O5.  ^he  greatest  precautions  are  necessary 
in  working  with  phosphorus  on  account  of  its  highly  inflammable 
character.  It  must  always  be  cut,  as  well  as  kept,  under  water, 
and  must  not  be  rubbed  either  in  contact  with  the  skin,  or  when 
it  is  being  dried  with  blotting  paper,  as  burning  phosphorus 
produces  deep  wounds,  which  heal  only  with  great  difficulty. 
Phosphorus  does  not,  however,  combine  with  oxygen  in  absence 
of  moisture,  and  may  be  distilled  in  the  perfectly  dry  gas  without 
undergoing  any  change. 1 

Phosphorus  combines  directly  with  the  elements  of  the  chlorine 

and  sulphur  groups  of  elements,  but  not  directly  with  hydrogen. 

If  it  is  heated  with  aqueous  vapour  to  a  temperature  of  250°, 

the  water  is  decomposed  with  formation  of   phosphorous  acid 

and  phosphuretted  hydrogen.     It  also  combines  with  most  of 

.  the  metals  at  a  high  temperature,  and  on  account  of  its  easy 

I  oxidizibility,  it  acts  as  a  powerful  reducing  agent,  precipitating 

/  certain    metals,  such  as   gold,  silver,  and    copper,  when   it   is 

I  brought  into  solutions  of  their  salts.     This  reaction  may  also 

[   be  made  use  of  for  the  purpose  of  detecting  free  phosphorus. 

Thus,  if  the  material  under  examination  be  boiled  with  water, 

and  the  escaping  vapour  allowed  to  come  in  contact  with  a 

piece    of    paper   which   has   been   wetted   with   a   solution   of 

nitrate  of  silver,   any  phosphorus  present  will  cause  a  black 

stain  of  metallic  silver  to  appear  on  the  paper.     No  reduction, 

however,  takes  place  of  a  lead  salt  under  similar  circumstances, 

whilst  if  the  black  stain  be  due  to  the  presence  of  sulphuretted 

1  H.  B.  Baker,  Journ.  Chem.  Soc.  1885,  i.  349. 


DETECTION  OF  PHOSPHORUS 


559 


hydrogen,  the  lead  as   well  as   the    silver   paper  will  become 
stained. 

316  Detection  of  Phosphorus. — The  water  in  which  phosphorus 
has  been  kept  is  also  luminous  in  the  dark,  as  phosphorus  is 
soluble,  though  only  very  slightly  so,  in  water.  When  phos- 


FIG.  161. 


phorus  is  boiled  with  water,  it  is  partially  volatilized,  issuing  in 
the  state  of  vapour  together  with  the  steam.  This  property  of 
phosphorus  is  made  use  of  for  its  detection  in  cases  of  phos- 
phorus poisoning.  The  apparatus  which  is  used  for  this 
purpose  is  shown  in  Fig.  161.  The  contents  of  the  stomach 
supposed  to  contain  the  poison  are  diluted  with  water  and 


560  THE  NON-METALLIC  ELEMENTS 


placed  in  the  flask  A.  This  flask  is  connected  by  the  tube  b, 
with  a  condensing  tube  ccc,  surrounded  by  cold  water.  As 
soon  as  the  liquid  contained  in  the  flask  is  heated  to  boiling, 
some  of  the  phosphorus,  if  present,  is  volatilized  together  with  the 
steam,  and  if  the  whole  of  the  apparatus  be  placed  in  the  dark, 
a  distinct  luminosity,  usually  in  the  form  of  a  ring,  is  observed 
-at  the  point  where  the  steam  is  condensed.  If  the  quantity  of 
phosphorus  is  not  too  small,  some  of  it  is  found  in  the  receiver  in 
the  form  of  small  solid  globules  (Mitscherlich). 

317  Action  of  Phosphorus  as  a  Poison. — Ordinary  phosphorus 
is  a  powerfully  poisonous  substance  capable  of  inducing  death 
in  a  few  hours,  or,  when  given  in  small  doses,  of  producing 
a  remarkable  train  of  poisonous  symptoms  lasting  for  many 
days,  or  even  for  weeks.  Red  phosphorus  appears,  on  the 
other  hand,  to  be  without  action  on  the  animal  economy, 
when  introduced  into  the  stomach. 

Although  cases  occur  in  which  the  administration  of  phos- 
phorus is  followed  by  death  in  a  few  hours,  more  commonly 
some  days  elapse  between  the  date  of  administration  and 
death. 

In  the  more  common  cases  of  phosphorus  poisoning,  some 
time  after  the  poison  has  been  taken  there  supervenes  pain  in 
the  stomach,  with  vomiting  of  garlic-smelling  substances,  and  not 
unfrequently  diarrhoea  ;  all  these  symptoms  of  gastro-iritestinal 
irritation  may  be,  and  often  are,  absent.  Whether  they  are 
present  or  absent  the  patient  soon  becomes  very  weak,  a  febrile 
condition  ensues,  and  the  skin  assumes  a  jaundiced  hue. 
Hemorrhages  may  occur,  and,  towards  the  end,  convulsions  or 
coma  usually  make  their  appearance. 

The  appearances  observed  in  the  bodies  of  animals  and  men 
poisoned  with  phosphorus  are  very  interesting,  and  indicate  that 
this  substance  produces  a  powerful  effect  upon  the  nutrition  of 
the  body.  Minute  extravasations  of  blood  are  frequently  seen 
in  the  lining  membrane  of  the  stomach  and  intestines,  and 
not  unfrequently  small  ulcers  occur  in  those  organs.  The 
common  and  remarkable  appearances  are  fatty  degeneration 
of  the  liver,  kidneys,  heart  and  voluntary  muscles.  It  is  to 
be  remarked  that  the  same  changes  are  observed,  although  in 
a  less  marked  degree,  after  chronic  poisoning  by  arsenic,  an- 
timony, and  vanadium.  These  fatty  degenerations  probably 
indicate  that  phosphorus  and  the  allied  poisons  exert  an  in- 
fluence whereby  the  oxidation  changes,  which  have  their  seat 


RED  PHOSPHORUS 


561 


in  the  animal  tissues,  are  more  or  less  slowed  or  arrested 
(Gamgee). 

Death  has  in  man  followed  the  administration  of  doses  of 
phosphorus  not  exceeding  a  decigram. 

318  Red  Phosphorus. — This  peculiar  modification  of  phos- 
phorus, frequently  termed  amorphous  phosphorus,  was  dis- 
covered by  v.  Schr otter  in  1845.1  Other  chemists  had  indeed 
previously  noticed  the  existence  of  this  substance,  but  its  nature 
had  been  misunderstood.  It  is  obtained  by  the  action  of  light 


FIG.  162. 


and  heat  on  ordinary  phosphorus.  This  change  occurs  with 
tolerable  rapidity  when  the  yellow  phosphorus  is  heated  from 
240°  to  250°.  At  a  higher  temperature  red  phosphorus  begins 
to  undergo  the  opposite  change,  yellow  phosphorus  being  formed. 
Hence  the  passage  from  one  allotropic  modification  to  another 
is  more  readily  shown  in  the  case  of  phosphorus  than  in  that 
of  any  other  element.  For  this  purpose  all  that  is  needed  is  a 
glass  tube  containing  three  bulbs  (Fig.  162),  and  having  the 
open  end  bent  at  right  angles  and  dipping  under  mercury.  In 

.      l  Pogg.  Ann.  81,  276. 
37 


562  THE  NON-METALLIC  ELEMENTS 

the  last  of  these  three  bulbs  is  placed  a  piece  of  phosphorus. 
The  phosphorus  is  then  heated,  the  whole  of  the  oxygen  con- 
tained in  the  apparatus  being  very  soon  absorbed,  and  the 
remainder  of  the  phosphorus  is  distilled  from  the  last  into  the 
middle  bulb.  By  gently  heating  this,  it  is  transformed  into  the 
red  modification,  which  by  a  further  application  of  heat,  is  re- 
converted into  the  ordinary  modification,  which  distils  into  the 
third  bulb.  This  experiment  should  be  made  on  a  leaden  table 
or  on  a  surface  covered  with  a  coating  of  sand,  in  case  the  bulbs 
should  burst. 

Red  phosphorus  is  also  formed  when  ordinary  phosphorus  is 
heated  in  closed  vessels  to  300°,  or  about  10°  above  the 
boiling  point.  In  this  case  the  change  takes  place  in  a  few 
minutes.  The  conversion  of  ordinary  into  red  phosphorus 
can  also  be  brought  about  by  certain  chemical  actions.  E. 
Kopp1  found,  in  1845,  that  by  the  action  of  iodine  on  phos- 
phorus a  red  body  is  formed,  which  on  heating  yields  the 
ordinary  modification  of  phosphorus,  and  B.  C.  Brodie2  has 
shown  that  only  a  trace  of  iodine  is  needed  to  bring  about  the 
change  from  the  yellow  to  the  red  modification,  and  that  when 
common  phosphorus  is  heated  with  a  trace  of  iodine  to  200°, 
a  very  violent  reaction  takes  place  and  the  red  modification  is 
formed. 

Red  phosphorus  is  usually  obtained  as  a  compact  solid  sub- 
stance, which  has  a  dark  reddish  brown  colour,  and  generally  pos- 
sesses a  metallic  iron-grey  lustre.  It  has  a  specific  gravity  of 
2*106,  exhibits  a  conchoidal  fracture,  and  yields  a  reddish  brown 
coloured  powder  closely  resembling  finely  divided  oxide  of  iron ; 
its  hardness  lies  between  that  of  calcspar  and  fluorspar.  It  was 
formerly  supposed  to  be  amorphous,  but  Pedler3  and  Retgers4 
have  shown  that  it  is  at  any  rate  partially  crystalline.  Accord- 
ing to  Muthmann 5  red  phosphorus  is  a  mixture  of  an  orange 
red  amorphous  powder  with  small  crystals  having  a  violet  tint. 
It  is  a  tasteless,  odourless  substance,  insoluble  in  all  those 
solvents  which  dissolve  common  phosphorus,  and  when  intro- 
duced into  the  system  in  the  ordinary  manner  is  not  poisonous, 
the  whole  being  excreted  unchanged  ;  if,  however,  it  is  injected 
into  the  blood  the  usual  symptoms  of  phosphorus  poisoning 
occur.6 

1  Compt.  Rend.  18,  871.  2  Journ.  Chem.  Soc.  1853,  289. 

3  Journ.  Chem.  Soc.  1890,  i.  599.  4  Zeit.  Anorg.  Chem.  3,  399. 

5  Zeit.  Anorg.  Chem.  4,  303.  6  Neumann,  Ber.  21,  748c. 


RED  PHOSPHORUS  563 


It  is  usually  stated  that  red  phosphorus,  when  perfectly  free 
from  the  ordinary  modification,  is  unalterable  in  the  air,  but 
Pedler1  has  shown  that  at  any  rate  in  hot  moist  climates,  this 
is  not  the  case,  but  that  it  slowly  undergoes  oxidation,  with 
formation  of  phosphorous  and  phosphoric  acids.  When  heated 
by  itself  in  absence  of  air,  it  does  not  undergo  any  alteration 
until  a  temperature  of  350°  is  reached,  when  it  is  slowly  con- 
verted into  ordinary  phosphorus,  the  change  taking  place  more 
quickly  at  a  higher  temperature.  When  heated  in  the  air  it 
takes  fire  at  about  260°.  Ordinary  phosphorus  takes  fire  spon- 
taneously when  brought  into  chlorine  gas,  but  the  red  phos- 
phorus requires  heating  before  ignition  takes  place.  Similar 
differences  between  the  two  modifications  present  themselves 
in  a  large  number  of  other  chemical  reactions,  and  the  red 
modification  conducts  electricity,  although  but  feebly,  whilst 
the  yellow  does  not  do  so  at  all. 

The  mode  of  manufacturing  red  phosphorus  is  simple. 
Ordinary  phosphorus  is  placed  in  an  iron  vessel  and  heated  to  a 
temperature  of  240°.  This,  vessel  is  closed  by  means  of  a  cover, 
through  which  passes  a  long  narrow  pipe  open  at  both  ends,  so 
that  the  air  has  limited  access  to  the  phosphorus  contained  in 
the  vessel.  Thus  all  danger  of  explosion  is  avoided,  and  the 
air  in  the  narrow  tube  undergoing  but  little  change,  only  a 
little  of  the  phosphorus  takes  fire  as  soon  as  the  oxygen 
has  been  withdrawn  from  it  by  the  combustion  of  the  first 
portion  of  the  phosphorus.  The  red  phosphorus  thus  prepared 
is  ground  under  water  and  freed  from  common  phosphorus 
by  boiling  with  a  solution  of  caustic  soda,  washing  and  drying. 
The  commercial  red  phosphorus,  when  in  large  compact 
masses,  almost  always  contains  a  small  quantity  of  enclosed 
yellow  phosphorus,  and  not  unfrequently  takes  fire  when 
it  is  rubbed  or  broken.  In  consequence  it  is  usually  packed 
in  vessels  containing  water,  whilst  the  ground  substance,  as 
above  described,  may  be  sent  in  the  dry  state  in  tin  boxes. 
It  frequently  also  contains  traces  of  graphite,  originating 
from  the  iron  pots  in  which  it  is  heated. 

319  Metallic  or  Rhortibohedral  Phosphorus. — When  phosphorus 
is  heated  in  sealed  tubes  in  contact  with  metallic  lead  for  ten 
hours  at  a  temperature  approaching  a  red  heat,  a  third  modi- 
fication of  phosphorus  is  formed.  On  cooling,  the  whole  mass 
of  lead  is  found  to  be  permeated  with  small  crystals,  which 

1  Loc.  cit. 


564  THE  NON-METALLIC  ELEMENTS 

have  been  formed  by  the  phosphorus  dissolving  in  the 
melted  lead  at  a  high  temperature  and  crystallizing  out  on 
cooling.1  In  order  to  separate  these  crystals  from  the  metallic 
lead,  the  mass  is  placed  in  dilute  nitric  acid,  when  the  lead  is 
dissolved.  The  crystals  of  phosphorus  are  still  further  purified 
by  subsequent  boiling  in  strong  hydrochloric  acid.  Metallic 
phosphorus  is  a  brightly  lustrous  dark  crystalline  mass,  which 
in  thin  plates  possesses  a  red  colour  and  consists  of  micro- 
scopic rhombohedra.  Its  specific  gravity  at  15°'5  is  2'34 ;  it 
is  stated  to  conduct  electricity  better  than  the  red  variety, 
and  requires  to  be  heated  to  a  temperature  of  358°  before  it 
is  converted  into  ordinary  phosphorus.  This  variety  is  also 
formed  when  red  phosphorus  is  heated  under  pressure  to  a 
temperature  of  580°  (Troost  and  Hautefeuille). 

The  recent  investigations  of  the  properties  of  red  phosphorus 
have  shown  that  its  properties  agree  more  nearly  with  those 
of  metallic  phosphorus  than  was  formerly  supposed,  and  it 
appears  probable  that  the  latter  is  simply  a  better  crystallized 
variety  of  the  red  modification  (Pedler,  Retgers). 

According  to  Thenard  a  fourth  modification  of  phosphorus 
exists.  This  substance  has  a  black  colour,  and  is  obtained  when 
melted  phosphorus  is  quickly  cooled.  Recent  observations  have, 
however,  shown  that  this  black  phosphorus  is  only  formed  when 
foreign  bodies,  especially  mercury  or  other  metals,  are  present, 
these  bodies  uniting  with  phosphorus  to  form  a  black  metallic 
phosphide.  A  farther  modification  has  been  described  by 
Vernon,2  but  its  existence  is  as  yet  doubtful. 

When  hydrogen  is  passed  over  phosphorus  or  when  phos- 
phates, hypophosphites,  or  phosphites,  or  the  corresponding  acids, 
are  brought  into  a  vessel  in  which  hydrogen  is  being  evolved, 
the  hydrogen  is  seen  to  burn  with  an  emerald  green  flame ;  and 
if  the  quantity  of  phosphorus  be  not  too  small,  a  white  porce- 
lain plate  held  in  the  flame  is  stained  with  a  red  deposit.3  This 
reaction  does  not  occur  in  the  presence  of  alcohol,  ether,  or 
animal  matter.4  The  spectrum  of  the  phosphorised  hydrogen 
flame  exhibits  three  bright  green  lines,  of  which  one  is  almost 
coincident  with  one  of  the  lines  of  the  barium  spectrum,  the 
third  being  not  quite  so  bright,  and  lying  between  the  two 
bright  ones  and  the  sodium  line.5 

1  Hittorf,  Pogg.  Ann.  126,  193.  2  Proc.  Chem.  Soc.  1891,  3. 

3  Dusart,  Compt.  Rend.  43,  1126.  4  Blondlot,  Compt.  Rend.  52,  1197. 

5  Christofle  and  Beilstein,  Ann.  Chem.  Phys.  [4]  3,  280. 


LUCIFER  MATCHES  565 


Phosphorus  is  frequently  used  in  the  laboratory.  It  is  largely 
employed  in  the  manufacture  of  the  iodides  and  bromides  of  methyl 
and  ethyl,  bodies  much  used  in  the  preparation  of  certain  aniline 
colours.  The  main  purpose  for  which  phosphorus  is  employed  in 
the  arts  is,  however,  the  manufacture  of  lucifer  matches,  for  which 
purpose  more  than  1,000  tons  are  employed  every  year. 

320  Lucifer  Matches. — The  application  of  this  substance  to 
the  artificial  production  of  heat  and  light  is  only  of  recent  date. 
The  oldest  mode  of  artificially  obtaining  fire  is  that,  still  made 
use  of  by  certain  rude  tribes,  of  rubbing  together  a  piece  of 
hard  wood  and  a  piece  of  soft  wood,  turning  the  former  quickly 
on  the  latter  until  it  takes  fire.  At  a  later  time  it  was  found 
that,  when  a  piece  of  iron  pyrites  was  struck  with  a  mass  of 
iron,  sparks  flew  off,  by  means  of  which,  dry  inflammable 
materials,  such  as  tinder,  might  be  ignited.  In  place  of  iron 
pyrites  flint  was  next  used,  and  the  iron  replaced  by  a  rough 
piece  of  steel.  The  tinder  employed  was  made  of  charred  linen, 
and  the  glowing  tinder  was  made  use  of  to  ignite  a  match,  con- 
sisting of  a  splint  of  wood,  the  ends  of  which  were  coated  with  sul- 
phur. Up  to  the  year  1829  this  was  the  usual  method  employed 
for  obtaining  a  light.  The  first  lucifer  matches  consisted  of  pieces 
of  wood  the  ends  of  which  had  been  dipped  into  sulphur,  and 
which  were  coated  in  addition  with  a  mixture  of  sugar  and 
chlorate  of  potash.  In  order  to  bring  about  the  ignition 
of  these  matches,  they  were  dipped  into  a  bottle  containing 
asbestos  moistened  with  fuming  sulphuric  acid.  Friction 
matches  were  invented  in  the  year  1832,  the  material  composing 
the  inflammable  mixture  consisting  of  two  parts  of  sulphide  of 
antimony  and  one  part  of  chlorate  of  potash,  mixed  together  to 
a  paste  with  gum  and  water.  The  matches,  which  had  been 
previously  coated  with  sulphur,  were  then  dipped  into  this 
mixture  and  dried.  In  order  to  ignite  them,  these  matches 
were  drawn  through  two  layers  of  sand-paper,  held  between  the 
thumb  and  first  finger.  The  antimony  sulphide  was  soon 
replaced  by  phosphorus,  and  the  first  matches  which  were  made 
in  this  way  were  sold  in  boxes  containing  from  50  to  60  for 
twopence.  Chlorate  of  potash  is  now  supplanted  by  nitre, 
especially  in  the  case  of  Continental  makers,  inasmuch  as  the 
latter  substance  is  less  liable  to  give  rise  to  an  explosive  ignition. 
A  further  improvement  consisted  in  the  replacement  of  sulphur, 
which  produces  a  disagreeable  smell,  by  wax  or  paraffin. 

The  discovery  of  red  phosphorus  naturally  led   to   the   idea 


566  THE  NON-METALLIC  ELEMENTS 

of  the  employment  of  this  substance  in  the  manufacture 
of  lucifer  matches,  and  this  improvement  was  especially  valu- 
able, as,  in  spite  of  all  care,  the  phosphorus  disease  made 
its  appearance  in  match  manufactories,  where  ordinary  phos- 
phorus was  employed.  The  substitution  of  the  red  phos- 
phorus for  the  white  modification  rendered  its  recurrence 
impossible. 

Many  difficulties  had  to  be  overcome  in  the  employment  of 
this  new  substance,  and  it  was  only  after  some  time  that  the 
following  mixture  applied  to  the  head  of  the  matches  was  found 
to  serve  the  required  purpose  : 

Potassium  Chlorate .  32 

Potassium  Bichromate 12 

Red  Lead ' 32 

Sulphide  of  Antimony 24 

This  mixture  contains  no  phosphorus,  and,  as  a  rule,  it  will 
only  ignite  on  a  surface  strewn  with  a  mixture  of  amorphous 
phosphorus  and  sulphide  of  antimony.  If,  however,  these  so- 
called  safety  matches  be  quickly  rubbed  over  a  non-conducting 
surface  such  as  that  of  glass  or  a  smooth  sheet  of  paper  they 
can  be  made  to  take  fire. 


PHOSPHORUS  AND  HYDROGEN. 

321  Three   compounds    of   phosphorus    with    hydrogen   are 
known — 

1.  Gaseous  Hydrogen  Phosphide,  PH3. 

2.  Liquid  „  „  P9H4. 

3.  Solid  „  „  P"4H2. 


GASEOUS  HYDROGEN  PHOSPHIDE  OR  PHOSPHINE,  PH3  =  33*8. 

By  heating  together  phosphorus  and  caustic  potash  Gengembre 
in  17831  obtained  a  gas  which  was  spontaneously  inflammable. 
Some  years  later  Davy  2  and  Pelletier  3  prepared  a  very  similar 
gas  by  heating  phosphorous  acid.  This  gas  differed,  however,  from 
the  former,  inasmuch  as,  although  very  easily  inflammable,  it  did 

1  Crell.  Ann.  1,  450.  2  Phil.  Trans.  1809,  i.  67. 

3  Crell.  Ann.  1796,  ii.  148. 


PHOSPHINE  567 


not  take  fire  spontaneously  on  coming  in  contact  with  the  air 
Both  compounds  were  at  that  time  recognized  to  be  compounds 
of  hydrogen  and  phosphorus.  The  true  explanation  of  the 
difference  between  these  two  gases  was  given  by  Paul  The'nard.1 
He  showed  that  the  spontaneous  inflammation  of  the  one  gas 
was  due  to  the  presence  in  it  of  small  traces  of  the  vapour  of  a 
liquid  hydride  of  phosphorus. 

Preparation. — (1)  In  order  to  prepare  spontaneously  inflam- 
mable phosphuretted  hydrogen,  as  the  impure  gas  has  been  called, 
phosphorus  is  heated  with  milk  of  lime  or  with  a  solution  of 
caustic  potash.  The  spontaneously  inflammable  gas  is  evolved, 
and  calcium  or  potassium  hypophosphite  left  behind  ;  thus  : — 

3KOH  +  4P  +  3H2O  =  3KH2PO2  +  PH3. 

(2)  The  same  mixture  of  gases  is  also  readily  formed  when 
phosphide  of  calcium  is  thrown  into  water.     Each  bubble  of  the 
gas  ignites,  on  coming  to  the  surface  of  the  water,  with  a  sharp 
explosion,  burning  with  a  bright  white  flame,  and  a  ring-like  cloud 
of  phosphorus  pentoxide  is  formed,  which  on  ascending  shows  the 
remarkable  vortex  motions.    In  order  to  exhibit  this  phenomenon, 
a  small  flask  (a  Fig.  163)   is  three-quarters  filled  with  strong 
potash  solution,  a  few  pieces  of  phosphorus  are  thrown  in,  and 
the  whole  is  gently  warmed.     As  soon  as  small  flames  are  seen 
at  the  mouth  of  the  flask,  a  gas  delivery-tube  (c)  is  fixed  in  with 
cork,  the  lower  end  dipping  under  water. 

When  the  spontaneously  inflammable  phosphuretted  hydrogen 
is  exposed  to  the  light,  or  when  it  is  passed  through  a  freezing 
mixture,  or  left  in  contact  with  carbon  or  potassium,  it  loses  its 
power  of  spontaneous  inflammability,  inasmuch  as  the  liquid 
hydride  contained  in  it  is  either  decomposed  or  condensed. 

(3)  The  non-spontaneously  inflammable  phosphuretted  hydro- 
gen  is   obtained   by    warming   phosphorus   with    an   alcoholic 
solution  of  potash,  or  by  decomposing  phosphide  of  calcium  by 
means  of  hydrochloric  acid. 

(4)  Phosphine  may  also  be  prepared  by  the  action   of  dilute 
acids  on  the  phosphides  of  zinc,  iron,  tin,  or  magnesium.2 

The  phosphine  prepared  by  any  of  these  methods  is,  however, 

not  pure,  but  contains  more  or   less   hydrogen  mixed  with  it 

In  order  to  obtain  pure  phosphuretted  hydrogen  we  make  use 

of  its  property  of  combining  with  hydriodic  acid  to  form  the 

1  Ann.  Chim.  Phys.  [3]  14,  5.  2  Liipke,  Journ.  Chcm.  Soc.  1891,  ii.  397. 


568 


THE  NON-METALLIC  ELEMENTS 


crystalline  compound  termed  phosphonium  iodide,  PH4I.  This 
substance,  when  thrown  into  water,  decomposes  into  its  con- 
stituents, namely,  phosphine,  PH3.  and  hydriodic  acid,  HI.  The 
solid  iodide,  the  preparation  of  which  will  be  hereafter  described, 
is  employed  as  follows  for  the  preparation  of  the  pure  gas.1 
Some  pieces  of  the  iodide  of  the  size  of  peas,  mixed  with  broken 
glass,  are  brought  into  a  small  flask.  The  flask  is  closed  by  a 
cork  having  two  holes  bored  through  it,  in  one  of  which  is 
placed  a  stoppered  funnel-tube,  and  in  another  a  gas-delivery 
tube.  The  funnel  is  filled  with  concentrated  solution  of  potash, 


FIG.  163. 


and  this  is  allowed  to  run    into   the  flask    slowly,  when   the 
following  decomposition  occurs  :— 


PH4I  +  KOH  =  PH3  +  KI 


H20. 


According  to  Messinger  and  Engels  2  it  is  preferable  to  mix  the 
phosphonium  iodide  with  ether  and  gradually  add  water,  a 
regular  stream  of  phosphine  being  thus  obtained.  The  gas 
prepared  in  this  manner  is  not  spontaneously  inflammable,  at  any 
rate  at  the  beginning  of  the  operation,  although  if  the  evolution 


Hofmann,  Ber.  4,  200. 


2  Ber.  21,  326. 


PHOSPHINE  569 


be  carried  on  for  a  considerable  length  of  time  the  spontaneously 
inflammable  gas  is  formed  (Rammelsberg). 

322  Properties. — Phosphine  is  a  colourless  gas,  smelling  like 
rotten  fish ;  it  liquefies  at  —  85°, solidifying  at—  133°'5  (Olszewski). 
The  pure  gas  takes  fire  only  above  a  temperature  of  100°,  and  is 
so  inflammable  that  the  heat  evolved  by  -the  friction  of  the 
stopper  on  opening  the  bottle  containing  the  gas  is  sometimes 
sufficient  to  produce  its  inflammation.  It  may  be  mixed  with 
oxygen  without  undergoing  any  alteration,  but  if  this  mixture 
be  suddenly  exposed  to  diminished  pressure  an  explosion  occurs. 
This  remarkable  phenomenon  reminds  one  of  the  non-luminosity 
of  phosphorus  in  pure  oxygen  and  its  luminosity  in  diluted 
oxygen  at  the  same  temperature. 

Phosphine~also  takes  fire  when  a  few  drops  of  dilute  nitric 
acid  are  brought  in  contact  with  At,  or  when  it  is  mixed  with 
the  vapours  evolved  from  chlorine-  or  bromine-water.  If  the 
gas  free  from  air  be  led  through  common  nitric  acid  containing 
nitrous  fumes,  it  becomes  spontaneously  inflammable,  and  it 
explodes  in  chlorine  gas  with  great  violence  and  with  the  evo- 
lution of  a  bright  greenish-white  light.  Phosphine  is  somewhat 
soluble  in  water,  and  imparts  to  it  a  peculiar  and  disagreeable 
taste~7tfr^  solution  decomposes  in  the  light  with  the  evolution 
of  hydrogen  and  the  separation  of  red  phosphorus.  When 
a  series  of  electric  sparks  is  passed  through  the  gas  it  also 
decomposes  into  phosphorus  and  hydrogen,  the  volume  of  the 
latter  bearing  to  that  of  the  original  gas  the  proportion  of  three 
to  two.  In  order  to  show  this  a  eudiometer  similar  to  the  one 
already  described  (Fig.  135)  may  be  employed,  but  instead 
of  platinum  wires  pieces  of  gas-coke  are  melted  through  the 
glass,  inasmuch  as  platinum  and  phosphorus  in  contact  unite, 
forming  a  silver  white  compound  which  is  brittle  and  easily 
fusible  (Hofmann). 

Phosphine  combines,  like  ammonia,  with  certain  metallic 
chlorides ;  thus,  for  instance,  with  aluminium  chloride  A1C13, 
tin  chloride  SnCl4,  titanium  chloride  TiCl4,  and  antimony 
chloride  SbCl5. 

Phosphine  is  a  very  poisonous  gas,  producing,  when  present 
in  small  proportions  in  respired  air,  in  turn  dyspnoea  and  death. 
It  possesses  the  power  of  combining  with  the  respiratory  oxygen 
linked  to  hsemoglobin  ;  in  this  way  its*' toxic  action  has  been 
explained,  although  in  reality  it  is  almost  entirely  due  to  more 
complex  operations. 


570 


THE  NON-METALLIC  ELEMENTS 


323  Phosphonium  Compounds. — Phosphine  possesses  feebly  basic 
properties,  and  combines  with  hydrobromic  acid  and  hydriodic 
acid  to  form  salts,  in  a  similar  manner  to  ammonia.  These 
salts  contain  the  compound  radical  PH4,  which  is  usually 
termed  phosphonium,  just  as  the  compound  radical  NH4  (p.  470) 
is  termed  ammonium. 

Phosphonium  Bromide,  PH4Br,  crystallizes  in  colourless  cubes, 
which  boil  at  30°.  The  vapour  possesses  a  specific  gravity  of 
1'906,  so  that  we  conclude  that  it  is  a  mixture  of  phosphine 
and  hydrobromic  acid. 

Phosphonium  Iodide,  PH4I. — This  beautiful  compound,  which 
crystallizes  in  large  transparent  glittering  quadratic  prisms,  can 
easily  be  obtained  by  placing  in  a  retort  of  a  litre  capacity  (see 
Fig.  164)  400  grams  of  common  phosphorus,  allowing  an  equal 


FIG.  164. 

weight  of  dry  carbon  bisulphide  to  run  in,  and  gradually  adding 
680  grams  of  pure  iodine,  care  being  taken  to  keep  the  retort 
well  cooled.  The  carbon  bisulphide  is  next  completely  removed 
by  distillation  in  a  water  bath,  and  the  retort  connected  with 
a  long  wide  tube  placed  in  a  slightly  slanting  position,  and 
furnished  at  its  lower  end  with  a  tubulated  receiver.  This, 
again,  is  connected  by  a  series  of  bulb  tubes  with  two  absorp- 
tion-vessels, the  first  of  which  contains  a  dilute  solution  of 
hydriodic  acid,  and  the  second  water.  The  object  of  this 
arrangement  is  to  absorb  the  hydriodic  acid  formed  during  the 
reaction,  and  at  the  same  time  to  prevent  the  liquid  from  entering 
the  wide  tube  into  which  the  iodide  of  phosphonium  is  sub- 
limed. The  apparatus  is  then  filled  with  pure  carbon  dioxide, 
a  slow  current  of  the  gas  being  passed  through  during  the 


LIQUID  HYDROGEN  PHOSPHIDE  571 


operation.  The  experiment  being  thus  far  arranged,  240  grams 
of  water  are  allowed  to  drop  slowly  by  means  of  a  stoppered 
tube-funnel  into  the  retort,  which  is  slightly  warmed.  The 
heat  evolved  by  the  action  then  taking  place  is  sufficient  to 
sublime  the  greater  part  of  the  iodide  of  phosphonium  into  the 
wide  tube.  Towards  the  end  of  the  operation,  which  usually 
requires  about  eight  hours  for  its  completion,  the  retort  is  heated 
somewhat  more  strongly.  When  no  further  increase  in  the 
amount  of  sublimate  takes  place,  the  apparatus  is  dismounted, 
the  end  of  the  long  tube  closed  with  corks,  and  the  thick  crust 
of  phosphonium  iodide  loosened  by  means  of  a  stout  iron 
wire,  and  preserved  in  stoppered  bottles.1  The  formation  of 
phosphonium  iodide  is  represented  by  the  following  equation  :  — 


51  +  9P  +  15H2O  =  5PH4I  +  4H3PO4. 

An  excess  of  phosphorus  is,  in  practice,  employed  because  a  part 
of  this  substance  is  converted,  during  the  reaction,  into  the 
red  modification.  The  formation  of  the  hydriodic  acid  which 
escapes  is  due  to  the  decomposition  of  the  iodide  of  phos- 
phonium in  the  presence  of  warm  water.  Phosphonium 
iodide  boils  at  about  80°,  but  it  easily  vaporizes  at  a  lower 
temperature.  It  is  used  in  the  laboratory  as  a  powerful  reducing 
agent,  as  well  as  for  the  preparation  of  many  organic  phos- 
phorus compounds. 


LIQUID  HYDROGEN  PHOSPHIDE,  P2H4  =  65-60. 

324  This  substance,  discovered  by  Thenard  in  the  year  1845, 
is  obtained  by  the  action  of  water  on  calcium  phosphide.  The 
latter  is  prepared  by  the  action  of  phosphorus  on  lime  at  a  red 
heat  and  is  best  converted  into  liquid  hydrogen  phosphide  by 
carefully  adding  it  in  small  quantities  at  a  time  to  water  heated 
to  60°,  the  air  in  the  apparatus  having  previously  been  displaced 
by  hydrogen.  The  gas  evolved  is  passed  through  a  small  con- 
denser placed  in  ice  water,  in  which  a  portion  of  the  liquid 
phosphide  condenses,  the  remainder  passing  on  with  the  gaseous 
hydride  simultaneously  formed.  The  liquid  phosphide  is  colour- 
less, has  a  sp.  gr.  of  about  I'Ol,  and  boils  at  57 — 58°  under  a 
pressure  of  735  mm.,  and  leaves  no  residue  if  not  too  strongly 
heated.  It  has  the  empirical  formula  PH2,  but  its  molecular 
1  Hofmann,  Ber.  6,  286. 


572 


THE  NON-METALLIC  ELEMENTS 


formula  is  in  all  probability  P2H4,  corresponding  to  that  of 
hydraziue,  N2H4 ;  no  determination  of  its  vapour  density  could, 
however,  be  made  as  it  so  readily  undergoes  decomposition.1 

Liquid  hydrogen  phosphide  is  spontaneously  inflammable, 
taking  fire  at  once  on  exposure  to  the  air  and  burning  with  a 
bright  phosphorus-like  flame.  On  exposure  to  light,  or  when 
heated  above  its  boiling  point  it  decomposes  into  phosphine 
and  solid  hydrogen  phosphide,  according  to  the  following  equa- 
tion :  — 

5P2H4  =  2P2H  +  6PH3. 

The  same  reaction  takes  place  in  contact  with  hydrochloric  acid 
and  hydriodic  acid,  1  cc.  of  hydrochloric  acid  being  sufficient, 
according  to  Thenard,  to  decompose  an  indefinite  quantity  of  the 
phosphide. 


FIG.  165. 

•• 

In  order  to  exhibit  the  properties  of  the  liquid  phosphuretted 
hydrogen,  Hofmann  employs  a  U-tube  made  of  strong  glass, 
3  to  4  mm.  in  diameter,  each  of  the  limbs  of  which  is  furnished 
with  a  glass  stopcock  (Fig.  165).  This  tube  is  placed  in  a  freez- 
ing mixture  of  pounded  ice  and  salt  and  connected  with  a  flask, 
into  which  from  30  to  50  grams  of  freshly  prepared  calcium 
phosphide  are  gradually  thrown.  This  being  decomposed  by 
water,  the  phosphuretted  hydrogen  evolved  passes  through  the 
U-tube,  in  which  the  liquid  hydride  condenses,  whilst  the 
spontaneously  inflammable  gas  escapes.  As  soon  as  all  the 
calcium  phosphide  has  been  decomposed,  a  current  of  dry 
carbon  dioxide  is  led  through  the  apparatus;  the  flame  of 
the  issuing  gas  is  then  changed  to  a  faintly  luminous  cone. 
If,  however,  the  carbon  dioxide  be  replaced  by  a  current  of 
hydrogen,  the  luminous  flame  is  again  seen. 

1  Gattermann  and  Haussknecht,  Ber.  23,  1174. 


FLUORIDES  OF  PHOSPHORUS  573 


SOLID  HYDROGEN  PHOSPHIDE,  P2H. 

325  This  compound  is  formed  in  the  manner  already  de- 
scribed, and  may  be  readily  obtained  by  passing  the  uncondensed 
vapours  obtained  in  preparing  the  foregoing  compound  into  a 
large  flask  containing  concentrated  hydrochloric  acid ;  it  forms 
a  yellow  powder,  having  the  empirical  formula  P2H,  but  its 
molecular  formula  is  as  yet  unknown.  On  heating  this  in  a  stream 
of  carbon  dioxide  to  70°  it  decomposes  into  phosphorus  and 
hydrogen.  It  does  not  take  fire  in  the  air  until  it  attains  a 
temperature  of  160°. 


PHOSPHORUS  AND  FLUORINE. 

PHOSPHORUS  TRIFLUORIDE,  PF3. 

326  This  compound  is  obtained  by  the  action  of  copper 
phosphide  on  lead  fluoride,  or  by  allowing  arsenic  trifluoride 
to  drop  into  phosphorus  trichloride,  moisture  being  excluded. 
It  is  a  colourless  'gas  which  does  not  fume  in  the  air,  and 
condenses  to  a  colourless  liquid  at  —  10°  under  a  pressure 
of  40  atmospheres.  It  is  only  slowly  absorbed  by  water,  but 
forms  an  explosive  mixture  with  oxygen,  and  unites  with 
ammonia  and  bromine.  The  dry  gas  is  dissociated  by  the 
passage  of  electric  sparks  into  fluorine  and  phosphorus  penta- 
fluoride. 

PHOSPHORUS  PENTAFLUORIDE,  PF5, 

Was  discovered  by  Thorpe,1  who  prepared  it  by  the  action 
of  arsenic  trifluoride  on  phosphorus  pentachloride. 

5  AsF3  +  3PC15  =  5  AsCl3  +  3PF5. 

It  is '  also  formed  by  heating  phosphorus  trifluorodibromide 
(p.  579)  which  splits  up  at  15°  into  the  peutabromide  and 
pentafluoride.2  It  is  a  colourless  gas  and  decomposes  in  contact 
with  water  into  phosphoric  and  hydrofluoric  acids  ;  it  possesses 
a  strongly  irritating  smell,  and  attacks  the  mucous  membrane. 

1  Proc.  Roy.  Soc.  25,  122. 

2  Moissan,  Compt.  Rend.  99,  655,  570  ;  100,  272,  403. 


574  THE  NON-METALLIC  ELEMENTS 

It  partially  condenses  to  a  liquid  at  16°  under  a  pressure  of  46 
atmospheres,  and  solidifies  at  a  very  low  temperature.  Accord- 
ing to  Thorpe  the  gas  is  unaffected  by  passing  a  series  of  electric 
sparks  through  it  whether  alone  or  mixed  with  hydrogen  or 
oxygen,  whilst  Moissan  l  found  that  with  sparks  of  high  tension 
a  partial  dissociation  into  fluorine  and  the  trifluoride  occurs. 
On  the  other  hand  when  phosphorus  trifluoride  comes  in 
contact  with  free  fluorine  it  takes  fire  and  burns  with  a  yellow 
flame  forming  the  pentafluoride.  With  dry  ammonia  it  yields 
a  white  solid  compound  having  the  composition  2PF5,  5NH3. 

The  vapour  density  of  the  compound  is  63  (H  =  l)  and  the 
molecular  formula  is  therefore  PF5.  The  existence  of  this 
gaseous  pentafluoride  taken  in  conjunction  with  its  stability  even 
at  high  temperatures  is  of  great  theoretical  interest,  inasmuch 
as  it  shows  that  phosphorus  can  form  pentavalent  derivatives 
capable  of  existing  in  a  state  of  vapour. 


PHOSPHORUS  AND   CHLORINE. 

Ordinary  phosphorus  takes  fire  in  dry  chlorine  gas,  and  burns 
with  a  pale-greenish  flame,  with  formation  of  phosphorus  tri- 
chloride, PC13,  or,  with  an  excess  of  chlorine,  phosphorus  penta- 
chloride,  PC15. 


PHOSPHORUS  TRICHLORIDE,  PC13  =  136-37. 

327  This  compound  was  discovered  by  Gay-Lussac  and 
Thenard  in  1808.  It  is  best  prepared  by  placing  red  phos- 
phorus in  a  retort  (D,  Fig.  166),  and  heating  it  whilst  a 
stream  of  chlorine  gas  evolved  in  the  flask  (A),  and  dried  by 
passing  through  the  tube  (c),  is  led  over  it.2  The  distillate  is 
purified  from  any  pentachloride  which  is  formed  by  allowing  it 
to  remain  in  contact  with  ordinary  phosphorus  for  some  time 
and  then  rectifying. 

The  trichloride  is  a  mobile  colourless  liquid,  which  has  a  very 
pungent  smell,  boils  at  76°,  and  does  not  solidify  at  —  115°. 
The  specific  gravity  of  the  liquid  at  0°  is  T61294.3  When 

1  Bull.  Soc.  Chim.  [3]  5,  880. 

2  Dumas,  Ann.  Chim.  Phys.  [3],  55,  172. 

3  Thorpe,  Proc.  Roy.  Soc.  24,  295. 


PHOSPHORUS  PENTACHLORIDE 


575 


exposed  to  the  air  it  evolves  white  fumes,  absorbing  the  atmo- 
spheric moisture,  and  decomposing  into  hydrochloric  and  phos- 
phorous acids  ;  thus  :  — 


3H2O 


3HC1  +  P(OH)3. 


Sulphur  trioxide  acts  violently  on  this  compound,  with 
formation  of  phosphorus  oxychloride,  and  sulphur  dioxide. 
Heated  with  concentrated  sulphuric  acid,  chlorosulphonic  acid 


FIG.  166. 


and   phosphorus    pentoxide    are    produced    according    to   the 
equation  :  — 


2PC13  4-  3S02(OH)2  =  2S02  +  5HC1 


SO  +  P0 


It  also  unites  with  ammonia  forming  additive  compounds.1 


PHOSPHORUS  PENTACHLORIDE,  PC15  =  206-75. 

328  This  compound  was  discovered  by  Sir  Humphry  Davy 
in  the  year  1810,  though  it  was  first  analysed  by  Dulong  in 
1816.  It  is  easily  formed  by  the  union  of  phosphorus 
trichloride  with  chlorine.  To  prepare  it,  a  current  of  dry 
chlorine  is  led  through  a  wide  tube  on  to  the  surface  of  the 
liquid  trichloride  contained  in  a  flask  surrounded  by  cold  water. 

1  Besson,  Compt.  Rend.  HI,  972. 


576  THE  NON-METALLIC  ELEMENTS 

As  the  absorption  of  the  chlorine  is  accompanied  by  the  evolu- 
tion of  much  heat,  it  is  necessary  to  take  care,  in  the  beginning 
at  least,  that  the  liquid  is  well  cooled.  The  reaction  is  finished 
as  soon  as  the  product  assumes  the  condition  of  a  perfectly  dry 
mass. 

Phosphorus  pentachloride  is  a  white  or  yellowish -white 
lustrous  crystalline  powder,  possessing  a  very  sharp  unpleasant 
smell,  and  violently  attacking  the  eyes  and  the  mucous  mem- 
brane. On  heating,  it  is  found  to  sublime  below  100°,  but  it 
cannot  be  fused  under  the  ordinary  pressure  of  the  atmosphere. 
When,  however,  it  is  heated  under  increased  pressure,  it  melts 
at  148°,  solidifying  on  cooling  in  transparent  prisms.  When 
heated  still  more  strongly  it  boils,  emitting  a  colourless  vapour, 
which  becomes  coloured  on  further  heating,  the  coloration 
increasing  with  the  temperature.  This  is  due  to  the  fact  that 
the  vapour  gradually  undergoes  dissociation  into  equal  molecules 
of  free  chlorine  and  phosphorus  trichloride.  That  this  is  the 
case  is  fully  proved  by  Dumas'  determination  of  the  density  of 
this  mixture  at  different  temperatures : — 

Temperature      ....     182°     200°     250°     300°     336° 
Density 73'3     TOO     57'6     52*4     52'5 

These  numbers  clearly  exhibit  the  gradual  dissociation  of  the 
vapour,  the  density  undergoing  a  continuous  diminution  until  the 
temperature  of  300°  has  been  reached.  Above  that  point  it 
remains  constant.  The  vapour  at  these  temperatures  consists  of 
a  mixture  of  ^n  equal  number  of  molecules  of  the  trichloride  and 
chlorine,  possessing  the  density :  136*37  -f  2  x  35'19_  ^-i.™ 

4 

That  the  vapour  thus  obtained  contains  free  chlorine  was 
proved  by  Wanklyn  and  Robinson  in  the  following  way.1  The 
vapour  of  the  pentachloride  was  allowed  to  diffuse  into  an 
atmosphere  of  carbon  dioxide.  Chlorine  gas  being  lighter  than 
the  vapour  of  the  trichloride,  must  diffuse  more  quickly  than 
the  latter.  Accordingly,  if,  after  the  experiment,  the  vessels 
containing  the  pentachloride  were  found  to  contain  the  trichlo- 
ride, whilst  the  atmosphere  of  carbon  dioxide  was  admixed  with 
free  chlorine,  the  fact  of  dissociation  would  be  proved.  This 
was  the  result.  The  dissociation  of  the  pentachloride  may  be 
prevented,  or  at  any  rate  much  diminished,  by  allowing  it  to 
volatilize  in  a  space  saturated  with  the  vapour  of  the  trichloride. 
1  Proc.  Roy.  Soc.  12,  507. 


PHOSPHORUS  PENTACHLORIDE  577 

Wurtz 1  obtained,  under  these  circumstances,  a  vapour  possessing 
a  density  close  upon  103'4  (the  normal  density  of  PC15)  at 
temperatures  varying  from  160°  to  175°. 

In  perfectly  dry  air  pentachloride  of  phosphorus  undergoes  no 
alteration  ;  on  exposure,  however,  to  moist  air,  it  decomposes 
with  formation  of  phosphorus  oxychloride  ;  thus : — 

PC15  +  H20  =  POC13  +  2HC1. 

This  compound  as  well  as  the  pentachloride,  dissolves  in  water 
with  evolution  of  heat  and  formation  of  phosphoric  and  hydro- 
chloric acids ;  the  two  reactions  being  : — 

POC13 + 3H2O  =  PO(OH)3  +  3HC1, 
PC15+4H20  =  PO(OH)3  +  5HC1. 

One  of  the  most  important  properties  of  phosphorus  penta- 
chloride is  its  action  on  the  acid-forming  oxides  and  on  the  acids. 
These  it  converts  into  the  acid  chlorides.  Many  examples  of 
this  kind  of  decomposition  have  already  been  given.  For  in- 
stance, it  forms,  with  sulphur  trioxide,  the  chloride  of  sulphuric 
acid  or  sulphuryl  chloride  ;  thus  : — 

S03  4-  PC16  =  S02C12  +  POC13. 

In  this  reaction  two  atoms  of  chlorine  are  replaced  by  one 
atom  of  oxygen.  When,  on  the  other  hand,  the  pentachloride  is 
allowed  to  act  upon  a  hydroxy-acid,  chlorine  replaces  the  radical 
hydroxyl,  OH  ;  thus  : — 

S02  {  OH  +  PC15  =  S02 1  ^j  +  POC13  4-  HC1. 

In  a  similar  way  the  pentachloride  acts  upon  organic  acids 
and  other  compounds  containing  hydroxyl ;  and  in  consequence 
of  this  property  it  is  much  used  in  the  preparation  of  organic 
chlorides. 

Phosphorus  pentachloride  forms  a  crystalline  compound  with 
iodine  monochloride  ;  thus  : — PC15  +  IC1,  as  also  with  different 
metallic  chlorides ;  thus : — PC15  +  FeCl3,  &c. 

At  the  ordinary  temperature  ammonia  acts  upon  phosphorus 
pentachloride  with  formation  of  chloramido-derivatives  (p.  610) ; 
if,  however,  ammonia  be  passed  into  a  cooled  solution  of  the 
1  Compt.  Rend.  76,  601. 


578  THE  NON-METALLIC  ELEMENTS 

pentachloride  in  carbon  tetrachloride,  the  two  combine  forming 
the  additive  compound  PCI5,  8NH3,  which  separates  out  as  a 
white  powder  and  is  stable  in  the  air.1 

PHOSPHORUS  TRIFLUORODICHLORIDE,  PF3C12, 

Is  formed  by  the  direct  combination  of  chlorine  and  phos- 
phorus trifluoride,  and  is  a  colourless  uninflammable  gas  which  is. 
instantaneously  absorbed  and  decomposed  by  water  and  alkalis, 
It  has  a  vapour  density  of  5*4  (air  =  1)  corresponding  to  the  above 
formula.  It  condenses  to  a  liquid  at  —8°  under  atmospheric 
pressure,  and  is  decomposed  at  250°  or  by  the  passage  of  electric 
sparks  into  phosphorus  pentafluoride  and  pentachloride.  When 
treated  with  a  small  quantity  of  water  it  yields  phosphorus 
oxyfluoride  POF3  and  with  sulphur  forms  the  corresponding 
thiofluoride  PSF3.2 


PHOSPHORUS  AND  BROMINE. 

PHOSPHORUS  TRIBROMIDE,  PBr3  =  268-88. 

329  Bromine  and  phosphorus  act  so  violently  upon  each  other 
that  small  pieces  of  phosphorus  thrown  upon  bromine  may 
cause  a  dangerous  explosion.  In  order  to  prepare  the  tribro- 
mide,  dry  carbon  dioxide  is  allowed  to  pass  through  bromine,  a 
small  portion  of  the  vapour  of  which  is  carried  over  and  is  then 
brought  into  contact  with  dry  phosphorus  (Lieben).  Another 
method  of  preparation  consists  in  dissolving  both  of  these 
elements  separately  in  dry  carbon  bisulphide,  and  then  gradually 
pouring  the  bromine  solution  into  that  containing  the  phos- 
phorus and  distilling,  when  the  carbon  bisulphide  boiling  at  43° 
comes  off,  leaving  behind  the  phosphorus  tribromide  which  boils 
at  175°  (Kekule). 

The  simplest  process  for  preparing  the  tribromide  is  to 
place  red  phosphorus  in  a  flask  closed  by  a  doubly  bored 
cork,  one  opening  of  which  is  connected  with  an  inverted  con- 
denser, and  round  which  cold  water  is  allowed  to  flow,  whilst 
through  the  second  opening  a  funnel  with  glass  stopcock  is 
placed,  by  means  of  which  bromine  is  allowed  to  fall  slowly  on 
the  phosphorus.  The  first  drops  combine  with  evolution  of 

1  Besson,  Compt.  Rend.  HI,  972.  2  Poulenc,  Compt.  Rend.  H3,  75. 


PHOSPHORUS  PENTABROMIDE  579 

light  and  heat,  but  this  rapid  combination  soon  ceases,  and  the 
bromine  can  be  allowed  to  drop  in  without  causing  any  violent 
action.  The  product  is  then  separated  by  distillation  from  the 
excess  of  phosphorus  which  must  be  present  (Schorlemmer). 

The  tribromide  is  a  colourless  mobile  liquid  possessing  at  0°  a 
specific  gravity  of  2*925.  It  has  a  strong  unpleasant  pungent 
odour,  and  is  decomposed  in  presence  of  water  into  phosphorous 
acid  and  hydrobromic  acid. 

PHOSPHORUS  PENTABROMIDE,  PBr5  =  427*6. 

This  body  is  obtained  when  bromine  is  added  to  the  cold 
tribromide.  It  is  a  lemon-yellow  crystalline  body  which  on 
heating  melts,  forming  a  red  liquid  which  decomposes  at  100° 
into  the  tribromide  and  bromine  (Gladstone).  When  a  current  of 
carbon  dioxide  is  led  through  the  melted  compound,  the  bromine 
is  carried  over  and  the  tribromide  remains  behind.  The  penta- 
bromide  possesses  an  extremely  pungent  odour,  and  forms, 
when  brought  into  contact  with  a  small  quantity  of  water, 
phosphorus  oxybromide  and  hydrobromic  acid. 

PHOSPHORUS  TRIFLUORODIBROMIDE,  PF3Br2  =  246'2 

Is  obtained  by  the  union  of  phosphorus  trifluoride  and  bromine, 
and  forms  an  amber  yellow  fuming  liquid,  which  solidifies  at 
—  20°,  forming  pale  yellow  crystals  (Moissan). 

PHOSPHORUS  CHLOROBROMIDE,  PCl3Br2  =  305-1. 

330  If  bromine  and  phosphorus  trichloride  are  brought 
together  in  molecular  proportions,  heat  is  evolved  and  the  liquid 
separates  into  two  layers.  The  upper  layer  consists  of  a  solution 
of  bromine  in  phosphorus  trichloride,  whilst  the  lower  consists  of 
a  solution  of  trichloride  in  bromine.  If  the  mixture  be  cooled 
down  to  a  temperature  of —20°  the  whole  solidifies  to  a  yellowish- 
red  crystalline  mass  which  again  splits  up  at  ordinary  tempera- 
tures into  two  distinct  layers  (Wichelhaus).  If  this  mixture, 
placed  in  a  sealed  tube,  be  exposed  for  some  weeks  to  about 
15°,  a  crystalline  compound  is  formed  which  is  more  stable,  in- 
asmuch as  it  does  not  decompose  into  its  constituents  until  a 
temperature  of  35°  is  reached. 

Phosphorus  chlorobromide  combines  with  bromine,  and  forms 


580  THE  NON-METALLIC  ELEMENTS 

the  compound  PCl3Br2  +  Br9  which  solidifies  in  large  crystals,  red 
by  transmitted,  but  blue  by  reflected,  light.  A  second  compound 
PCl3Br2  +  2Br2  is  also  formed  at  the  same  time,  crystallizing  in 
needle-shaped  crystals  of  a  greenish  lustre,  which  soon  turn 
brown.  These  compounds  correspond  to  those  formed  by  the 
union  of  phosphorus  trichloride  with  iodine  monochloride  and 
with  the  metallic  chlorides  (Michaelis). 


PHOSPHORUS  AND  IODINE. 

PHOSPHORUS  DI-IODIDE,  P2I4  =  565-24. 

331  This  compound,  which  has  an  analogous  composition  to 
liquid  phosphuretted  hydrogen,  is  obtained  by  dissolving  one 
part  of  phosphorus  in  carbon  bisulphide  and  then  adding 
gradually  8'2  parts  of  iodine.  On  gently  warming  the  solution 
so  a&  to  distil  off  the  carbon  bisulphide,  the  di-iodide  remains 
behind  as  a  yellow  crystalline  mass.  When  the  bisulphide  is 
cooled  down  to  0°,  the  same  compound  separates  out  in  long 
orange-red  crystals  (Corenwinder).  The  crystals  melt  at  110° 
and  are  decomposed  by  water  with  formation  of  red  phosphorus, 
phosphorous  acid,  and  hydriodic  acid ;  thus  :—?• 

3P2I4  +  12H2O  =  2P  +  4P(OH)3  +  12HI. 
Its  vapour  density  corresponds  to  the  formula  P2I4-1 

PHOSPHORUS  TRI-IODIDE,  PI3  =  408-53. 

This  compound  is  obtained  in  a  similar  way  to  the  foregoing, 
but  using  1^  times  as  much  iodine.  By  gently  heating  the 
solution,  the  greater  portion  of  the  bisulphide  of  carbon  is  got 
rid  of,  and  the  residue  is  cooled  down  by  a  mixture  of  salt  and 
ice.  Red  six-sided  crystals  separate  out  which  melt  at  55°  and 
which  on  gently  cooling  may  be  obtained  of  large  size.  On  the 
addition  of  water  to  this  compound  hydriodic  and  phosphorous 
acids  are  formed  ;  thus  . — 

PI3  +  3H2O  =  P(OH)3  +  3HI. 

Phosphorus  pentiodide  PI5  has  been  described  by  Hampton,2 
but  its  existence  cannot  as  yet  be  regarded  as  definitely  proved. 
1  Troost,  Compt.  Rend.  95,  293.  2  Chem.  News,  42,  180. 


PHOSPHORUS  SUBOXIDE  581 


OXIDES  AND  OXYACIDS  OF  PHOSPHORUS. 

Oxygen   forms  with   phosphorus   four  compounds,  and  with 
hydrogen  and  oxygen  a  large  number  of  acids : — 

Oxides.  Acids. 

Phosphorus  suboxide,  P4O.       Hypophosphorous  acid,  PH(OH)2. 
Phosphorous  oxide,  P4O6.          Phosphorous  acid,  P(OH)3. 
Phosphorus  tetroxide,  P2O4.      Hypophosphoric  acid,  P2O2(OH)4. 
Phosphorus  pentoxide,  P2O5.     Phosphoric  acid,  PO(OH)3. 

Pyrophosphoric  acid,  P2O3(OH)4. 

Metaphosphoric  acid,  P02OH. 


PHOSPHORUS  SUBOXIDE,  P40. 

332  This  oxide  was  first  obtained  by  Leverrier  by  allowing 
a  solution  of  phosphorus  in  phosphorus  trichloride  to  oxidise 
in  the  air,1  and  is  also  obtained  by  the  action  of  zinc  on  phos- 
phorus oxychloride,2  and  together  with  the  other  oxides  when 
phosphorus  is  burned  in  an  insufficient  supply  of  air.3     It  is  a 
red  apparently  amorphous  compound  and  closely  resembles  red 
phosphorus  in  appearance. 

HYPOPHOSPHOROUS  ACID,  H3PO2. 

333  The  salts  of  this  acid  were  discovered  by  Pulong  in  1816. 
They  are  formed   when  the  phosphides  of  the  metals  of  the 
alkaline  earths  are  decomposed  by  water,  or  when  phosphorus  is 
boiled  with  an  alkali  or  an  alkaline  earth.     For  the  purpose  of 
preparing  the  acid,  baryta  is  best  employed,  as  it  is  from  the 
barium  salt  that  not  only  the  other  salts  but  also  the  free  acid 
is  easily  prepared.     The   formation   of  barium    hypophosphite 
is  shown  in  the  following  equation : — 

3Ba(OH)2  +  8P  +  6H2O  =  3Ba(PH2O2).2  +  2PH3. 

At  the  same  time  a  small  quantity  of  barium  phosphate  is 
formed,  but  this  can  readily  be  separated  from  the  hypophos- 
phite by  filtration.  To  the  clear  solution,  the  requisite  quantity 
of  dilute  sulphuric  acid  is  added,  and  the  filtered  solution 

1  Annalen,  27,  167.  2  Reinitzer  and  Goldschmidt,  Bcr.  13,  847. 

3  Thorpe  and  Tutton,  Journ.  Chem.  Soc.  1890,  i.  549  ;  1891,  i.  1019. 


582  THE  NON-METALLIC  ELEMENTS 

evaporated  to  a  syrupy  consistency.  On  cooling  the  solution, 
the  hypophosphorous  acid  is  obtained  in  the  form  of  a  thick  very 
acid  liquid.  Hypophosphorous  acid  can  also  be  obtained  in  the 
form  of  a  white  crystalline  mass  melting  at  17°'4  as  follows.1 
The  tolerably  concentrated  solution  is  gently  evaporated  in  a 
platinum  dish  at  a  temperature  below  its  boiling  point  and  then 
gradually  heated  from  110°  up  to  130°,  at  which  temperature 
it  is  allowed  to  remain  for  ten  minutes.  The  solution  thus 
obtained  is  cooled,  poured  into  a  stoppered  bottle,  and  this  placed 
in  a  freezing  mixture  at  a  few  degrees  below  0°.  The  acid 
is  then  found  either  to  crystallize  spontaneously  or  to  do  so  on 
being  touched  with  a  glass  rod. 

Hypophosphorous  acid  when  strongly  heated  decomposes  into 
phosphuretted  hydrogen  and  phosphoric  acid  ;  thus  :  — 


Its  aqueous  solution  precipitates  gold  and  silver  from  solutions 
of  their  salts,  phosphoric  acid  being  formed  ;  thus  :  — 

4AgN03  +  2H2O  +  H3P02  =  4Ag  +  4HN03  +  H3P04. 

When  a  solution  of  this  acid  is  added  to  mercuric  chloride 
solution,  either  calomel  (mercurous  chloride)  or  metallic  mercury 
is  precipitated  according  to  the  proportions  in  which  the  acid  is 
present. 

Hypophosphorous  acid  is  also  oxidized  by  chlorine  and  other 
oxidizing  agents  to  phosphoric  acid,  and  when  exposed  to  the 
air  it  takes  up  oxygen  with  the  formation  of  phosphorous  acid. 
Nascent  hydrogen  reduces  it  to  phosphuretted  hydrogen. 
Although  hypophosphorous  acid  contains  the  group  hydroxyl 
twice,  only  one  atom  of  hydrogen  can  be  replaced  by  metals, 
This  is  easily  explained  when  we  remember  that  this  acid  may 


rOH 

X 


be   considered  as   dihydroxyl-phosphine  PK  OH,  or  as  a  weak 

IH 

basic  body  which  has  become  a  weak  acid  by  the  addition  of 
two  atoms  of  oxygen. 

The  Hypophosphites. — Most  of  the  salts  of  hypophosphorous 
acid  are  soluble  in  water,  some  being  also  soluble  in  alcohol, 
and  crystallizable.  In  the  dry  state  they  do  not  undergo  altera- 
tion in  the  air  and  may  be  boiled  in  water,  free  from  absorbed 
oxygen,  without  decomposition.  Like  the  free  acid,  all  the 

1  Thomsen,  Ber.  7,  994,  996. 


PHOSPHOROUS  OXIDE 


583 


hypophosphites  possess  strong  reducing  properties,  giving  with 
solutions  of  gold,  silver,  and  mercury,  the  same  reactions  as 
the  acid  itself. 


PHOSPHOROUS  OXIDE  OR  PHOSPHOROUS  ANHYDRIDE,  P406. 

334  This  oxide  was  first  obtained  by  Sage  in  1777,  and 
was  also  observed  by  Cabell,  but  was  first  thoroughly  examined 
by  Thorpe  and  Tutton.1  In  order  to  prepare  it  the  apparatus 
shown  in  Fig.  167  is  employed.  Pieces  of  phosphorus  about 
an  inch  in  length  are  placed  in  the  wide  combustion  tube 
(a),  which  is  open  at  one  end  to  admit  air  and  bent  into  the 
shape  shown  to  prevent  escape  of  melted  phosphorus.  The 


FIG.  167. 


other  end  of  the  tube  is  fitted  into  a  brass  tube  enclosed 
in  a  second  wider  brass  tube  (b),  water  being  introduced  into 
the  space  between  the  tubes  by  means  of  d.  A  loose  plug  of 
glass  wool  is  placed  at  the  further  end  of  the  brass  tube  be- 
fore the  connection  with  the  U-shaped  condenser  (c),  surrounded 
by  ice  and  salt,  which  is  in  turn  connected  with  the  wash  bottle 
(/)  containing  sulphuric  acid.  The  outlet  of/  is  in  connection  with 
a  water  pump,  by  means  of  which  a  slow  current  of  air  is  drawn 
through  the  apparatus.  The  phosphorus  is  ignited  by  carefully 
heating,  and  the  temperature  of  the  water  in  the  jacket  allowed 
to  reach  50°  and  maintained  at  that  temperature  till  nearly  the 
end  of  the  experiment,  when  it  is  allowed  to  rise  to  60°.  The 
red  oxide  formed  condenses  in  the  portion  of  the  tube  nearest 
the  phosphorus,  and  the  passage  of  the  pentoxide  is  prevented 
by  the  plug  of  glass  wool,  whilst  the  phosphorous  oxide  passes 

1  Loc.  cit. 


584  THE  NON-METALLIC  ELEMENTS 

through  to  the  condenser.  The  reaction  is  stopped  as  soon  as 
four-fifths  of  the  phosphorus  has  been  burnt,  and  the  U  tube 
detached  and  warmed,  when  the  phosphorous  oxide  melts  and 
falls  into  the  small  bottle  below. 

Phosphorous  oxide  is  thus  obtained  as  a  wax-like  mass,  but 
may  also  be  condensed  in  the  form  of  feathery  crystals,  and 
when  the  melted  substance  is  allowed  to  cool  it  separates  out 
in  thin  prisms  capped  by  pyramids,  probably  belonging  to  the 
monosymmetric  system.  It  has  an  unpleasant  garlic  like  odour, 
melts  at  22'5°  and  resolidifies  at  21°  but  frequently  exhibits  the 
property  of  superfusion  ;  the  liquid  has  a  specific  gravity  of 
1-9358  at  24-8°/4°.  It  boils  at  173'1  (corr.)  in  an  atmosphere  of 
nitrogen,  its  vapour  density  being  7'74 ;  its  molecular  formula, 
like  that  of  the  corresponding  oxide  of  arsenic,  is  therefore  P406. 
When  quite  pure  it  is  unaltered  in  the  light,  but  it  usually  assumes 
a  dark  red  colour  owing  to  the  separation  of  red  phosphorus. 

Phosphorous  oxide  is  only  attacked  very  slowly  by  cold  water, 
with  the  gradual  formation  of  phosphorous  acid  ;  with  hot  water 
however  a  violent  reaction  takes  place,  spontaneously  inflam- 
mable hydrogen  phosphide  being  evolved,  and  phosphoric  acid 
and  either  red  phosphorus  or  the  suboxide  remaining  behind. 
On  exposure  to  the  air  or  oxygen  it  spontaneously  oxidises  to 
phosphorus  pentoxide,  and  when  heated  to  50 — 60°  ignites  and 
burns  with  great  brilliancy.  It  is  acted  upon  with  great 
violence  by  alcohol,  ignition  taking  place,  but  dissolves  without 
alteration  in  ether,  carbon  bisulphide,  benzene,  and  chloroform. 
When  heated  in  a  sealed  tube  to  210°  it  becomes  turbid  and  at 
440°  is  completely  converted  into  red  phosphorus  and  phos- 
phorus tetroxide.  When  thrown  into  chlorine  it  burns  with  a 
green  flame,  but  by  the  slow  action  of  the  gas  it  yields  a  mixture 
of  phosphoryl  and  metaphosphoryl  chlorides.  With  sulphur  it 
yields  phosphorus  sulphoxide,  and  with  ammonia  the  diamide  of 
phosphorous  acid  HOP(NH2)2. 

PHOSPHOROUS  ACID,  H3P03. 

335  This  acid  is  formed  together  with  hypophosphorous  and 
phosphoric  acids  when  phosphorus  is  oxidised  in  moist  air.  In 
order  to  prepare  pure  phosphorous  acid  the  reaction  employed  by 
Davy  in  1812  is  employed.  This  consists  in  the  decomposition  of 
the  trichloride  by  water  ;  thus : — 

PC13  +  3H2O  =  P(OH)3  +  3HC1. 


PHOSPHOROUS  ACID  585 


For  this  purpose  it  is  not  necessary  to  prepare  the  pure 
trichloride  separately,  for,  if  chlorine  be  led  through  melted 
phosphorus  under  water,  the  trichloride  is  first  formed,  and,  on 
coming  in  contact  with  the  water,  this  is  decomposed  as  shown 
in  the  above  equation.  Care  must  be  taken  to  stop  passing  the 
chlorine  in  before  all  the  phosphorus  has  disappeared,  as  the 
chlorine  would  otherwise  oxidize  the  phosphorous  to  phosphoric 
acid.  It  is  difficult  to  prevent  this  altogether  even  if  phosphorus 
is  present  in  excess. 

By  evaporating  the  solution  until  the  residue  attains  a  tem- 
perature of  180°  a  thick  syrupy  substance  is  obtained  which  is 
transformed  more  or  less  rapidly  on  cooling  into  a  crystalline 
mass  melting  at  70°'1.]  Phosphorous  acid  has  an  acid,  garlic- 
like  taste,  absorbs  moisture  rapidly  from  the  air  and  deliquesces. 
When  strongly  heated,  phosphorous  acid  decomposes  into 
phosphuretted  hydrogen  and  phosphoric  acid  ;  thus : — 

4H3P03=3H3P04  +  PH3. 

And  when  treated  with  phosphorus  pentachloride,  phosphorus 
trichloride  is  formed  : — 

P(OH)3  +  3PC15  =  PC13  +  3POC1, 4  3HC1. 

Hence  the  trichloride  is  the  chloride  of  phosphorous  acid.2 

The  aqueous  solution  of  the  acid  also  slowly  absorbs  oxygen 
from  the  air,  and  in  presence  of  nascent  hydrogen  it  is  reduced 
to  phosphuretted  hydrogen.  It  acts  as  a  strong  reducing 
agent,  precipitating  gold,  silver,  and  mercury  from  their  solutions 
like  hypophosphorous  acid. 

The  Phosphites. — Although  a  weak  acid,  phosphorous  acid  is 
tribasic,  but  under  ordinary  circumstances,  -only  two  atoms  of  its 
hydrogen  can  be  replaced  by  metals.  A  normal  tribasic  salt, 
P(ONa)3,  is  known,  but  this  has  not  yet  been  obtained  in  the 
anhydrous  state.3  On  the  other  hand,  ethers  of  phosphorous 
acid  are  known  in  which  the  three  atoms  of  hydrogen  are  replaced 
by  a  radical,  such  as  ethyl,  C2H5,  and  the  resulting  tri-ethyl 
phosphite,  P(OC2H5)3,  is  a  body  which  can  be  distilled  without 
decomposition. 

The  phosphites  which  are  soluble  in  water  possess  an  acid  and 

1  J.  Thomsen,  Ber.  7,  996.  2  Geuther,  /.  Pr.  Chem,  [2]  8,  359. 

3  Zimmermann   Annalen,  175,  21. 


586  THE  NON-METALLIC  ELEMENTS 

garlic-like  taste.  They  act  upon  the  salts  of  the  noble  metals 
like  the  hypophosphites,  from  which,  however,  they  are  distin- 
guished by  giving  a  precipitate  with  baryta-  or  lime-water. 


PHOSPHORUS  TETROXIDE,  P2O4. 

336  This  compound  is  prepared  by  heating  the  mixture 
of  oxides  obtained  by  the  combustion  of  phosphorus  in  an 
insufficient  supply  of  air  to  290°  in  a  sealed  tube.  Phosphorus 
tetroxide  then  volatilises  and  condenses  in  the  cool  portions  of 
the  tube,  in  the  form  of  clear  transparent  crystals,  belonging  to 
the  rhombic  system.  It  may  also  be  prepared  from  phos- 
phorous oxide  by  heating  it  to  440°  when  it  decomposes  into  red 
phosphorus  and  phosphorus  tetroxide. 

The  tetroxide  is  a  deliquescent  substance  and  dissolves  in 
water  with  evolution  of  heat  yielding  a  strongly  acid  solution 
which  is  unaltered  by  boiling,  reduces  mercuric  to  mercurous 
chloride,  and  gives  with  silver  nitrate  a  white  precipitate  rapidly 
changing  to  black.  It  only  decolourises  permanganate  solution 
slowly,  and  on  evaporation  in  vacuo  yields  a  thick  colourless 
syrup  containing  phosphorous  and  phosphoric  acids.  It  is  not  the 
anhydride  of  hypophosphoric  acid,  as  the  latter  reduces  per- 
manganate quickly,  and  also  forms  a  sparingly  soluble  sodium  salt, 
which  cannot  be  obtained  from  the  solution  of  the  tetroxide.1 


HYPOPHOSPHORIC  ACID,  H4P2O6. 

337  It  was  formerly  supposed  that  the  acid  obtained  by  the 
slow  oxidation  of  phosphorus  in  moist  air,  prepared  by  exposing 
thin  strips  of  phosphorus  to  a  limited  supply  of  moist  air  as 
shown  in  Figs.  168  and  169,  is  phosphorous  acid.  Salzer2  has, 
however,  shown  that  in  addition  to  phosphoric  and  phosphorous 
acids  this  liquid  contains  hypophosphoric  acid.  On  neutralising 
with  caustic  soda,  a  slightly  soluble  salt,  sodium  hypophosphate, 
H2Na.2P2O6,  separates  out.  A  solution  of  this  salt  yields  with 
lead  acetate  an  insoluble  salt,  Pb2P2O6,  from  which  on  decom- 
position with  sulphuretted  hydrogen,  the  acid  can  be  prepared, 
as  an  odourless  acid  liquid.  The  aqueous  solution  decomposes 

1  Thorpe  and  Tutton,  Journ.  Chem.  Soc.  1886,  i.  833. 

2  Annalen,  187,  222  ;  194,  28  ;  211,  1. 


PHOSPHORUS  PENTOXIDE 


587 


on  concentration.     Hypophosphoric  acid  is  tetrabasic,  and  yields 
two  classes  of  salts.     The  sodium  salts  crystallize  well : 

Na4P2O6  4-  10H2O     Normal  sodium  hypophosphate. 
H2Na2P2O6  +  9H2O     Hydrogen  sodium  hypophosphate. 


FIG.  168. 


FIG.  169. 


PHOSPHORUS  PENTOXIDE,  P2O5. 

338  The  thick  white  clouds  which  are  formed  when  phosphorus 
burns  brightly  in  the  air  consist  of  this  oxide.  If  a  small  piece  of 
phosphorus  is  burnt  on  a  dry  plate  covered  with  a  bell-jar,  these 
fumes  condense  partly  on  the  sides  of  the  glass  and  partly  on  the 
plate,  in  the  form  of  a  white  flocculent  powder,  whilst  the  por- 
tion near  the  burning  phosphorus  forms  a  glassy  mass.  This 
white  powder  is  amorphous,  and  may  be  sublimed  in  a  test- 
tube  heated  over  a  gas  lamp.  The  pure  powder  is  perfectly 
colourless  and  odourless.  If  it  should  possess  any  garlic-like 
smell  it  contains  phosphorous  oxide,  and  if  it  has  a  yellowish  or 
reddish  colour  it  is  mixed  with  red  phosphorus  or  phosphorus 
suboxide.  It  possesses  no  action  upon  dry  blue  litmus  paper, 
and  is  excessively  hygroscopic,  deliquescing  very  quickly  when 
exposed  to  the  air,  with  formation  of  metaphosphoric  acid  : — 

P205  +  H20==2HP03. 

When  thrown  into  water  it  dissolves  with  a  hissing  noise 
and  with  the  evolution  of  a  large  amount  of  heat.  On  heating 
with  carbon,  or  other  reducing  agents,  phosphorus  is  formed  : — 


Phosphorus   pentoxide    is    often    used    in    the  laboratory  as  a 
desiccating  agent,  especially  for  the  purpose  of  removing  the  last 


588 


THE  NON-METALLIC  ELEMENTS 


traces  of  moisture  from  gases  or  from  liquids.  Owing  to  its 
power  of  combining  with  water  it  is  able  to  withdraw  the 
elements  of  water  from  many  compounds  containing  oxygen 
and  hydrogen.  Thus  it  is  used  for  the  preparation  of  nitrogen 
pentoxide  and  other  compounds. 

In  order  to  prepare  larger  quantities  of  the  pentoxide  the  ar- 
rangement shown  in  Fig.  170  is  employed.  (A)  is  a  large  dry 
glass  balloon,  provided  with  three  necks  (a,  d,  and  y) ;  the  neck  (g) 
is  connected  with  a  powerful  water-aspirator  by  means  of  which 


FIG.  170. 


air,  dried  by  passing  through  the  drying  tube  (/),  is  drawn  into 
the  balloon  ;  g  communicates  with  a  large  wide-necked  bottle 
(B)  into  which  a  portion  of  the  light  powder  is  driven  by  the 
current  of  air  ;  through  the  neck  (a)  passes  a  straight  glass  tube, 
closed  with  a  cork  at  the  top  but  open  at  the  bottom,  reaching 
nearly  to  the  centre  of  the  balloon  and  having  a  small  copper 
crucible  (c)  fixed  to  its  lower  end.  A  piece  of  phosphorus  is 
dropped  down  the  straight  tube  into  the  crucible  and  ignited  by  a 
hot  wire,  a  good  current  of  air  being  kept  up  until  the  operation  is 
complete  ;  a  second  piece  of  phosphorus  is  then  dropped  down 


PHOSPHORUS  PENTOXIDE 


589 


into  the  crucible  (c),  and  when  this  is  burnt  a  third  piece  is 
introduced,  and  so  on,  until  a  sufficient  quantity  of  the  pentoxide 
has  been  obtained. 

A  more  practically  useful  arrangement  for  preparing  the  pen- 
toxide in  quantity  consists  of  a  cylinder  (a,  Fig.  171),  open  at 
both  ends,  made  of  common  sheet-iron,  fourteen  inches  high  and 
twelve  inches  in  diameter,  having  a  cover  provided  with  a  bent 
chimney  (b),  one  inch  in  diameter,  closed  by  a  cork.  The  cylinder 
is  supported  by  a  wooden  tripod,  and  rests  in  a  sheet-iron 


FIG.  171. 

funnel  (h\  fitting  into  the  neck  of  a  wide-mouthed  bottle  (g).  A 
copper  spoon  (<£),  fixed  to  an  iron  rod,  serves  to  receive  the 
phosphorus  which  is  from  time  to  time  renewed  by  drawing 
back  the  spoon  to  the  opening  (e)  and  dropping  in  a  fresh 
piece.  In  order  to  renew  the  supply  of  air,  which  during  dry 
weather  does  not  require  desiccation,  the  board  (i)  is  occasionally 
removed,  and  air  allowed  to  enter  between  the  funnel  and  the 
cylinder.1  On  account  of  its  being  so  exceedingly  hygroscopic, 

1  V.  Grabowski,  Annalen,  136,  119. 


590  THE  NON-METALLIC  ELEMENTS 

the  pentoxide  thus  obtained  must  be  preserved  in  well-stoppered 
bottles,  or,  better,  in  hermetically-sealed  flasks. 

In  order  to  free  phosphorus  pentoxide  from  all  traces  of  lower 
oxides,  it  must  be  distilled  over  platinum  sponge  in  a  current  of 
oxygen.1 

PHOSPHORIC  ACID. 

339  The  history  of  this  acid  possesses  a  peculiar  interest  for 
the  scientific  chemist.  In  the  year  1746,  Marggraf  observed 
that,  when  fusible  salt  of  urine,  NH4NaHPO4,  is  mixed  with  a 
solution  of  nitrate  of  silver,  a  yellow-coloured  silver  salt  is 
precipitated.  It  was  afterwards  noticed  that  other  salts  of 
phosphoric  acid,  as,  for  instance,  the  ordinary  phosphate  of  soda, 
gave  the  same  reaction ;  but  Clark,  in  the  year  1828,  pointed 
out  that  when  the  salt  is  heated  and  then  dissolved  in  water, 
the  solution  so  obtained  gives  with  nitrate  of  silver  a  white 
precipitate.  He,  therefore,  distinguished  the  acid  contained  in 
the  heated  salt  from  that  contained  in  the  common  phosphate, 
and  termed  the  former  pyrophosphoric  acid.  In  1829  Gay- 
Lussac  proved  that  the  acid  thus  prepared  can  be  converted 
into  other  salts  without  losing  its  peculiar  properties;  and 
Berzelius  and  Engelhardt  had  previously  found  that  freshly- 
prepared  solution  of  well-ignited  phosphoric  acid  is  able  to 
coagulate  clear  solutions  of  albumen,  whilst  this  is  not  the  case 
when  the  solution  of  the  acid  has  been  standing  for  any  con- 
siderable length  of  time.  These  and  similar  observations  led  to 
the  conclusion  that  phosphoric  acid,  with  which  the  pentoxide 
was  then  classed,  can  exist  in  several  isomeric  conditions. 

The  classical  researches  of  Thomas  Graham2  first  threw  a 
clear  light  on  this  subject.  He  showed,  in  the  first  place,  that, 
in  addition  to  ordinary  phosphoric  acid  and  pyrophosphoric 
acid,  a  third  modification  exists,  to  which  he  gave  the  name  of 
metaphosphoric  acid,  and  that  this  is  the  substance  which  has 
the  power  of  coagulating  albumen.  He  also  ascertained  that 
common  phosphates,  when  they  are  saturated  with  a  base, 
contain  three  times  as  much  of  that  base,  in  proportion  to  the 
same  weight  of  phosphoric  acid  as  the  metaphosphates,  whilst 
the  pyrophosphates  contain  twice  as  much  as  the  metaphosphates. 
Graham  likewise  proved  that,  when  the  acids  are  liberated  from 

1  Threlfall,  Phil.  Mag.  1893  ;  Shenstone  and  Beck,  Journ.  Chem.  Soc.  1893, 
i.  473.  2  Phil.  Trans.  1833,  ii.  253. 


PHOSPHORIC  ACID  59L 


these  different  salts,  they  may  be  regarded  as  containing  different 
quantities  of  water.  Hence  the  composition  of  the  three  acids 
is  as  follows  : — 

Old  notation.       New  notation. 
Common,  or  Orthophosphoric  acid    P2O5  +  3H2O      H3P04 

Pyrophosphoric  acid P2O5  +  2H2O      H4P2O7 

Metaphosphoric  acid P2O5  +  H2O       HPO3. 

The  exact  constitution  of  these  acids  has  been  the  subject 
of  a  considerable  amount  of  discussion.  The  supporters  of  the 
theory  of  the  constant  valency  of  the  elements  regarded  these 
three  acids  as  well  as  hypophosphorous  and  phosphorous  acids 
as  derivatives  of  phosphine  PH3  and  ascribed  to  them  the 
following  constitutional  formulae : 

Hypophosphorous  Acid.  Phosphorous  Acid. 

H<\  RO. 

)P— OH  )P— OH 

H/  HO/ 


Orthophosphoric  Acid.              Pyrophosphoric  Acid.  Metaphosphoric  Acid. 

OH— P— O— OH 

HOV  /O 

>P— 0— OH                  O  HO— P/  | 

HO/                                     |  XO 
OH— P— O— OH 


Other  chemists  however  believed  that  in  the  phosphoric  acids 
phosphorus  behaves  as  a  pentad,  and  represented  them  by  the 
following  formulae  : — 

Orthophosphoric  Acid.          Pyrophosphoric  Acid.  Metaphosphoric  Acid 

HO. 

>P  =  0 

H0\  HO/  |  O   =  P  =  O 

HO-^P  =  0  O 

HO/  HOX    |  OH 

>P  =  0 
HO/ 

Since  it  has  been  shown  that  phosphorus  forms  a  stable  pen- 
tafluoride,  the  vapour  density  of  which  remains  constant  through 
a  considerable  range  of  temperature  and  corresponds  to  the 
molecular  formula  PF5,  the  pentad  nature  of  phosphorus  in 
many  of  its  derivatives  has  been  generally  admitted,  and  the 


592  THE  NON-METALLIC  ELEMENTS 


second  series  of  formulae  for  the  phosphoric  acids  is  now  almost 
universally  accepted. 

Poisonous  Action  of  the  Acids  of  Phosphorus. — When  ad- 
ministered in  an  uncombined  condition,  the  various  oxides  of 
phosphorus  produce  apparently  the  same  symptoms  which 
follow  the  administration  of  other  mineral  acids.  Sufficient 
data  do  not  exist  as  to  the  specific  physiological  action  of  all 
these  compounds. 

In  the  case  of  ortho-  meta-  and  pyrophosphoric  acids  it  would 
appear  that  the  first,  when  in  combination  with  inactive  bases, 
acts  as  a  perfectly  inert  body.  The  second  possesses  some 
activity  as  a  poison,  while  the  pyro-salts  when  introduced 
directly  into  the  blood  are  found  to  be  very  powerful  poisons 
(Gamgee). 

ORTHOPHOSPHORIC  ACID,  H3PO4. 

340  In  order  to  prepare  this  acid,  red  phosphorus  is  heated  in 
a  retort  with  common  concentrated  nitric  acid.  The  phosphorus 
is  oxidized  at  the  expense  of  the  nitric  acid,  and  red  fumes  are 
slowly  evolved.  When  the  phosphorus  has  dissolved,  the  residue 
is  evaporated  in  a  porcelain  dish,  and  the  concentrated  solution 
repeatedly  treated  with  nitric  acid,  in  order  to  oxidize  com- 
pletely any  phosphorous  acid  which  may  have  been  formed.  As 
soon  as  the  further  addition  of  nitric  acid  is  unaccompanied  by 
the  evolution  of  red  fumes,  the  operation  is  concluded,  and  the 
residue  only  requires  to  be  evaporated  again,  in  order  to  get 
rid  of  the  excess  of  nitric  acid  (v.  Schrotter).  Common  phos- 
phorus was  formerly  employed  instead  of  red  phosphorus  for 
this  purpose ;  but  this  undergoes  oxidation  much  more  slowly 
than  the  red  variety,  inasmuch  as  it  melts  forming  round 
globules,  which  are  only  slowly  attacked  by  the  nitric  acid.  In 
addition  to  this,  when  common  phosphorus  is  employed,  weak 
nitric  acid  can  alone  be  used,  as  the  strong  acid  is  apt  to 
produce  an  explosion  when  brought  in  contact  with  it. 

The  residue  obtained  in  the  preparation  of  hydriodic  acid 
by  means  of  iodine  and  phosphorus  consists  of  a  mixture 
of  phosphoric  and  phosphorous  acids,  containing  a  small  quan- 
tity of  hydriodic  acid.  In  order  to  prepare  pure  orthophosphoric 
acid  from  this  residue,  it  may  be  heated  with  a  little  fuming 
nitric  acid,  and  filtered,  in  order  to  separate  it  from  the  solid 
iodine  which  is  liberated.  More  nitric  acid  is  then  added,  in 


PHOSPHORIC  ACID  593 


order  to  oxidize  phosphorous  acid,  and  the  liquid  is  evaporated 
to  a  syrupy  consistency. 

Orthophosphoric  acid  is  prepared  on  the  large  scale  from 
bone-ash,  which  consists  chiefly  of  tri-calcium  phosphate, 
together  with  a  small  quantity  of  magnesium  phosphate  and 
calcium  carbonate.  According  to  Liebig,  equal  weights  of 
sulphuric  acid  and  bone -ash  are  taken ;  the  sulphuric  acid 
is  diluted  with  ten  times  its  weight  of  water,  and  is  allowed 
to  remain  in  contact  with  the  bone  ash  for  some  time.  The 
acid  solution  is  then  filtered  through  linen,  and  the  filtrate 
evaporated  to  a  small  bulk.  On  the  addition  of  strong  sul- 
phuric acid,  the  calcium,  still  present  in  solution,  is  precipitated 
as  gypsum.  The  clear  solution  is  then  poured  off,  evaporated  to 
dryness,  and  freed  from  an  excess  of  sulphuric  acid  by  ignition. 
The  residue  is  free  from  lime  and  sulphuric  acid,  but  it  contains 
small  quantities  of  magnesia,  which  can  only  with  difficulty  be 
removed. 

In  order  to  prepare  a  pure  acid  from  bone-ash,  the  powdered 
ash  is  dissolved,  in  the  smallest  possible  quantity  of  nitric 
acid,  and  to  the  clear  liquid  a  solution  of  acetate  of  lead 
is  added.  A  precipitate  of  lead  phosphate  falls  down,  which 
must  be  warmed  for  some  time  with  the  liquid,  in  order  to  free 
the  precipitate  from  any  calcium  phosphate  which  is  thrown 
down  with  the  lead  salt.  It  is  well  washed  with  boiling  water, 
and  decomposed  by  sulphuretted  hydrogen.  According  to 
Berzelius's  method,  the  lead  salt  is  decomposed  by  dilute 
sulphuric  acid,  and  the  excess  of  acid  removed  by  ignition  of 
the  evaporated  filtrate.  The  residue  is  then  dissolved  in  water 
and  freed  from  traces  of  lead  by  sulphuretted  hydrogen. 

Another  method  which  may  be  employed  is  to  dissolve  the 
bone-ash  in  its  own  weight  of  hydrochloric  acid  of  specific 
gravity  1*18,  diluted  with  four  times  its  weight  of  water.  To 
this  solution  one-and-a-half  parts  of  dry  sodium  sulphate  is 
added,  whereby  a  precipitate  of  gypsum  is  produced  ;  this  is 
then  filtered  off,  and  the  boiling  solution  neutralized  with 
sodium  carbonate  ;  the  solution  is  again  filtered,  to  separate 
any  calcium  carbonate  which  may  fall  down,  and  the  whole 
precipitated  with  barium  chloride.  The  precipitate  thus  formed 
consists  of  a  mixture  of  barium  sulphate  and  barium  phosphate 
and  this  is  decomposed  by  one  part  of  sulphuric  acid,  having  a 
specific  gravity  of  171  (Neustadt). 

A  further  method  is  to  treat  calcium  phosphate  with  hydro- 

39 


594  THE  NON-METALLIC  ELEMENTS 

fluoric  acid  in  a  leaden  dish ;  the  excess  of  acid  is  then  evapor- 
ated off  and  the  solution  filtered  from  separated  calcium  fluoride 
and  evaporated.1 

Commercial  phosphoric  acid  frequently  contains  arsenic  acid, 
derived  from  the  sulphuric  acid  or  hydrochloric  acid  employed 
in  its  manufacture.  In  order  to  free  it  from  arsenic,  it  must  be 
dissolved  in  water,  sulphur  dioxide  led  through  the  warm 
solution,  in  order  to  reduce  the  arsenic  acid  to  arsenious  acid, 
then  the  solution  boiled  to  remove  the  excess  of  sulphur  dioxide, 
and  sulphuretted  hydrogen  passed  through,  by  which  means  the 
whole  of  the  arsenic  is  precipitated  as  the  insoluble  trisulphide. 

341  Phosphoric  acid  is  extremely  soluble  in  water  ;  it  has  a 
pleasant   purely   acid   taste,   and  is  perfectly  free  from  smell. 
When  the  aqueous  acid  is  evaporated  down  until  the  residue 
possesses  the  composition,  H3P04,  it  presents  the  appearance  of 
a   thick    syrup,  from  which,   on   standing,  a   crystalline   mass 
is  deposited.     If  a  crystal  of  this  acid  be  dropped  into  a  freshly- 
prepared   solution  of  the  requisite  strength,  crystals  begin   to 
form   at   once,  and  soon  spread  throughout  the  mass.     These 
crystals  belong  to  the  rhombic  system,  forming  six-sided  prisms 
terminated  by  six-sided  pyramids,  which  melt  at  38'6°.2      The 
crystallized  acid  may  be  heated  to  160°  without  undergoing  any 
alteration,  but  above  this  temperature  it  loses  water,  and  at  213° 
is  completely  converted  into  pyrophosphoric  acid,  H4P2O7.     This 
substance,  in  its  turn,  loses  water  when  heated  to  redness,  with 
formation  of  metaphosphoric  acid,  HPO3. 

The  table  on  the  following  page  gives  the  variation  of  the 
specific  gravity,  with  the  percentage  composition  of  aqueous 
solutions  of  ortho-phosphoric  acid  : — 3 

342  The    Orthophosphates. — Orthophosphoric   acid,  being  tri- 
basic,  forms  three  classes  of  salts  according  as  one,  two,  or  three 
atoms  of  hydrogen  are  replaced  by  their  equivalent  of  metal. 
Thus  we  know  three  orthophosphates  of  sodium  : — 

Trisodium  or  normal  sodium  phosphate,  Na3P04  +  2H20. 
Hydrogen  disodium  phosphate,  HNa9PO4  + 1 2H2O. 
Dihydrogen  sodium  phosphate,  H2NaPO4  +  H2O. 

Of  the  normal  salts  those  of  the  alkalis  with  the  exception  of 
the  lithium  salt  are  easily  soluble  in  water  and  their  solutions 

1  Nicolas,  Oompt.  Rend.  HI,  975.  2  J.  Thomsen,  Ber.  7,  997. 

3  John  Watts,  Chem.  Neics,  12,  160. 


THE  ORTHO PHOSPHATES 


595 


have  a  strong  alkaline  reaction.  The  normal  orthophosphates 
which  are  insoluble  in  water  are  easily  soluble  in  dilute  acids 
by  which  means  they  are  converted  into  the  soluble  hydrogen 


Specific 
Gravity. 

Per  Cent. 

PA. 

Specific 
Gravity. 

Per  Cent. 
P205. 

Specific 
Gravity, 

Per  Cent. 
PA 

1-508 

49-60 

1-328 

36-15 

1-144 

17-89 

1-492 

48-41 

1-315 

34-82 

1-136 

16-95 

1-476 

47-10 

1-302 

33-49 

1-124 

15-64 

1-464 

45-63 

1-293 

32-71 

1-113 

14-33 

1-453 

45-38 

1-285 

31-94 

1-109 

13-25 

1-442 

44-13 

1-276 

31-03 

1-095 

1218 

1-434 

43-95 

1-268 

30-13 

1-081 

10-44 

1-426 

43-28 

1-257 

29-16 

1-073 

9-53 

1-418 

42-61 

1-247 

28-24 

1-066 

8-62 

1-401 

41-60 

1-236 

27-30 

1-056 

7-39 

1-392 

40-86 

1-226 

26-36 

1-047 

6-17 

1-384 

40-12 

1-211 

24-79 

1-031 

415 

1-376 

39-66 

1-197 

23-23 

1-022 

3-03 

1-369 

39-21 

1-185 

22-07 

1-014 

1-91 

1-356 

38-00 

1-173         20-91 

1-006 

0-79 

1-347 

37-37 

1-162         19-73 

1-339 

36-74 

1-153 

18-81 

orthophosphates.  These  latter  salts  are  readily  obtained  from 
the  normal  compounds;  even  carbon  dioxide  brings  about  the 
change ;  thus,  if  this  gas  be  led  into  a  solution  of  trisodium 
phosphate  the  following  reaction  takes  place  : — 

Na3PO4  +  C02  +  H2O  =  HNa2PO4  +  HNaCO3. 

The  hydrogen  disodium  phosphate  which  is  here  formed 
together  with  hydrogen  sodium  carbonate  is  the  common 
phosphate  of  soda  of  the  shops.  This  salt,  although  according 
to  its  constitution  it  must  be  considered  as  an  acid  salt, 
inasmuch  as  it  still  contains  hydrogen  replaceable  by  a 
metal,  has  a  slightly  alkaline  reaction,  and,  like  other  soluble 
hydrogen  orthophosphates,  is  easily  obtained  by  adding  a  solution 
of  soda  to  phosphoric  acid  until  the  liquid  has  a  weak  alkaline 
reaction.  The  hydrogen  disodium  orthophosphate  is  converted 
on  heating,  with  loss  of  water,  into  the  pyro phosphate. 

The  dihydrogen  orthophosphates  of  the  alkalis  are  soluble  in 


596  THE  NON-METALLIC  ELEMENTS 

water,  and  possess  a  slight  acid  reaction.  Dihydrogen  potassium 
phosphate,  H2KPO4,  forms  large  well  defined  crystals,  and  this 
salt  may  be  heated  to  a  temperature  of  400°  without  losing 
water.  At  a  higher  temperature,  however,  one  molecule  of 
water  is  driven  off  and  potassium  metaphosphate,  KPO3,  is 
formed. 

The  orthophosphates  can  readily  be  recognized  by  the  follow- 
ing reactions.  The  normal  as  well  as  the  acid  salts  give  with 
nitrate  of  silver  a  yellow  precipitate  of  silver  orthophosphate, 
Ag3P04;  thus:— 

Na3PO4  +  3AgNO3  =  3NaN03  +  Ag8PO4. 
HNa2PO4  +  3AgNOs=  2NaNO8  +  HNO3  +  Ag3PO4. 
H2NaP04  +  3AgN08=  NaNOs  +  2HNO3  +  Ag3PO4. 

In  the  case  of  common  phosphate  of  soda  and  nitrate  of  silver, 
we  have  the  singular  fact  of  two  neutral  or  slightly  alkaline 
solutions,  when  mixed,  yielding  a  strongly  acid  liquid. 

When  to  a  solution  of  an  ortho-salt  a  mixture  of  sal-ammoniac, 
ammonia,  and  magnesium  sulphate  solutions  is  added,  a  crystal- 
line precipitate  of  ammonium  magnesium  phosphate, 

(NH4)MgP04  +  CH205 

is  thrown  down.  In  order  to  detect  orthophosphoric  acid  in  a 
substance  insoluble  in  water,  the  body  may  be  dissolved  in 
nitric  acid  and  an  excess  of  a  solution  of  molybdate  of  am- 
monia in  nitric  acid  added  to  the  liquid.  If  phosphoric  acid  be 
present,  this  solution  on  slightly  warming,  or  on  standing  in  the 
cold,  yields  a  dense  yellow  precipitate.  The  composition  of 
this  precipitate  is  approximately  represented  by  the  following 
formula : — 

14Mo03  +  (NH4)3PO4  +  4H2O. 


METAPHOSPHORIC  ACID,  HP03. 

343  This  modification  of  phosphoric  acid  was  discovered  by 
Graham  in  1833.  It  is  obtained  when  a  solution  of  phosphoric 
acid  is  heated  until  the  residue  does  not  give  off  any  more 
water.  The  acid  thus  prepared  solidifies  on  cooling  to  a  soft 
pasty  mass  which  on  exposure  to  the  air  readily  absorbs 
moisture  and  deliquesces.  The  glacial  phosphoric  acid  of  the 


METAPHOSPHORIC  ACID  597 

shops  is  metaphosphoric  acid  which  usually  contains  soda  as 
impurity.1  Metaphosphoric  acid  is  also  formed  when  crystalline 
phosphorous  acid  is  heated  in  a  sealed  tube  with  bromine 
(Gustavson) ;  thus  : — 

H3P03  +  Br2  =  HP03  +  2HBr. 

When  phosphorus  pentoxide  is  allowed  to  deliquesce  in  moist 
air  or  when  it  is  dissolved  in  cold  water  metaphosphoric  acid  is 
formed.  This  aqueous  solution  on  standing  at  the  ordinary 
temperature  of  the  air,  gradually  undergoes  change  with  forma- 
tion of  common  phosphoric  acid  and  this  conversion  takes  place 
quickly  without  the  formation  of  the  intermediate  pyrophos- 
phoric  acid  if  the  liquid  be  boiled  (Graham).  Metaphosphoric 
acid  is  volatile  at  a  bright  red  heat  and  when  heated  with 
sulphates  expels  sulphuric  acid  from  them,  for  although  sul- 
phuric acid  is  a  stronger  acid  than  phosphoric  acid,  the  former 
is  more  easily  volatile  than  the  latter.  An  aqueous  solution  of 
metaphosphoric  acid  is  also  obtained  by  passing  a  current  of 
sulphuretted  hydrogen  gas  through  a  liquid  containing  lead 
metaphosphate  in  suspension. 

The  solution  of  metaphosphoric  acid  is  distinguished  from 
those  of  the  other  two  modifications  inasmuch  as  it  produces 
a  white  precipitate  with  solutions  of  calcium  chloride,  barium 
chloride  and  albumen. 

344  The  Metaphosphatcs. — The  salts  of  metaphosphoric  acid 
are  obtained  by  neutralizing  the  aqueous  solution  of  the  acid  by 
a  base  or  by  heating  a  dihydrogen  orthophosphate ;  thus  : — 

KH2P04  =  KP03  +  H20. 

No  less  than  five  distinct  modifications  of  the  metaphosphates 
are  known  to  exist.2 

(1)  tfonometaphosphates. — Of  this   class   only   those   of  the 
alkali  metals  are  known,  such  as  KPO3.     The  monometaphos- 
phates  are  remarkable  as  being  insoluble  in  water.     The  potas- 
sium    salt     is    formed,    as     above    shown,    when    dihydrogen 
potassium  phosphate  is  heated.     The  monometaphosphates  are 
distinguished  from  the  other  modifications  inasmuch  as  they  do 
not  form  any  double  salts. 

(2)  Dimetaphosphates. — These  salts  are  formed  when  aqueous 
phosphoric  acid  is  heated  to  a  temperature  of  350°  (Fleitmann) 


1  Brescius,  Zeit.  Analyt.  Chem.  6,  187. 

2  Maddrell,  Mem.  Chem.  Soc.  3,  373. 


598  THE  NON-METALLIC  ELEMENT 

or  316°  (Maddrell)  with  the  oxides  of  zinc,  manganese,  or  copper. 
If  the  copper  salt  be  then  decomposed  by  potassium  sulphide  a 
soluble  potassium  dimetaphosphate,  K2P2O6,  is  obtained  and  if 
sodium  sulphide  be  employed  a  soluble  sodium  dimetaphosphate 
is  in  like  manner  produced.  In  addition  to  the  dimetaphosphates 
containing  only  one  metal,  double  salts  such  as  CuK2(P2O6)2 
can  be  prepared.  Only  the  dimetaphosphates  of  the  alkali 
metals  are  soluble  in  water  and  are  crystallizable  ;  the  others 
are  insoluble  or  only  very  slightly  soluble  (Fleitmann). 

(3)  Trimetapliosphates. — The  sodium  salt,  Na3P3O9,  is  obtained 
together  with  the  monometaphosphate  when  microcosmic  salt, 
(NH4)HNaP04,  is  gently  heated  until  the  fused  mass  becomes 
crystalline  (Lindbom).     By  double  decomposition  other  trimeta- 
phosphates    can   be   obtained   from   this   salt.     These   are   all 
soluble  in  water,  including  the  silver  salt,  and  they  form  double 
salts  such  as  NaBaP309.    The  silver  salt  may  be  obtained  in  the 
form  of  large  transparent  monosyin metric  crystals  by  allowing  a 
mixture  of  the  sodium  salt  and  nitrate  of  silver  solution  to  stand 
for  some  days.     In  the  same  way  the  crystalline  lead  salt  may 
be  prepared  by  the  substitution  of  nitrate  (but  not  of  acetate) 
of  lead  for  the  silver  salt. 

(4)  Tctrametaplwsphates. — The  lead  salt,  Pb2P4O12,  is  formed 
by  treating  oxide  of  lead  with  an  excess  of  phosphoric  acid  and 
heating  up  to  a  temperature  of  300°.     If  this  is  then  decom- 
posed  by  sodium  sulphide   the   sodium   salt  is  obtained  as   a 
tetrametaphosphate.     This,  however,  is   not   a  crystalline  salt 
but  forms  with  a  small  quantity  of  water  a  viscid  elastic  mass 
and  on  the  addition  of  a  larger  quantity  of  water  a  gum-like 
solution  which  will  not  pass  through  a  filter.     The  tetrameta- 
phosphates  of  the  alkalis  produce  viscid  precipitates  with  the 
soluble  salts  of  the  alkaline  earths.     If  sodium  dimetaphosphate 
is  fused  with  copper  dimetaphosphate  and  the  mixture  allowed 
gradually  to  cool  a  double  compound  having  the  composition 
CuNa2P4O12  is  formed  (Fleitmann  and  Henneberg). 

(5)  Hcxametapliosphates. — The     sodium    salt,    Na6P6O18,    is 
obtained   when  fused    sodium    metaphospliate    is    allowed    to 
cool  slowly.     It  is  a  crystalline  mass,  deliquesces  on  exposure 
to   the    air,  and  produces  with  barium    chloride    a    flocculent 
precipitate,  and  with  the   salts  of  the   heavy  metals  gelatinous 
precipitates. 

It  also  forms  characteristic  double  salts  in  which  fine  equiva- 
lents of  the  monad  metal  are  replaced  by  the  corresponding 


PYROPHOSPHORIC  ACID  599 

amount  of  a  dyad  metal,  the  calcium  salt,  for  example,  having 
the  composition  Ca"5Na2(PO3)12  or  (NaPO3)2(Ca"P2O6)5. 

The  metaphosphates  which  are  soluble  in  water  have  a  neutral 
or  slightly  acid  reaction.  When  their  solutions  are  boiled  they 
are  converted  into  orthophosphates.  All  the  metaphosphates 
undergo  this  change  on  boiling  with  nitric  acid  or  when  they 
are  fused  with  an  alkali. 

The  different  varieties  of  metaphosphates  are  derived  from 
acids,  all  of  which  possess  the  same  composition,  but  differ,  as 
the  double  salts  show,  from  one  another  in  molecular  weight. 
Compounds  of  this  description  are  termed  polymeric  bodies.  If 
we  assume  the  constitution  formula  now  usually  accepted  for 
metaphosphoric  acid,  namely  HO — P~O 

O 

the  constitution  of  these  hypothetical  acids  may  be  represented 
graphically  as  follows  : — 

O     OH 

V 

0  =  P— OH  P 


and 
O  =  P— OH 


HO     O     OH. 

Dimetaphosphoric  Acid.  Trimetaphosphoric  Acid. 


PYROPHOSPHORIC  ACID,  H4P2O7. 

345  Common  phosphate  of  soda,  or  hydrogen  disodium  phos- 
phate, HNa.2PO4,  when  heated  to  a  temperature  of  240°  loses 
water,  and  is  converted  into  sodium  pyrophosphate  (Graham)  : 

2HNa2PO4  =  H2O  +  Na4P2O7. 

The  salt  thus  obtained  dissolves  in  water,  but  does  not  again 
form  an  orthophosphate,  and  is  distinguished  from  the  original  salt 
inasmuch  as  its  solutions  yield  on  the  addition  of  silver  nitrate,  a 
white,  and  not  a  yellow,  precipitate.  This  fact  was  first  observed 
by  Clark,  of  Aberdeen,  in  the  year  1828.1  It  is,  however,  to 
Graham  that  we  are  indebted  for  a  knowledge  of  the  fact  that 
1  Edinburgh  Journal  of  Science,  7,  298. 


600  THE  NON-METALLIC  ELEMENTS 

the  heated  sodium  salt  as  well  as  the  silver  salt  obtained  from 
it  is  derived  from  an  acid  having  the  composition  H4P2O7,  and 
that  this  can  be  obtained  from  orthophosphoric  acid  by  heating 
it  for  a  considerable  length  of  time  to  a  temperature  of  215°. 
Pyrophosphoric  acid  is  also  formed  when  equal  molecules  of 
ortho-  and  metaphosphoric  acids  are  heated  together  at 
1000.1 

/OH  HOV  /OH 

HO— P=0+H0— P<  =  >P— 0— P< 

||XOH  HCK||  ||XOH 

00  00 

Pyrophosphoric  acid  forms  either  a  soft  glassy  mass  (Graham) 
or  an  opaque  indistinctly  crystalline  mass  (Peligot). 

An  aqueous  solution  of  pyrophosphoric  acid  is  obtained  by 
precipitating  the  sodium  salt  with  a  solution  of  acetate  of  lead 
and  decomposing  the  well-washed  lead  pyrophosphate  with  sul- 
phuretted hydrogen.  The  acid  solution  undergoes  no  change 
at  the  ordinary  temperature,  even  after  standing  for  some 
time,  but  when  heated  it  is  converted  into  orthophosphoric 
acid. 

Pyrophosphoric  acid  may  be  distinguished  from  the  ortho- 
modification  inasmuch  as  its  solution  produces  a  white  granular 
precipitate  with  silver  nitrate,  and  from  the  meta- variety  inas- 
much as  it  does  not  produce  a  precipitate  either  with  a  solution 
of  chloride  of  barium  or  with  one  of  albumen. 

346  Pyrophosphates. — These  salts  are  prepared  from  the  mono- 
hydrogen  orthophosphates  by  heat,  or  by  neutralizing  a  freshly 
prepared  solution  of  the  acid  by  means  of  a  base.  Both  normal 
pyrophosphates,  such  as  Na4P2O7,  and  acid  or  hydrogen  pyro- 
phosphates, such  as  H2Na2P2O7  exist ;  the  first  have  an  alkaline 
reaction,  the  second  a  slightly  acid  one.  The  pyrophosphates  of 
the  alkali  metals  are  soluble  in  water,  those  of  the  other  metals 
insoluble,  but  many  of  them  dissolve  in  an  excess  of  sodium 
pyrophosphate.  The  pyrophosphates  in  solution  remain  un- 
altered in  the  cold  and  even  on'  heating  do  not  change,  but 
when  boiled  with  an  acid  are  decomposed,  the  orthophosphates 
being  formed.  The  same  change  takes  place  on  fusion  with 
an  alkali. 

TetrapJwsphates. — A  sodium  salt  having  the  composition 
Na6P4O13,  was  obtained  by  Fleitmann  and  Henneberg  by  fusing 

1  Geuther,  J.  Pr.  Chcm.  [2],  8,  359. 


ESTIMATION  OF   PHOSPHORIC  ACID  601 

sodium  hexametaphosphate  with  pyrophosphate  or  orthophos- 
phate,  in  quantities  represented  by  the  following  equations : — 

Na6P6O18  +  3Na4P207  =  3Na6P4013 ;  or 
Na0P6018  +  2Na3P04  =  2Na6P4013. 


The  salt  thus  obtained  can  be  crystallized  from  solution  in  warm 
water  and  gives  with  silver  nitrate  a  white  precipitate,  Ag6P4013 
which  does  not  dissolve  in  an  excess  of  the  sodium  salt.  The 
constitution  of  the  pyrophosphates,  or  the  diphosphates  as  they 
may  be  called,  and  of  the  tetraphosphates  may  be  exhibited  as 

follows : — 

Diphosphoric  Acid.  Tetraphosphoric  Acid. 

OH 

OH  /PO— OH 

I  0< 

XPO-OH  >PO— OH 

°\  °\ 

\PO-OH  >PO-OH 

I  o< 

OH  \PO-OH 

I 
OH 

347  Quantitative  Determination  of  Phosphoric  Acid. — Phos- 
phoric acid  is  best  determined  in  the  soluble  phosphates  by 
adding  to  the  solution  a  mixture  of  sal-ammoniac,  ammonia  and 
magnesium  sulphate  solutions,  when  a  precipitate  of  ammonium 
magnesium  phosphate  occurs,  NH4MgP04  4-  6H2O.  After  this 
has  stood  for  some  time,  the  solution  is  filtered  off,  and  the 
precipitate  washed  with  dilute  ammonia,  dried,  converted  by 
ignition  into  magnesium  pyrophosphate,  Mg2P2O7,  and  weighed. 

The  insoluble  phosphates  must  be  converted  into  soluble  salts 
before  the  determination  of  the  phosphoric  acid.  If  they  are 
soluble  in  nitric  acid,  the  method  proposed  by  Sonnenschein 
may  be  adopted ;  namely,  to  precipitate  the  nitric  acid  solution 
of  the  phosphate  by  an  excess  of  ammonium  molybdate  dis- 
solved in  nitric  acid.  On  standing  for  some  time  at  a  moderate 
temperature,  a  yellow  precipitate,  containing  all  the  phosphoric 
acid,  is  formed ;  the  excess  of  molybdenum  salt  is  removed  by 
washing  with  water,  the  precipitate  dissolved  in  ammonia,  and 
the  phosphoric  acid  precipitated  by  magnesium  sulphate  as 
described  above. 


002  THE  NON-METALLIC  ELEMENTS 

If  the  phosphates  will  dissolve  in  acetic  acid  the  phosphoric 
acid  may  be  precipitated  with  uranium  acetate  as  uranium 
phosphate.  This  method  may  also  be  employed  for  the  volu- 
metric determination  of  phosphoric  acid,  the  point  of  complete 
precipitation  of  the  phosphoric  acid  being  ascertained  by  the 
addition  of  a  drop  of  the  solution  to  a  drop  of  a  solution  of  ferro- 
cyanide  of  potassium  with  which  the  slightest  excess  of  uranium 
acetate  produces  a  brown  colour. 

Metaphosphates  and  pyrophosphates  must  first  be  converted 
into  orthophosphates  before  precipitation.  In  like  manner 
phosphorus  itself  as  well  as  the  hypophosphites  and  phosphites 
may  also  be  quantitatively  determined  in  the  form  of  phosphoric 
acid  by  previously  oxidizing  them  with  nitric  acid. 

HALOGEN  DERIVATIVES  OF  PHOSPHORIC  ACID. 

348  The  hydroxyl  groups  contained  in  the  three  modifications  of 
phosphoric  acid  may  be  replaced,   as  is   the  case   with   other 
hydroxyacids,  by  chlorine  or  bromine  thus  giving  rise  to  the  oxy- 
chlorides  and  oxybromides  of  phosphorus,  as  they  are  commonly 
termed. 

PHOSPHORUS  OXYFLUORIDE,  POF3  =  103-4, 
was  first  obtained  by  Moissan  1  by  exploding  a  mixture  of 
phosphorus  trifluoride  and  oxygen  and  is  also  prepared  by 
heating  a  mixture  of  two  parts  of  cryolite  and  three  parts  of 
phosphorus  pentoxide  in  a  brass  tube,2  or  by  dropping  phosphorus 
oxychloride  on  to  dried  zinc  fluoride,3  or  by  the  action  of 
anhydrous  hydrogen  fluoride  on  phosphorus  pentoxide.4  It  is 
a  gas  which  fumes  in  the  air  and  condenses  to  a  liquid  at — 50° 
or  at  16°  under  a  pressure  of  15  atmospheres  ;  it  solidifies  at  a 
very  low  temperature. 

PHOSPHORUS  OXYCHLORIDE  OR  PHOSPHORYL  CHLORIDE, 
POC13.  =  152-25. 

349  This   compound    was  discovered    in    the  year  1847  by 
Wurtz,5  who    obtained    it    by  decomposing    the  pentachloride 
with  the  requisite  quantity  of  water : 

PC15  +  H20  =  POC13  +  2HC1. 

1  Compt.  llend.  102,  1245. 

2  Thorpe  and  Harably,  Journ.  Chem.  Soc.  1889,  i.  759. 

3  Bull.  Soc.  Chim.  (3),  4,  260.  4  Bull.  Soc.  Chim.  (3),  5,  458. 
5  Ann.  Chim.  Phys.  [3]  20,  472. 


PHOSPHORUS  OXYCHLORIDE  603 

The  best  method  of  preparing  this  substance  is  the  one  pro- 
posed by  Gerhardt ; l  namely,  by  heating  dried  oxalic  acid  with 
phosphorus  pentachloride,  when  the  following  reaction  takes 
place : — 

PC15  +  H2C2O4  =  POC13  +  2HC1  +  CO2  +  CO. 

Instead   of  oxalic  acid,  boric    acid    may  be  employed,  thus : — 

3PC15  +  2B(OH)3  =  3POC13  +  B2O3  +  6HC1. 

Pure  phosphorus  oxychloride  may  also  be  easily  obtained  by 
heating  the  pentachloride  and  the  pent  oxide  together  in  sealed 
tubes  in  the  proportion  of  3  molecules  of  the  former  to  one 
molecule  of  the  latter : 2 

3PC15  +  P205  =  5POC13. 

The  same  compound  is  also  obtained  by  distilling  phosphorus 
pentoxide  with  common  salt :  3 

2P2O5  +  SNaCl  =  POC13  +  3NaP03. 

Phosphorus  oxychloride  is  also  formed  as  a  product  of  decom- 
position in  the  preparation  of  many  acid  chlorides.  Several  ex- 
amples of  this  mode  of  formation  have  already  been  mentioned 
and  many  more  will  have  to  be  described. 

Phosphorus  oxychloride  is  a  colourless  mobile  liquid  boiling 
at  107°-2  and  having  a  specific  gravity  of  1-7118  at  0°  (Thorpe). 
When  strongly  cooled  it  solidifies  in  the  form  of  tabular  or 
needle-shaped  crystals  which  melt  at  -1°'5  (Geuther  and 
Michaelis).  It  fumes  strongly  in  the  air  and  has  a  very  pene- 
trating and  acrid  smell  resembling  that  of  phosphorus  trichloride. 
According  to  Cahours  the  specific  gravity  of  the  vapour  is  5'334. 
The  oxychloride  when  thrown  into  water  sinks,  and  slowly 
dissolves  with  the  formation  of  phosphoric  and  hydrochloric 
acids : — 

POC13  +  3H2O  =  PO(OH)3  +  3HC1. 

When  brought  in  contact  with  many  metallic  chlorides  it  forms 
crystalline  double  compounds. 

1  Ann.  Chim.  Phys.  [3]  44,  102. 

2  Gerhardt  and  Chiozza,  Annalen,  87,  290. 

3  Kolbe  and  Lautemami,  Annalcn,  113,  240. 


604  THE  NON-METALLIC  ELEMENTS 


PYROPHOSPHORYL  CHLORIDE,  P2O3C14. 

350  When  the  four  hydroxyl-groups  in  pyrophosphoric  acid, 
H4P2O7,  are  replaced  by  chlorine,  pyrophosphoryl  chloride  is 
formed.  It  was  first  prepared  by  Geuther  and  Michaelis,1  by 
the  action  of  nitrogen  peroxide  on  phosphorus  trichloride.  The 
reaction  which  takes  place  in  this  case  is  a  somewhat  compli- 
cated one,  nitrogen,  nitrosyl  chloride,  phosphoryl  chloride,  and 
phosphorus  pentoxide  being  formed. 

Pyrophosphoryl  chloride  is  a  colourless  strongly  fuming  liquid 
boiling  between  210°  and  215°  and  then  undergoing  partial 
decomposition  into  pentoxide  and  common  oxychloride  : 


At  7°  the  specific  gravity  of  the  liquid  is  1*78,  it  decomposes 
violently  in  contact  with  water  without  sinking  in  it,  and  in 
this  decomposition  orthophosphoric  acid  and  not  pyrophosphoric 
acid  is  formed.  When  it  is  treated  with  pentachloride  of 
phosphorus,  phosphoryl  chloride  is  produced  : 

P2(XC14  +  PC15  =  3POC13. 

By  the  action  of  phosphorus  pentoxide  on  phosphorus  oxy- 
chloride, Gustavson  2  obtained  a  syrupy  mass  which  he  regarded 
as  metaphosphoryl  chloride  PO2C1.  Hambly3  has  however 
shown  that  this  is  a  mixture  of  at  least  two  compounds,  one  of 
which  is  pyrophosphoryl  chloride  ;  the  other  constituent  has  a 
constant  composition  but  cannot  from  the  analysis  have  a  simpler 
formula  than  P7O15C15  and  is  probably  itself  a  mixture. 


PHOSPHORUS  OXYBROMIDE,  OR  PHOSPHORYL  BROMIDE. 

POBr3  =  28476. 

351  This  body  is  formed  by  the  action  of  a  small  quantity  of 
water  upon  the  pentabromide,  but  it  is  best  prepared  by  distil- 
ling pentabromide  of  phosphorus  with  oxalic  acid  (Baudrimont). 
It  forms  a  mass  of  flat  tabular  crystals  which  have  a  specific 
gravity  of  2'822  (Hitter),  melt  at  46°  and  boil  at  195°.  Water 
decomposes  the  oxybromide  into  phosphoric  and  hydrobromic 
acids. 

1  Ber.  4,  766.  2  £a\  4,  853.  3  Journ.  Chcm.  Soc.  1891,  i.  202. 


SULPHIDES  OF  PHOSPHORUS  605 


PHOSPHORYL  BROMOCHLORIDE.  POBrCl2.  =  196-42. 

When  phosphorus  trichloride  is  allowed  to  fall  drop  by 
drop  into  absolute  alcohol,  C2H5(OH),  the  first  action  that  takes 
place  is  the  formation  of  the  compound  (OC2H5)PC12.  This  body 
is  decomposed  by  the  addition  of  bromine  into  ethyl  bromide, 
C2H5Br,  and  phosphorus  oxybromochloride,  POBrCl2.  This 
latter  compound  is  a  highly  refractive  liquid,  boiling  at  136°  and 
having  at  0°  a  specific  gravity  of  2'049  (Menschutkin).  When 
the  liquid  is  cooled  it  solidifies  in  the  form  of  tabular  crystals 
melting  at  11°,  and  probably  isomorphous  with  the  crystals  of 
phosphoryl  chloride  and  phosphoryl  bromide  (Geuther  and 
Michaelis). 


PHOSPHORUS   AND   SULPHUR. 

352  The  compounds  formed  by  these  two  elements  were 
formerly  divided  into  two  classes  according  as  they  contained  a 
greater  number  of  atoms  of  phosphorus  or  sulphur.  Two  com- 
pounds of  the  former  class  have  been  described,  namely,  sulphur 
tetraphosphide  SP4  and  sulphur  diphosphide  SP2,  but  later 
researches1  have  shown  that  these  substances  are  in  reality 
simply  solutions  of  sulphur  in  phosphorus,  for  no  heat  is 
evolved  in  their  formation  from  the  elements,  and  a  current 
of  an  indifferent  gas  removes  the  whole  of  the  phosphorus  at 
temperatures  below  its  boiling  point. 

The  sulphides  of  phosphorus  are  solid  bodies  formed  when 
phosphorus  and  sulphur  are  gently  heated  together,  the  reaction 
being  accompanied  by  the  evolution  of  heat.  If  common 
phosphorus  is  employed  for  this  purpose  violent  explosions 
may  occur,  and  hence  it  is  advisable  to  employ  red 
phosphorus.  Even  in  the  latter  case,  when  finely  divided 
sulphur,  such  as  flowers  of  sulphur,  is  employed,  the  reaction  is 
often  very  violent,  and  for  this  reason  the  sulphides  of  phos- 
phorus are  best  prepared  by  mixing  the  necessary  quantity  of 
red  phosphorus  with  small  lumps  of  roll  sulphur.  The 
mixture  is  effected  in  a  flask,  the  cork  loosely  placed  in,  and  the 
flask  then  heated  on  a  sand  bath  by  means  of  a  Bunsen  flame, 

1  Schultze,  J.  Pr.  Chem.  [2]  22,  H3  ;  Ber.  16,  2066  ;  Isambert,  Compt. 
Rend.  96,  1499,  1628,  1721, 


606  THE  NON-METALLIC  ELEMENTS 

until  the  reaction  begins,  when  the  flame  is  removed.  After 
the  flask  has  cooled  it  may  be  broken  to  obtain  the  solid  mass 
which  is  then  preserved  in  dry  well-closed  bottles  (Kekul4). 

TETRAPHOSPHORUS  TRISULPHIDE,  P4S3. 

This  compound  is  obtained  as  a  yellow  mass,  crystallizing 
from  solution  in  carbon  bisulphide  or  phosphorus  trichloride  in 
the  form  of  rhombic  prisms.  It  melts  at  166°  (Ramme),  forming 
a  reddish  liquid,  which  boils  about  380°  (Isambert).  When 
heated  in  a  current  of  carbon  dioxide  it  sublimes  at  260°  and 
condenses  to  form  crystals,  which  appear  to  belong  to  the  regular 
system.  The  specific  gravity  of  its  vapour  is  7'90.  It  is  very 
easily  inflammable  and  is  slowly  decomposed  in  contact  with 
boiling  water  with  formation  of  sulphuretted  hydrogen,  phos- 
phuretted  hydrogen,  and  phosphorous  acid  : — 

P4S3  +  9H20  =  3SH2  +  3P(OH)3  +  PH3. 

TETRAPHOSPHORUS  HEXASULPHIDE,  P4SC, 

Is  obtained  according  to  Isambert1  by  heating  the  calculated 
quantities  of  the  elements  in  an  atmosphere  of  carbon  dioxide. 
It  crystallizes  in  thin  needles,  and  boils  at  490°,  the  vapour 
density  corresponding  to  the  above  formula. 


TRIPHOSPHORUS  HEXASULPHIDE,  P3S6. 

This  is  obtained  in  crystalline  needles,  melt-ing  from  296 — 
298°,  by  heating  yellow  phosphorus  and  sulphur  with  carbon 
bisulphide  for  some  hours  to  210°.  The  above  molecular 
formula  was  established  by  a  vapour  density  determination 
(Ramme).2 

PHOSPHORUS  PENTASULPHIDE,  P2S5. 

353  In  order  to  prepare  this  compound  in  the  pure  state,  the 
sulphide  prepared  by  Kekule's  method  is  distilled  in  a  current  of 
carbon  dioxide.  A  pale  yellow  crystalline  mass  is  thus  obtained, 
and  frequently  distinct  crystals.3  When  a  mixture  of  yellow 

1  Compt.  Rend.  102,  1386.  2  Ber.  12,  940,  1350. 

3  Carl  Meyer  and  V.  Meyer,  Ber.  12,  610. 


THIOPHOSPHORIC  ACID  607 

phosphorus  and  sulphur  in  the  proper  proportions  is  heated  with 
carbon  bisulphide  to  210°,  fine,  pale,  yellow  crystals  separate 
out  (Ramme).  Phosphorus  pentasulphide  melts  at  274  —  276°, 
boils  at  518°  (Goldschmidt),  and  yields  a  brown  vapour  having  a 
specific  gravity  of  7'67  (C.  and  V.  Meyer).  Water  decomposes 
this  substance  as  follows  :  — 

P2S6  +  8H20  =  2PO(OH)3  +  5H2S. 

Phosphorus  pentasulphide  is  often  used  in  making  organic 
preparations  for  the  purpose  of  replacing  oxygen  in  organic 
compounds  by  sulphur.  Thus,  for  instance,  if  common  alcohol, 
C2H.OH,  be  heated  with  this  body,  thioalcohol  or  mercaptan, 
C2H5SH,  is  formed. 

PHOSPHORUS  SULPHOXIDE,  P4O6S4, 

Is  obtained  by  carefully  heating  phosphorous  oxide  with  the 
calculated  quantity  of  sulphur  to  150  —  170°,  the  product  when 
heated  in  vacuo  subliming  in  colourless  strongly  refractive  crystals. 
It  melts  at  about  102°,  and  boils  at  295°,  yielding  a  vapour 
whose  density  corresponds  to  the  formula  P4O6S4.  It  rapidly 
deliquesces  in  the  air  and  is  quickly  dissolved  by  water,  form- 
ing sulphuretted  hydrogen  and  metaphosphoric  acid,  which  is 
eventually  converted  into  orthophosphoric  acid  :  — 

6H20  =  4HP03  +  4H2S. 


MONOTHIOPHOSPHORIC   ACID,  H3PSO3. 

354  This  substance  may  be  regarded  as  phosphoric  acid  in 
which  one  atom  of  sulphur  replaces  one  atom  of  oxygen.  It  is 
however,  not  known  in  the  free  state.  The  sodium  salt,  Na3PSO3, 
is  produced  when  the  chloride  (described  below)  is  heated  with 
caustic  soda.  The  salt  forms  distinct  crystals,  which  have  an 
alkaline  reaction,  and  are  decomposed  by  the  weakest  acids,  the 
thiophosphoric  acid  which  is  thus  liberated  being  at  once 
decomposed  into  sulphuretted  hydrogen  and  phosphoric  acid  ; 
thus  :  — 

PS(OH)3  +  H20  =  PO(OH)3  +  SH2. 

Dithiophosphoric   acid,    H3PS.2O2,   and    trithiophosphoric   acid, 
H3PS30,  are  also  known  in  their  salts.1 

1  Kubierschky,  J.  Pr.  CJtcm.  [2],  31,  93. 


608  THE  NON-METALLIC  ELEMENTS 


THIOPHOSPHORYL   FLUORIDE,    PSF3. 

355  This  compound  is  obtained  by  the  action  of  thiophos- 
phoryl  chloride  on  arsenic  fluoride,  but  is  best  prepared  by 
heating  a  mixture  of  carefully  dried  lead  fluoride  and  phos- 
phorus pentasulphide  to  170 — 250°,  the  following  reaction  taking 
place : — 

P2S5  +  3PbF2  =  3PbS  +  2PSF3. 

At  the  ordinary  temperature  it  is  a  transparent  colourless 
gas,  which  condenses  to  a  liquid  under  a  pressure  of  10 — 11 
atmospheres.  It  does  not  attack  dry  glass,  but  is  spontaneously 
inflammable  in  air  or  oxygen,  and  is  decomposed  by  heat  or  the 
electric  spark  with  separation  of  sulphur  and  phosphorus  and 
formation  of  phosphorus  fluorides.  The  products  obtained  by 
the  action  of  oxygen  consist  of  phosphorus  pentafluoride,  phos- 
phorus pentoxide  and  sulphur  dioxide,  and  with  water,  of 
sulphuretted  hydrogen,  phosphoric  and  hydrofluoric  acids.1 


THIOPHOSPHORYL  CHLORIDE,  PSC13. 

This  compound,  which  corresponds  to  oxychloride  of  phos- 
phorus is  best  obtained  by  acting  upon  pentachloride  of  phos- 
phorus with  phosphorus  pentasulphide  (Weber)  : 

P2S5  =  5PSC13. 


It  is  a  colourless,  mobile,  very  refractive  liquid,  whi(5h  fumes 
strongly  in  the  air,  possesses  a  powerful  pungent  odour,  boils  at 
125°,  and  has  a  specific  gravity  at  0°  of  1-16816  (Thorpe).  The 
specific  gravity  of  the  vapour  according  to  Cahours  is  5  '878  at 
298°.  It  is  decomposed  by  water  into  hydrochloric  acid  and 
thiophosphoric  acid,  which  is  then  further  decomposed  as 
described  above. 


THIOPHOSPHORYL  BROMIDE,  PSBr3. 

According  to  Baudrimont  this  body  is  obtained  by  distilling 
the  tribrornide  of  phosphorus  with  flowers  of  sulphur.     It  is 
however  best  prepared  by  dissolving  equal  parts  of  sulphur  and 
1  Thorpe  and  Rodger,  Journ.  Chcm.  Soc.  1889,  i.  306. 


PHOSPHOROUS  DIAMIDE  609 

phosphorus  in  carbon  bisulphide,  and  adding  gradually  to  the 
well-cooled  liquid  eight  parts  of  bromine.  On  distillation  the 
carbon  bisulphide  first  comes  over  and  then  the  thio-brornide. 
This  is  purified  by  shaking  it  up  several  times  with  cold  water ; 
a  hydrate  is  formed  in  this  way  having  the  composition 
PSBrs  +  H2O,  which  on  being  warmed  to  35°  separates  into 
its  constituents.  The  anhydrous  compound  is  obtained  by  the 
evaporation  of  the  solution  in  carbon  bisulphide  as  a  yellow 
liquid,  which,  when  touched  by  a  solid  body,  at  once  solidifies 
to  a  crystalline  mass.  This  compound  can  also  be  obtained 
in  the  crystalline  state  from  solution  in  phosphorus  tri- 
bromide,  when  it  separates  out  in  regular  octahedra,  having 
a  yellow  colour  and  melting  at  38°.  When  heated  with  water 
it  is  slowly  decomposed,  and  on  distillation  is  resolved  partly 
into  sulphur  and  into  a  compound,  PSBr3PBr3,  which  boils  at 
2050.1 

PYROPHOSPHORYL  THIOBROMIDE,  P2S3Br4. 

356  This  compound,  the  sulphur  analogue  of  pyrophosphoryl 
chloride,  is  obtained  by  pouring  carbon  bisulphide  over  phos- 
phorus trisulphide  and  adding  to  it,  drop  by  drop,  a  solution 
of  bromine  in  carbon  bisulphide.2  It  forms  a  light  yellow 
oily  liquid  which  fumes  in  the  air,  has  an  aromatic  and 
pungent  smell,  and  on  heating  decomposes  into  the  ortho- 
compound  and  phosphorus  pentasulphide  ;  thus  : — 


PHOSPHORUS  AND  NITROGEN. 

357  No  compound  of  phosphorus  and  nitrogen  is  known,  but 
derivatives  of  phosphorous  and  phosphoric  acids  containing  this 
element  have  been  prepared. 

Phosphorous  diamide  HO.P(NH2)2  is  obtained  by  the  action  of 
ammonia  on  a  solution  of  phosphorous  oxide  in  ether  or  benzene. 
It  is  a  white  powder  which  dissolves  in  water  instantly,  with 
such  violence  that  it  becomes  incandescent.  When  treated  with 
moderately  dilute  hydrochloric  acid,  a  violent  reaction  occurs 
with  liberation  of  non-spontaneously  inflammable  phosphine, 

1  Michaelis,  Annalcn,  104,  9.  2  Michaelis,  loc.  cit. 

40 


610  THE  NON-METALLIC  ELEMENTS 

separation  of  free  phosphorus,  and  formation  of  a  solution  of 
ammonium  chloride,  phosphorous  and  phosphoric  acids.1 

Amidophosphoric  acid  or  Phosphamidic  acid,  NH9.PO(OH)2. 
The  potassium  salt  of  this  acid  is  obtained  by  the  action  of 
potash  on  the  corresponding  phenyl  salt.  The  free  acid,  which 
is  prepared  by  decomposing  the  silver  salt  with  sulphuretted 
hydrogen  and  adding  alcohol  to  the  filtered  solution,  crystallises 
in  colourless  microscopic  crystals,  which  have  a  sweet  taste,  and 
are  not  hydrolysed  by  caustic  alkalis.  It  forms  both  normal 
and  acid  salts.2 

Diamidophosphoric  acid,  (NH2)2PO.OH,  is  obtained  in  a 
similar  manner  to  the  foregoing  compound.3  It  is  a  crystalline 
substance  and  is  converted  by  nitrous  acid,  first  into  monamido- 
phosphoric  acid  and  then  into  orthophosphoric  acid.  In  addi- 
tion to  the  normal  silver  salt,  (NH2)2PO.OAg,  it  forms  a 
remarkable  compound  of  the  formula  (NHAg)2P(OAg)3,  derived 
from  the  unknown  acid  (NH2)2  P(OH)3. 

358  When  dry  ammonia  is  passed  over  phosphorus  penta- 
chloride  as  long  as  it  is  absorbed,  a  white  mass  is  obtained 
consisting  probably  of  a  mixture  of  the  compound  PC13(NH2)2 
and  NH4CJ.  No  means  of  separating  these  are  known,  but  if 
the  product  be  treated  with  water  ammonium  chloride  dissolves, 
and  the  chloramido-compound  is  decomposed  with  loss  of  hydro- 
chloric acid,  and  is  converted  into  phosphamide  PO(NH)NH2. 
This  remains  behind  as  a  white  insoluble  powder,  which  is 
slowly  converted  by  the  boiling  liquid  into  acid  ammonium 
phosphate : — 

PO(NH)NH2  +  3H30  =  PO(OH)(ONH4)2. 

Phospham,  P3H3N6  (?).  If  the  product  of  the  reaction  of 
ammonia  and  phosphorus  pentachloride  is  heated  in  absence  of 
air  until  no  further  fumes  of  ammonium  chloride  are  evolved,  a 
light  white  powder  having  the  above  composition  remains  behind 
to  which  the  name  phospham  has  been  given.4  It  is  insoluble 
in  water  and  does  not  melt  at  a  red  heat,  but  oxidises  on  heating 
in  the  air  with  evolution  of  white  fumes.  If  it  be  moistened 
with  water  and  then  heated,  metaphosphoric  acid  and  ammonia 
are  formed : — 

P3H3N6  +  9H2O  =  3PO3H  +  6NH3. 

1  Thorpe  and  Tutton,  Journ.  Chem.  Soc.  1891,  i.,  1027. 

2  Stokes,  Amer.  Chem.  Journ.  15,  198.  3  Stokes,  Ber.  27,  565. 
4  Liebig  and  Wohler,  Annalcn,  H,  146  ;  Hofmann,  Ber.  17,  1909. 


IMIDODIPHOSPHORIC  ACIDS  611 


When  fused  with  potash  it  decomposes  with  evolution  of  light 
and  heat : — 

P3H3N6  +  9KOH  +  3H20  =  3PO(OK)3  +  6NH3. 

Its  exact  molecular  weight  has  not  yet  been  ascertained,  but  from 
analogy  with  the  corresponding  phosphorus  chloronitride  it  has 
probably  the  molecular  formula  given  above. 

Phosphoryl  nitride,  PON,  is  obtained  by  heating  phosphamide 
to  redness  in  absence  of  air,  or  by  subjecting  the  product  of  the 
reaction  of  ammonia  and  phosphorus  oxychloride  at  0°  to  the 
same  treatment.  It  forms  a  white  amorphous  powder,  which 
melts  at  a  red  heat,  and  resolidifies  to  a  glassy  mass.  It  is  not 
acted  upon  by  nitric  acid,  but  when  fused  with  caustic  alkalis  is 
converted  into  ammonia  and  potassium  orthophosphate. 

PON  +  3KOH  =  PO(OK)3  +  NH3. 


IMIDODIPHOSPHORIC  ACIDS. 

359  The  action  of  ammonia  on  phosphorus  oxychloride  was 
investigated  many  years  ago  by  Gladstone  l  and  by  Schiff.2  The 
former  succeeded  in  obtaining  from  the  product  a  number  of 
acids,  which  he  regarded  as  pyrophosphaminic  acids,  or  acids 
derived  from  pyrophosphoric  acid  by  the  replacement  of  hydroxyl 
by  the  group  NH2.  The  later  investigations  of  Mente  3  have 
however  shown,  that  these  acids  are  in  reality  formed  from 
diphosphoric  acid  by  the  replacement  of  one  or  more  oxygen 
atoms  by  the  imido  group  NH  and  they  may  therefore  be 
termed  imidodiphosphoric  acids. 

The  constitution  of  these  acids  is  probably  as  follows  :  — 

/NH\ 
Imidodiphosphoric   acid,   HO.PO<  >PO.OH, 

\  O  / 

XNH\ 
Diimidodiphosphoric  acid,  HO.PO(  >PO.OH, 


/NH\ 
Diimidodiphosphaminic  acid,  HO.POc  )PO.NH2. 


1  AnnaUn,  76,  74  ;  Journ.  Chem.  Soc.  1850,  121  ;  1851,  135,  353  ;  1864, 
215  ;  1866,  290  ;  1868,  64,  261  ;  1869,  15. 

2  Annalen,  101,  299  ;  102,  113  ;  103,  169.        3  Annalen,  248,  232. 


612  THE  NON-METALLIC  ELEMENTS 

In  addition  to  these  Mente  has  also  prepared  nitrilotrimeta- 

Po/OH 

phosphoric  acid,  which  has  the  constitution 


Phosphoryl  triamide,  PO(NH2)3  has  been  described  by  Schiff, 
but  later  investigators  have  failed  to  obtain  it,  his  compound  being 
probably  impure  diimidodiphosphaminic  acid. 

Phosphorus  chloronitride,  P3N3C16.  This  substance  was  first 
obtained  by  Lie  big  and  Wohler  among  the  products  of  the 
reaction  of  ammonia  and  phosphorus  pentachloride,  and  has 
been  more  closely  investigated  by  Gladstone  *•  and  Wichelhaus.2 
It  is  best  prepared  by  subliming  a  mixture  of  one  part  of 
phosphorus  pentachloride  and  two  parts  of  dried  ammonium 
chloride,  purifying  the  sublimate  by  washing  with  Avater  and 
distilling  in  a  current  of  steam.  It  crystallises  in  thin  trans- 
parent six-sided  rhombic  tablets,  melts  at  114°  and  boils  at  250°; 
its  vapour  density  is  12'05,  corresponding  to  the  formula  given 
above.  It  is  insoluble  in  water,  but  is  slowly  decomposed  by  it 
with  formation  of  diimidodiphosphoric  acid. 

The  substance  PNC12  obtained  by  Besson  by  heating  the 
additive  compound  PC15,  8NH3,  is  probably  identical  with  this 
substance.  Besson  has  obtained  phosphorus  bromonitride  in  a 
similar  manner  ;  it  forms  refractive  apparently  rhombohedral 
crystals,  melts  at  188-190°,  and  begins  to  sublime  at  150°  in 
vacuo.3 


ARSENIC.     As  =  74-4. 

360  The  yellow  and  red  sulphides  of  arsenic,  now  termed 
orpiment  and  realgar,  were  known  to  the  ancients,  although  they 
did  not  distinguish  between  them.  Aristotle  gave  to  them  the 
name  of  cravSapd^rj,  which  was  also  applied  to  cinnabar  and  red 
lead,  and  Theophrastus  mentioned  them  under  the  name  of 


White   arsenic   or  arsenious  oxide,  As406,  is  first  distinctly 

1  Journ.  Chem.  Soc.  1869,  15.  2  Ber.  3,  163. 

3  Compt.  Rend.  114,  1264,  1479. 


ARSENIC  613 


mentioned  in  the  writings  of  the  Greek  alchemist  Olympiodorus, 
who  describes  it  under  the  name  of  white  alum,  and  gives  a 
recipe  for  its  preparation  from  the  sulphide  by  roasting  in  the 
air.1  The  later  alchemists  were  all  acquainted  with  these  sub- 
stances. Thus,  for  instance,  in  the  works  attributed  to  Basil 
Valentine  they  are  described  as  follows :  "  In  its  colour  the 
arsenicum  is  white,  yellow,  and  red ;  it  is  sublimed  by  itself 
without  any  addition,  and  also  with  addition  according  to 
manifold  methods." 

The  alchemists  made  use  of  arsenic  especially  for  the  purpose 
of  colouring  copper  white  (see  p.  7).  The  change  thus  brought 
about  was  believed  to  be  the  beginning  of  a  transmutation 
although  Albertus  Magnus  was  aware  that  on  strongly  heating 
the  alloy,  the  arsenic  is  volatilized.  He  describes  this  fact  in  his 
work  "  De  rebus  metallicis "  as  follows :  "  Arsenicum  aeri  con- 
junctum  penetrat  in  ipsum,  et  convertit  in  candorem ;  si  tamen 
diu  stet  in  igne,  aes  exspirabit  arsenicum,  et  tune  redit  pristinus 
color  cupri,  sicut  de  facile  probatur  in  alchymicis." 

Free  arsenic  was  known  to  the  Greek  alchemists,  who  obtained 
it  as  a  sublimate  capable  of  turning  copper  white,  and  hence 
looked  upon  it  as  a  kind  of  mercury.2 

That  a  metal-like  substance  is  contained  in  white  arsenic  was 
probably  known  to  Geber,  but  Albertus  Magnus  was  the  first  to 
state  this  distinctly :  "  Arsenicum  fit  metallinum  fundendo 
cum  duabus  partibus  saponis  et  una  arsenici."  Metallic  arsenic 
was  considered  by  the  later  alchemists  and  chemists  to  be  a 
bastard  or  semi-metal,  and  was  frequently  termed  arsenicum  rex. 
It  was,  however,  Brandt  who  in  the  year  1773  first  showed  that 
white  arsenic  is  a  calx  of  this  substance.  After  the  overthrow 
of  the  phlogistic  theory  the  views  concerning  the  composition  of 
white  arsenic  were  those  which  are  now  held,  namely,  that  it  is 
an  oxide  of  the  elementary  substance. 

Arsenic  occurs  in  the  free  state  in  nature,  usually  in  mam- 
millated  or  kidney-shaped  masses,  which  readily  split  up  into 
laminae.  Occasionally,  however,  native  arsenic  is  met  with  in 
distinct  crystals.  It  occurs  in  large  quantity  at  Andreasberg  in 
the  Harz,  in  Joachimsthal  in  Bohemia,  at  Freiberg  in  Saxony, 
at  ZimeofT  in  Siberia,  in  Borneo,  and  at  Newhaven  in  the  United 
States. 

Arsenic  occurs  much  more  commonly  in  a  state  of  combination 
in  many  ores  and  minerals,  of  which  the  following  are  the  most 
1  Berthelot,  Introduction  ti  I'titude,  &c.,  p.  68.  2  Ibid.  p.  99. 


614  THE  NON-METALLIC  ELEMENTS 

important  :  arsenical  iron,  FeAs2  ;  tin  white  cobalt  (CoNiFe)As2  ; 
arsenical  nickel,  NiAs  ;  arsenical  pyrites  or  mispickel,  Fe2S2As  ; 
realgar,  As2S2,  and  orpiment,  As2S3.  Less  frequently  we  find 
white  arsenic  or  arsenious  oxide  as  arsenite,  As4O6,  and  several 
salts  of  arsenic  acid,  such  as 

Pharmacolite  (HCaAs04)2  +  5H2O, 
Cobalt  bloom  Co3(AsO4)2  +  8H2O, 
Mimetesite  2Pb3(AsO4)2  +  Pb2(P04)CL 

Small  quantities  of  arsenic  also  occur  in  many  other  minerals. 
Thus,  for  instance,  it  is  contained  in  almost  all  specimens  of  iron 
pyrites  so  that  it  is  often  found  in  the  sulphuric  acid  which  is 
manufactured  from  pyrites,  and  in  the  various  preparations  for 
which  this  acid  serves.  Especially  remarkable  is  the  occurrence 
of  arsenic  in  almost  all  mineral  waters,  in  which  of  course  it  is 
only  contained  in  traces  (Will,  Fresenius).  Similarly  it  has  been 
detected  in  sea-water. 

361  Preparation.  —  The  arsenic  occurring  in  commerce  is  either 
the  natural  product,  which  is  never  quite  pure  but  contains  iron 
and  other  metals  mixed  with  it,  or,  more  generally,  that  obtained 
by  heating  arsenical  pyrites  in  earthenware  tubes  in  a  furnace. 
These  tubes  are  1  metre  in  length  and  about  32  cm.  wide.  In 
the  open  end  of  this  earthenware  tube  another  is  placed,  made 
of  sheet  iron,  about  20  cm.  long,  so  that  half  of  the  iron  tube 
is  inside  the  earthenware  one.  The  arsenic  sublimes  into  the 
iron  tube,  from  which  it  is  obtained  by  unrolling  the  sheet  iron. 
Prepared  by  this  process,  arsenic  forms  a  compact,  brittle, 
crystalline  mass  having  a  strong  metallic  lustre.  The  decom- 
position which  takes  place  is  represented  by  the  equation  :  — 

FeSAs  = 


In  Silesia  it  is  prepared  from  arsenious  oxide,  which  is  heated 
with  charcoal  in  an  earthen  crucible  covered  with  a  conical  iron 
cap,  into  which  the  arsenic  sublimes. 

In  order  to  purify  the  commercial  arsenic  it  is  sublimed  with 
the  addition  of  a  small  quantity  of  powdered  charcoal.  On  the 
small  scale,  arsenic  may  be  purified  by  introducing  the  mixture 
into  a  glass  flask  which  is  then  placed  in  a  large  crucible,  sur- 
rounded by  sand  and  heated  to  redness.  As  soon  as  the 
sublimation  begins,  a  loosely-fitting  stopper  of  chalk  is  placed  in 
the  neck  of  the  flask,  a  second  crucible  is  placed  over  the  first, 
and  the  whole  heated  until  the  arsenic  is  sublimed  into  the 


PROPERTIES  OF  ARSENIC  615 

upper  portion  of  the  flask.  In  this  way  rhombohedral  crystals 
of  arsenic  are  obtained,  which  have  a  bright  metallic  lustre  and 
are  isomorphous  with  tellurium  and  antimony  (Mitscherlich). 

Properties. — Arsenic  has  a  steel-grey  colour.  Its  specific 
gravity  at  14°  is  5'727,  its  specific  heat  0'083  (Wiillner  and 
Bettendorf),  and  it  is  a  good  conductor  of  electricity.  If  pure 
arsenic  is  quickly  sublimed  in  a  stream  of  hydrogen  gas  it  is 
deposited  in  the  neighbourhood  of  the  heated  portion  of  the 
tube  in  crystals,  but,  at  a  little  distance,  as  a  black  glittering 
mass,  and,  still  further  on,  as  a  yellow  powder.  The  last 
two  modifications  of  arsenic  are  both  amorphous ;  the  first 
has  a  specific  gravity  of  4713,  the  last  of  S'70.1  When  heated 
to  360°  they  are  both  transformed  into  the  crystalline  variety. 
According  to  Berthelot  and  Engel  amorphous  and  crystalline 
arsenic  evolve  the  same  amount  of  heat  on  oxidation.2 

It  was  formerly  supposed  that  arsenic  could  not  be  melted, 
for  when  heated  under  ordinary  circumstances  to  about  450°  it 
passes  at  once  from  the  solid  to  the  gaseous  state.  Landolt 3 
has,  however,  shown  that  under  an  increased  pressure  it  melts 
at  500°.  On  cooling,  the  fused  mass  forms  a  dense  crystalline 
solid  which  has,  at  19°,  a  specific  gravity  of  5*709.  Its  melting 
point  lies  between  those  of  antimony  and  silver  (Mallet). 

The  vapour  of  arsenic  is  of  a  lemon  yellow  colour  and  smells 
disagreeably  of  garlic.  It  is,  however,  still  uncertain  whether 
this  smell  is  due  to  the  element  itself,  or  to  a  low  oxide  which 
has  not  yet  been  isolated.  The  specific  gravity  of  the  vapour  at 
8.60°  was  found  by  Deville  and  Troost  to  be  10 '2,  whilst  Meyer 
and  Biltz4  obtained  the  values  5'45  at  1714°,  and  5'37  at  1736°. 
The  molecule  of  arsenic  at  these  high  temperatures  therefore 
consists  of  two  atoms,  whilst  at  860°  the  vapour  density  points 
to  the  presence  of  four  atoms  in  the  molecule. 

Arsenic  oxidizes  somewhat  rapidly  in  moist  air  at  the  ordinary 
temperature,  becoming  covered  with  a  blackish-grey  coating. 
Heated  in  oxygen  it  burns  with  a  bright  white  flame,  forming 
arsenious  oxide,  which  is  also  produced  when  arsenic  is  heated  in 
the  air ;  at  the  same  time  the  alliaceous  smell  of  its  vapour  is 
perceived.  In  the  act  of  combination  with  oxygen,  1  gram  of 
arsenic  evolves,  according  to  Thomsen,  1031  thermal  units, 
whilst  by  its  combination  with  chlorine,  in  which  gas  finely 
powdered  arsenic  takes  fire  spontaneously,  it  evolves  953  thermal 

1  Geuther,  Annalen,  240,  208.  2  Compt.  Rend.  HO,  498. 

3  Jahrbuchf.  Min.  1859,  733.  4  Ber.  22,  725. 


616  THE  NON-METALLIC  ELEMENTS 

units.  It  is  easily  oxidized  by  nitric  acid,  and  also  by  con- 
centrated sulphuric  acid  with  evolution  of  sulphur  dioxide.  It 
combines  with  various  non-metals,  and  with  most  of  the  metals. 
It  stands  in  such  close  proximity  to  the  latter  class  of  elements, 
especially  in  its  physical  properties,  such  as  lustre,  specific 
gravity,  &c.,  that  some  chemists  have  placed  it  amongst  them. 
Metallic  arsenic  is  chiefly  used  for  the  purpose  of  hardening  lead 
in  the  manufacture  of  shot. 

The  atomic  weight  of  arsenic  was  first  determined  by  Berzelius,1 
who  heated  2'203  grm.  of  As4O6  with  sulphur,  and  obtained 
1-069  of  SO2,  hence  As  =  74'44.  By  the  analysis  of  the  tri- 
chloride Dumas 2  obtained  As  =  74'39,  whilst  Kessler 3  by  oxi- 
dizing As4O6  to  As2O5  found  As  =  74'7.  The  most  accurate 
value  of  the  atomic  weight  is  probably  74'4. 


ARSENIC  AND  HYDROGEN. 

These  two  elements  unite  to  form  a  gaseous  compound,  AsH3, 
and  a  solid  compound,  As2H2. 

HYDROGEN  ARSENIDE,  OR  ARSINE,  AsH3  =  77*4. 

362  The  existence  of  this  gas  was  first  noticed  by  Scheele  in 
1775  on  treating  a  solution  of  arsenic  acid  with  zinc.  He  found 
that  the  gas  thus  evolved  deposited  white  arsenic  on  burning,  and 
explained  this  as  being  due  to  the  fact  that  the  inflammable  air 
had  dissolved  some  arsenic.  Proust  showed  in  1799  that  the 
same  gas  is  given  off  when  arsenious  acid  and  dilute  sulphuric 
acid  are  brought  together  in  the  presence  of  zinc,  and  also  when 
sulphuric  acid  is  allowed  to  act  on  arsenical  metals.  The  gas 
which  is  thus  given  off  is  a  mixture  of  hydrogen  and  arsine. 

In  order  to  prepare  hydrogen  arsenide  in  the  pure  state, 
zinc  arsenide,  As2Zn3,  must  be  decomposed  by  dilute  sulphuric 
acid,4  thus : — 

As2Zn3  +  3H2S04  =  2AsH3  +  3ZnS04. 

Zinc  arsenide  is  obtained  by  heating  zinc  and  arsenic  together 
in  a  closed  crucible,  when  heat  enough  is  evolved  to  melt  the 
mass.  The  greatest  care  must  be  taken  in  the  preparation  of 
this  gas,  as  it  is  extremely  poisonous,  a  quantity  no  larger  than 

1  Pogg.  Ann.  8,  1.      2  Annalen,  113,  29.      3  Pogg.  Ann.  95,  204  ;  113,  134. 
4  Soubeiran,  Ann.  Chim.  Phys.  [2],  23,  307  ;  43,  407. 


ARSINE  617 


one  bubble  having  been  known  to  produce  fatal  effects.  Gehlen 
lost  his  life  in  this  way  in  the  year  1815. 

Arseniuretted  hydrogen,  as  the  gas  was  formerly  called,  is 
also  formed  by  the  electrolysis  of  arsenious  and  arsenic  acid 
(Bloxam)  and  in  small  quantity  when  arsenic  is  boiled  with 
water.1 

363  Properties. — Arseniuretted  hydrogen  has  a  very  peculiar 
and  disagreeable  smell,  and,  according  to  Dumas,  possesses  a 
specific  gravity  of  2'695.  It  liquefies  at  — 40°  (Stromeyer),  but 
it  does  not  solidify  when  cooled  to  a  temperature  of  — 110°. 
Arsine  is  formed  from  its  elements  with  absorption  of  heat 
(p.  232),  and  like  many  other  compounds  of  the  same  class  can  be 
made  to  undergo  explosive  decomposition  into  its  elements  when 
exposed  to  the  shock  produced  by  the  detonation  of  fulminate  of 
mercury,  although  it  does  not  explode  when  heated.2  It  burns 
with  a  pale  bluish  flame,  emitting  dense  clouds  of  arsenious  oxide. 
If  a  cold  piece  of  white  porcelain  be  held  in  the  flame,  metallic 
arsenic  is  deposited  as  a  brown  or  black  shining  mirror,  and 
when  the  gas  is  passed  through  a  glass  tube  which  is  heated  in 
one  or  in  several  places  by  means  of  a  gas  flame,  the  arsenic  is 
deposited  near  the  heated  portions  of  the  tube  in  the  form  of  a 
bright  shining  mirror,  the  hydrogen  being  liberated.  This 
decomposition  takes  place  at  2300.3 

Whenever  hydrogen  is  liberated  by  means  of  an  acid  from  any 
liquid  containing  arsenic  in  solution,  traces  of  arseniuretted 
hydrogen  are  evolved.  This  may  be  easily  detected  either  by 
the  smell  or  by  the  above-mentioned  reactions,  which  are  of 
such  delicacy  that  O'Ol  mgrm.  (T^Vo^  °f  a  grai*1)  can  with 
certainty  be  recognized  (Otto). 

Hydrogen  is  evolved  during  the  growth  of  moulds  and  of  cer- 
tain fungi,  and  it  is  possible  that  if  arsenic  compounds  are  present 
where  such  growths  are  going  on,  arseniuretted  hydrogen  may  be 
evolved.4  This  may,  perhaps,  explain  the  evil  effects  noticed 
when  arsenical  wall  papers  are  employed.5  At  the  same  time 
it  must  be  remembered  that  in  these  cases  arsenic  doubtless 
finds  its  way  into  the  system  in  the  form  of  dust,  which  in  such 
rooms  invariably  contains  it. 

When  arseniuretted  hydrogen  is  led  over  heated  oxide  of 
copper,  water  and  copper  arsenide  are  formed.  In  this  way 

1  Cross  and  Higgin,  Ber.  16,  1198.  2  Berthelot,  Gompt.  Rend.  93,  613. 

3  Brunn,  Ber.  22,  3205.  4  Selmi,  Ber.  7,  1642  ;  Giglioli,  Ber.  14,  2295. 

5  Fleck,  Dingl.  Polyt.  Journ.  207,  146. 


618  THE  NON-METALLIC  ELEMENTS 

arseniuretted  hydrogen  ean  easily  be  determined  quantitatively. 
When  metals  such  as  tin,  potassium,  or  sodium  are  heated  in 
the  gas  the  arsenides  of  the  metals  are  formed,  free  hydrogen, 
which  occupies  1J  times  the  volume  of  the  original  arseniu- 
retted hydrogen,  being  generated.  When  potassium  and  sodium 
are  heated  in  the  gas,  the  compounds  AsK3  and  AsNa3  are 
formed.  When  decomposed  by  dilute  acids  these  substances 
yield  arseniuretted  hydrogen  which  is  purer  than  that  obtained 
from  zinc  arsenide  (Janowsky). 

Arseniuretted  hydrogen  when  passed  into  a  dilute  solution 
of  a  gold  or  silver  salt  precipitates  the  metal,  the  arsenic 
entering  into  solution  in  the  form  of  arsenious  acid : — 

2AsH3  +  12AgNO3  +  6H20  =  2H3As03-f  12HNO3  +  12Ag. 

A  very  small  quantity  of  arseniuretted  hydrogen  can,  in  this 
way,  be  detected  by  the  precipitation  of  finely  divided  silver 
from  the  clear  solution,  the  liquid  becoming  acid  at  the  same 
time.  When  on  the  other  hand,  arsine  is  passed  into  a  strong 
solution  of  silver  nitrate,  a  yellow  coloration  Is  produced  which 
is  due  to  the  formation  of  a  double  salt,  As3Ag,3AgNO3 : — 

AsH3  +  6AgNO3  =  As3Ag,3AgNO3  +  3HNO3. 

On  the  addition  of  water  this  substance  is  decomposed  with 
formation  of  arsenious  acid  and  nitric  acid,  metallic  silver  being 
precipitated.1 

One  volume  of  water  absorbs  about  five  volumes  of  the  gas, 
and  the  solution  on  exposure  to  the  air  deposits  arsenic. 
Chlorine  decomposes  arseniuretted  hydrogen  with  great  violence, 
bromine  and  iodine  also  act  upon  it  less  energetically.  Sulphu- 
retted hydrogen  does  not  act  upon  the  pure  gas,  but  in  the 
presence  of  oxygen  or  at  a  temperature  above  230°  the 
yellow  sulphide  of  arsenic  is  formed.2 


SOLID  HYDROGEN  ARSENIDE.    As2H2. 

364  This  substance  is  formed  as  a  brown  silky  mass  when 
sodium  arsenide,  AsNa3,  is  decomposed  by  water.3  The  brown 
powder  produced  by  leaving  the  gaseous  compound  in  contact 
with  moist  air,  or  by  decomposing  one  part  of  arsenic  and  five 
of  zinc  by  hydrochloric  acid,  which  was  formerly  supposed  to 

1  Poleck  and  Thiimmel,  Ber.  16,  2438. 

2  Brunn,  Ber.  22,  3202.  *  Janowsky,  Ber.  6,  220. 


ARSENIC  TRICHLORIDE  619 

be  this  substance,  has  been  shown  to  be  nothing  but  the  element 
in  a  finely  divided  state. 

According  to  Ogier l  a  solid  hydride  of  the  formula  As2H  is 
formed  by  the  action  of  the  electric  current  on  arsine. 


ARSENIC  AND  FLUORINE. 

ARSENIC  TRIFLUORIDE,  AsF3  =  131-5. 

365  This  compound  was  obtained  by  Unverdorben  2  by  distil- 
ling a  mixture  of  four  parts  of  arsenious  oxide  and  five  parts  of 
fluor-spar  with  ten  parts  of  sulphuric  acid.  It  is  a  transparent 
colourless  liquid  boiling  at  63°  and  having  a  specific  gravity  of 
2'73  ;  it  fumes  strongly  in  the  air,  has  a  pungent  and  powerful 
odour,  and  when  brought  in  contact  with  the  skin  produces 
serious  wounds  which  only  heal  after  a  long  time  (Dumas.)  It 
attacks  glass,  and  is  decomposed  by  water  into  arsenious  and 
hydrofluoric  acids.  It  is  soluble  in  ammonia,  and  forms  with 
this  gas  a  crystalline  compound. 

Arsenic  Pent  a  fluoride  is  not  known  in  the  free  state.  Marignac 
obtained  a  double  compound  AsF5  +  KF  in  colourless  crystals 
by  dissolving  potassium  arsenate  in  hydrofluoric  acid. 


ARSENIC  AND  CHLORINE. 

ARSENIC  TRICHLORIDE.    AsCl3  =  180. 

366  This  is  the  only  known  compound  of  arsenic  and  chlorine. 
Glauber  first  prepared  this  substance  ;  his  work  "  Furni  novi 
philosophici,"  published  in  the  year  1648,  contains  the  follow- 
ing recipe  : — "  Ex  arsenico  et  auripigmento  to  distil  a  butter  or 
thick  oil.  As  has  been  described  under  antimony,  so  likewise 
from  arsenicum  or  auripigmentum  with  salt  and  vitriol  can  a 
thick  oil  be  distilled." 

Preparation. — In  order  to  prepare  arsenic  trichloride  accord- 
ing to  this  plan,  40  parts  of  arsenious  oxide  must  be  heated 
with  100  parts  of  sulphuric  acid  to  the  boiling  point  of  water 
in  an  apparatus  which  is  connected  with  a  well-cooled  receiver. 
Small  pieces  of  fused  chloride  of  sodium  are  then  carefully 

1  Compt.  Ecnd.  89,  1068. 

-  Pogg.  Ann.  7,  316  ;  see  also  Moissan,  Compt.  Rend.  99,  874. 


620  THE  NON-METALLIC  ELEMENTS 

thrown    in.       The    decomposition    which    takes   place    is    as 
follows  :  — 


12HC1  +  As406  =  4AsCl3  +  6H20. 

The  water,  which  is  formed  at  the  same  time,  remains  behind 
in  combination  with  the  sulphuric  acid,  whilst  the  trichloride 
distils  over. 

The  same  compound  is  easily  obtained  by  passing  dry  chlorine 
over  heated  arsenic  which  burns  to  chloride  of  arsenic.  In 
order  to  purify  it  from  excess  of  chlorine  it  must  be  rectified 
over  some  more  arsenic. 

Properties.  —  Arsenic  trichloride  is  a  colourless  oily  liquid  and 
has  a  specific  gravity  of  2*205  (0°/4°).  It  solidifies  at  —18°  in 
pearly  needles1  and  boils  at  130°'2  (Thorpe),  evolving  a  colour- 
less vapour,  which  has  a  specific  gravity  of  6*3  (Dumas).  It  is 
an  extremely  powerful  poison,  and  evaporates  in  the  air  with 
the  emission  of  dense  white  fumes.  When  arseniuretted  hydro- 
gen is  led  into  the  liquid,  arsenic  separates  out  (Janowsky)  ; 
thus  :  — 

AsCl3  +  AsH3  =  As2  +  3HC1. 

When  brought  in  contact  with  a  small  quantity  of  water, 
star-shaped  crystalline  needles  of  arsenic  oxychloride,  As(OH)2Cl 
separate  out  (Wallace).  In  contact  with  a  large  quantity  of 
water,  it  decomposes  into  arsenious  oxide  and  hydrochloric  acid, 
and  when  the  solution  is  distilled,  arsenic  -trichloride  comes 
over  together  with  the  vapour  of  water  ;  this  explains  the  fact 
that  the  hydrochloric  acid  prepared  from  arsenical  sulphuric 
acid  invariably  contains  arsenic.  Arsenic  trichloride  absorbs 
dry  ammonia,  forming  a  solid  compound  having  the  composition 
AsCl3+3NH3,  which  dissolves  in  alcohol,  being  deposited  from 
this  solution  in  white  crystals.2 


ARSENIC  AND  BROMINE. 

ARSENIC  TRIBROMIDE,  AsBr3  =  312-4. 

367  In  order  to  prepare  this  compound,  powdered  arsenic  is 
added  to  a  solution  of  one  part  of  bromine  in  two  parts  of  carbon 
bisulphide  until  the  solution  becomes  colourless.  Then  bromine 
and  arsenic  are  added  alternately  until  the  colour  of  the  first 

1  Besson,  Compt.  Rend.  109,  940.  2  Wallace,  Phil.  Mag.  [4],  16,  358. 


ARSENIC  TEI-IODIDE  621 


disappears ;  the  clear  liquid  is  poured  off,  and  the  bisulphide  of 
carbon  allowed  to  evaporate  spontaneously. 

Arsenic  tribromide  forms  colourless  deliquescent  crystals 
which  possess  a  strong  arsenical  odour  (Nickles),  and  melt  at 
about  20°.  It  has  a  specific  gravity  of  3'66,  and  boils  at  220°. 
By  the  action  of  water,  it  is  decomposed  in  a  similar  way  to  the 
chloride. 


ARSENIC  AND  IODINE. 

AESENIC  DI-IODIDE,  AsI2  =  326'2. 

368  When  arsenic  tri-iodide  is  heated  with  arsenic  together 
with  a  little  carbon  bisulphide  in  a  sealed  tube,  arsenic  di- iodide 
is  formed,  and  the  same  compound  is  produced  when  arsenic  is 
heated  with  iodine  in  the  proper  proportions  at  230°  in  a  sealed 
tube.  It  crystallises  from  carbon  bisulphide  in  cherry-red, 
brittle  prisms,  and  easily  oxidises  in  the  air.  When  heated 
with  water  or  alkalis  it  turns  black,  arsenic  and  arsenic  tri-iodide 
being  formed.1 

ARSENIC  TRI-IODIDE,  AsI3  =  452-1. 

When  arsenic  and  iodine  are  brought  together  they  com- 
bine with  considerable  evolution  of  heat.  For  the  purpose  of 
preparing  the  tri-iodide  a  method  is  adopted  similar  to  that  em- 
ployed for  the  preparation  of  the  tribromide.  It  is  obtained  in 
the  form  of  bright  red  hexagonal  tables,  which  have  a  specific 
gravity  of  4 '39.  It  may  also  be  prepared  by  passing  hydriodic 
acid  into  arsenic  trichloride,  when  hydrochloric  acid  is  evolved, 
and  the  .iodide  separates  out  in  the  form  of  crystals,  or  by  adding 
a  hot  solution  of  arsenious  oxide  in  hydrochloric  acid  to  a  con- 
centrated solution  of  potassium  iodide.2 

When  it  is  heated  in  the  air  or  in  oxygen  it  burns  with  forma- 
tion of  arsenious  oxide.  When  ammonia  is  passed  through  its 
solution  in  ether  or  benzene,  a  compound  of  the  formula 
2AsI3  +  9NH3  is  formed.  On  heating  with  alcohol  to  150°,  ethyl 
iodide,  C2H5I,  is  formed.3  It  is  used  in  medicine  as  a  remedy 
for  certain  skin  diseases. 

Arsenic  Penta-iodide,  AsI5,  has  been  prepared 4  by  heating  the 

1  Bamberger  and  Philipp,  Ber.  14,  2643.  2  Ibid.  3  Ibid. 

4  Sloan,  Chcm.  News,  46,  194. 


622  THE  NON-METALLIC  ELEMENTS 

tri-iodide  with  iodine  at  150°.  It  is  a  brown  crystalline  mass 
which  melts  at  70°,  has  a  sp.  gr.  of  3  93  and  is  soluble  in  water 
and  alcohol.  The  solutions  deposit  the  tri-iodide  on  standing. 


OXIDES  AND  OXYACIDS  OF  ARSENIC. 

369  Arsenic  unites  with  oxygen  in  two  proportions  producing 
two  acid-forming  oxides,  the  composition  of  which  corresponds 
with  the  oxides  of  phosphorus  :  viz. — 

Arsenious  Oxide,  As4O6. 
Arsenic  Pentoxide,  As.2O5. 

ARSENIOUS  OXIDE,  on  ARSENIC  TRIOXIDE,  As4O6  =  3611. 

Arsenious  oxide  has  long  been  known  under  the  names  of 
white  arsenic  and  arsenious  acid.  In  the  writings  of  Basil 
Valentine  the  name  Htittenrauch,  or  furnace-smoke,  is  given 
to  this  substance  because  it  is  obtained  by  roasting  arsenical 
pyrites,  and  is  emitted  during  the  process  in  the  form  of  a 
white  smoke  which  condenses  to  a  white  powder. 

Arsenious  oxide  is  prepared  on  the  large  scale  in  many 
metallurgical  processes  by  the  roasting  of  arsenical  ores.  The 
vapours  of  the  oxide  which  are  given  off  are  condensed  in  long 
passages  or  chambers  called  poison  chambers  (Giftkanale),  or 
in  towers  termed  poison-towers  (Giftthiirme)  in  the  form  of 
crude  flowers  of  arsenic  or  poison-flour  (Giftmehl).  For  the  pre- 
paration of  white  arsenic,  arsenical  pyrites  are  usually  employed, 
It  is  obtained  as  a  by-product  in  the  roasting  of  cobalt  ores, 
which  are  employed  in  the  manufacture  of  smalt.  The  crude 
sublimate  obtained  in  the  Freiberg  works  contains  about  75 
per  cent,  of  arsenious  oxide.  Open  roasters  are  now  supplanting 
the  muffle  furnaces  in  which  the  operations  were  formerly  con- 
ducted. The  hearth  of  such  a  furnace  is  about  four  metres  in 
length  and  about  2'8  metres  in  breadth,  and  in  it  900  kilos,  of 
the  ore  can  be  roasted  at  once  ;  four  charges  are  made  during 
the  day,  and  the  white  powder  which  comes  off  collects  in  long 
underground  passages  of  some  200  metres  in  length.  Large 
quantities  of  white  arsenic  are  manufactured  in  the  Harz,  and 
also  in  Devonshire  and  Cornwall.  At  the  Great  Devon  Consols 
and  other  mines  in  England  400  tons  are  manufactured  monthly 
by  roasting  tin-ore  containing  arsenical  pyrites,  and  also  the  grey 


ARSENIOUS  OXIDE  623 


flue  deposits  of  the  tin  mines.     In  the  year  1872  in  England  no 
less  than  5,171  tons  of  white  arsenic  were  made.1 

Of  late  years  Oxland's  self-acting  calciner  has  been  much 
used  for  the  manufacture  of  arsenious  oxide  from  the  Cornish 
and  Devonshire  ores.  This  furnace  consists  of  an  iron  tube, 
from  three  to  six  feet  in  diameter,  and  thirty  feet  long,  set  at  an 
inclination  of  from  half  to  one  inch  per  foot,  varying  according 
to  the  nature  of  the  ore.  This  tube  is  heated  by  a  fire  placed 
at  its  lower  end,  whilst  at  its  upper  it  is  placed  in  connection 
with  the  flues  in  which  the  white  arsenic  is  deposited.  The 
tube  is  made  to  revolve  by  suitable  machinery  at  the  rate  of 
about  one  revolution  in  four  minutes,  and  the  crushed  ore  is 
admitted  in  a  regular  stream  through  a  feed-pipe  at  the  back 
end  of  the  tube.  Great  economy  of  fuel  is  effected  by  this 
furnace ;  indeed,  if  properly  worked  with  a  good  ore,  the  heat  of 
combustion  of  the  sulphur  and  arsenic  is  itself  sufficient  to  carry 
on  the  process.  One  such  cylinder  turns  out  upwards  of  twenty- 
five  tons  of  ore  per  diem,  and  the  calcined  product  contains  less 
than  0'5  per  cent,  of  arsenic. 

A  part  of  the  arsenious  oxide  comes  into  the  market  in  the 
form  of  a  white  crystalline  powder,  the  rest  in  the  form  of 
arsenic  glass,  or  amorphous  arsenic  obtained  from  the  powder  by 
resublimation.  For  this  purpose  the  arsenic  powder  which  is 
not  white  enough  to  be  sent  into  the  market  is  in  the  Ger- 
man manufactories  placed  in  iron  pots  heated  by  a  furnace  and 
cylinders  placed  over  them,  in  which  the  arsenious  oxide  con- 
denses in  the  vitreous  form.  The  pots  hold  4J  cwt.  of  the 
crude  arsenious  oxide,  and  this  is  sublimed  in  from  ten  to 
twelve  hours.  In  order  to  produce  a  pure  product  two  sublima- 
tions are  necessary.  In  this  operation  care  must  be  taken 
that  a  reduction  to  the  state  of  metallic  arsenic  does  not  occur, 
as  this  not  only  colours  the  arsenic-glass  of  a  dark  tint,  but 
is  apt  to  form  a  fusible  alloy  with  the  iron  of  the  pots,  and 
thus  to  destroy  them ;  if  this  occurs  the  oxide  then  falls 
into  the  furnace  and  escapes  into  the  air,  and  this  is  a  continual 
source  of  danger  to  the  workmen  employed  in  the  operation. 
In  England  a  common  reverberatory  furnace  is  used  for  the 
resublimation  of  the  crude  white  arsenic,  but,  to  prevent  dis- 
coloration by  smoke,  either  coke  or  anthracite  is  used  (Phillips). 
Properties. — Arsenious  oxide  possesses  no  smell,  but  has  a  weak 
metallic  sweetish  taste ;  it  forms  a  colourless  and  odourless  vapour 

1  Phillips'  Metallurgy,  p.  463. 


624  THE  NON-METALLIC  ELEMENTS 

which  has  a  density  of  19 7' 7  at  temperatures  varying  from  570° 
(Mitscherlich)  to  1560°  (V.  and  C.  Meyer).1  Hence  its  molecular 
formula  is  As4O6.  The  vitreous  modification  of  arsenious  oxide  is 
translucent  or  transparent,  and  perfectly  amorphous.  Its  specific 
gravity  is  3*738.  On  heating,  it  melts  without  volatilization  at 
a  temperature  of  about  200°.  When  kept  for  any  length  of  time 
it  becomes  opaque,  being  changed  into  a  porcelain-like  mass. 
This  change  is  due  to  the  passage  from  the  amorphous  or 
vitreous  to  the  crystalline  condition ;  the  change  commences  at 
the  outside  of  the  mass,  and  gradually  penetrates  into  the  interior. 

Arsenious  oxide  is  slightly  soluble  in  water.  It  is  deposited 
on  cooling  from  a  hot  saturated  solution  in  transparent  regular 
octahedra.  It  is  very  much  more  soluble  in  hydrochloric  acid 
than  in  water,  and  it  may  be  easily  obtained  from  the  solution 
in  the  form  of  large  crystals.  It  also  occurs  in  this  form  as 
arsenic-bloom,  being  foun<?  together  with  native  arsenic,  having 
been  formed  by  the  oxidation  of  this  substance.  The  naturally 
occurring  crystals  sometimes  assume  the  form  of  octahedra  and 
sometimes  of  tetrahedra.  The  specific  gravity  of  octahedral 
arsenious  oxide  is  3'689,  and  in  its  passage  into  the  amorphous 
modification  an  amount  of  heat  is  evolved  represented  by  5,330 
thermal  units  (Deville  and  Troost).  When  the  crystals  are 
deposited  from  hydrochloric  acid  solution,  a  bright  and  con- 
tinuous luminosity  is  observed  in  the  dark ;  this,  however,  does 
not  occur  on  crystallizing  a  second  time.  Water  and  alcohol 
dissolve  different  quantities  of  the  amorphous  arid  of  the  crystal- 
line varieties  of  the  oxide.  Thus  eighty  parts  of  cold  water 
dissolve  nine  parts  of  the  crystalline  modification,  whilst  one 
part  of  the  amorphous  variety  dissolves  in  twenty-five  parts  of 
water  (Bussy).  One  part  of  the  crystallized  oxide  requires  400 
parts  of  absolute  alcohol  for  solution,  whilst  the  amorphous 
variety  dissolves  in  ninety-four  parts  of  alcohol  (Giradin).  A 
constant  solubility  of  each  variety  at  different  temperatures 
cannot,  however,  easily  be  obtained,  as  the  two  modifications 
pass  readily  from  one  into  the  other.  When  the  crystallized 
oxide  is  heated  it  evaporates  at  125 — 150°  without  melting, 
but,  under  an  increased  pressure,  it  melts  and  passes  into  the 
amorphous  modification. 

Arsenious  oxide  also  occurs  in  a  third  form  in  which  it  is 
found  crystallized  in  rhombic  prisms,  first  observed  by  Wohler 
in  a  deposit  from  a  cobalt  roasting-furnace.  Claudet  found 

1  Ber.  12,  1116. 


ARSENIOUS  ACID  625 


this  same  modification  in  a  mineral  occurring  at  San  Domingo 
in  Portugal,  and  hence  this  substance  has  received  the  name  of 
Claudetite.  According  to  Groth,  the  relations  of  the  axes  of  this 
form  are  0'3758 :  1 :  0'3500.  This  rhombic  form  of  the  oxide 
occurs  when  a  boiling  solution  of  potash  is  saturated  with  the 
amorphous  oxide,  and  then  the  solution  allowed  to  cool 
(Pasteur).  Debray  has  observed  that  both  crystalline  forms  may 
be  obtained  by  heating  the  oxide  in  a  closed  glass  tube,  half  of 
which  is  heated  to  a  temperature  of  400°.  On  cooling,  the  lower 
part  of  the  tube  is  found  to  contain  glassy  arsenious  oxide,  the 
middle  part  rhombic  crystals,  and  the  upper  part  regular 
octahedral  crystals. 

Arsenious  oxide  unites  with  many  substances  to  form  double 
compounds.  Thus,  when  dissolved  in  sulphuric  anhydride 
mixed  with  different  amounts  of  water,  compounds  of  one 
molecule  of  the  oxide,  As4O6,  with  16,  8,  4,  and  2  molecules 
of  sulphuric  anhydride  are  formed,1  whilst  when  heated  with 
sulphuric  anhydride  compounds  with  12  and  6  molecules  are 
obtained.2  These*  compounds  are  very  unstable  and  are 
decomposed  by  water. 

Many  other  double  compounds  with  other  substances  have 
also  been  described. 

Arsenious  oxide  serves  for  the  preparation  of  a  large  number 
of  other  arsenic  compounds,  especially  of  the  acids  of  arsenic 
and  their  salts.  It  is  also  employed  in  the  manufacture  of 
arsenical-pigments  and  is  largely  used  in  the  manufacture  of 
glass. 

ARSENIOUS  ACID,  As  (OH)3. 

370  An  aqueous  solution  of  arsenious  oxide  has  an  acid 
reaction,  and  contains  tribasic  arsenious  acid.  This  has,  however, 
not  been  prepared  in  the  pure  state,  although  a  large  number 
of  well-defined  salts  is  known.  The  salts,  which  are  very 
stable,  are  termed  the  arsenites.  Of  these  there  are  several 
series  known.  The  most  important  are :  the  ortho-arsenites, 
such  as  silver  ortho-arsenite,  Ag3AsO3,  and  calcium  ortho-arsenite, 
Ca3(AsO3)2.  Then  we  are  acquainted  with  metarsenites  such 
as  potassium  metarsenite,  KAsO2 ;  and  besides  these,  other  salts 
are  known  such  as  Ca2As2O5,  and  some  having  a  still  more 
complicated  constitution. 

1  Adie,  Jowrn.  Chetn.  Soc.  1889,  i.  157.  2  Weber,  Ber.  19,  3185. 

41 


626  THE  NON-METALLIC  ELEMENTS 

The  arsenites  of  the  alkali  metals  are  soluble  in  water ;  those 
of  the  other  metals  insoluble,  but  easily  soluble  in  acids.  A 
neutral  solution  of  an  arsenite  produces  with  ferric  chloride  a 
reddish-brown  precipitate,  with  silver  nitrate  a  yellow  precipitate, 
and  with  copper  sulphate  a  grass-green  precipitate.  The  last  is 
soluble  in  caustic  soda,  and  when  the  solution  is  boiled  cuprous 
oxide,  Cu2O,  is  precipitated.  Oxidizing  agents  convert  arsenious 
into  arsenic  acid.  Thus,  for  instance,  all  the  elements  of  the 
chlorine  group  effect  the  change  ; 

As(OH)3  +  I2  +  H20  =  AsO(OH)3  +  2HL 

Hence,  arsenious  acid  is  often  employed  for  the  volumetric 
determination  of  chlorine,  bromine,  and  iodine,  and  of  sub- 
stances which  are  capable  of  liberating  these  elements  from 
their  compounds. 

Arsenious  oxide  and  the  soluble  arsenites  act  as  very  powerful 
poisons,  a  dose  of  0*06  gram  (one  grain)  being  very  dangerous, 
but  from  0*125  to  O25  gram  (two  to  four  grains)  almost  always 
producing  fatal  effects  unless  speedily  ejected  from  the  system 
by  vomiting,  or  at  once  rendered  harmless  by  its  precipitation 
as  an  insoluble  compound.  In  small  doses,  however,  arsenious 
oxide  and  the  arsenites  of  the  alkali  metals  are  largely  used 
in  medicine,  especially  in  skin  diseases,  nervous  complaints,  and  in 
intermittent  fevers.  Fowler's  solution  or  the  liquor  arsenicalis 
of  the  pharmacopoeia  is  made  by  dissolving  eighty  grains  of 
arsenious  acid  (arsenious  oxide)  with  the  same  weight  of 
carbonate  of  potash,  in  one  pint  of  water.  The  solution  then 
contains  four  grains  of  arsenious  oxide  in  one  fluid  ounce. 

It  is  a  very  singular  fact  that  persons  can  accustom  themselves 
to  sustain  the  action  of  quantities  of  arsenic  which  if  taken  without 
preparation  would  certainly  have  proved  fatal.  Well-authenticated 
cases  of  such  arsenic  eating  occur  especially  in  Styria.1  In 
one  case,  a  woodcutter  was  seen  by  a  medical  man  to  eat  a  piece 
of  pure  arsenious  oxide  weighing  4'5  grains,  and  the  next  day  he 
crushed  and  swallowed  another  piece  weighing  5 '5  grains, living  on 
the  following  day  in  his  usual  state  of  health.  The  reasons  which 
the  arsenic-eaters  give  for  the  practice,  which  is  usually  carried 
on  in  secret,  is  that  it  enables  them  to  carry  heavy  weights  with 
ease  to  great  elevations.  The  workmen  in  the  arsenic  works 

1  Roscoe,  "On  the  alleged  Practice  of  Arsenic  Eating  in  Styria  " — Memoirs 
of  the  Lit.  and  Phil.  Soc.  of  Manchester.  1860. 


ARSENIC  ACID  627 


also  appear  to  possess  the  power  of  withstanding  doses  of  arsenic 
which,  given  to  ordinary  persons,  would  produce  fatal  effects. 

As  an  antidote  against  arsenic  poisoning,  sulphuretted 
hydrogen  was  formerly  employed,  but  this  substance  does 
not  act  satisfactorily.  Bunsen  and  Berthold  proposed  the 
best  antidote  for  poisoning  by  arsenic,  namely,  freshly  pre- 
cipitated hydrated  oxide  of  iron.1  This  converts  the  soluble 
arsenious  acid  or  even  an  alkaline  arsenite  into  basic  ferric 
arsenite  insoluble  in  water  and  in  the  liquids  of  the  stomach. 
The  hydrated  oxide  should  be  freshly  prepared,  as  when  kept 
it  becomes  crystalline,  and  loses  its  power. 


AKSENIC  PENTOXIDE,  As2O5  =  228-2. 

371  Arsenic  is  distinguished  from  phosphorus,  which  it  other- 
wise closety  resembles,  inasmuch  as  when  burnt  in  the  air  or  in 
oxygen  it  oxidizes  to  arsenious  oxide  which  does  not  directly 
combine  with  more  oxygen.  If,  however,  the  arsenious  oxide  be 
treated  with  an  oxidizing  agent  in  presence  of  water,  arsenic 
acid,  AsO(OH)3,  is  formed,  and  this,  when  heated  to  a  tempera- 
ture slightly  below  a  red-heat,  gives  off  water,  and  arsenic  pent- 
oxide  remains  behind  as  a  white  porous  mass : — 

2AsO(OH)3  =  As205  +  3H2O. 

If  the  substance  be  heated  still  more  strongly,  it  melts  and 
decomposes  into  arsenious  oxide  and  oxygen. 

The  specific  gravity  of  arsenic  pentoxide  is  3734  (Karsten)  ; 
it  dissolves  slowly,  but  to  a  considerable  extent,  in  water, 
and  deliquesces  in  moist  air  with  formation  of  arsenic  acid.  It 
is  easily  reduced  to  free  arsenic  when  heated  in  presence  of 
charcoal,  potassium  cyanide,  or  other  reducing  agents. 


ARSENIC  ACID,  AsO(OH)3. 

Arsenic  acid  was  first  prepared  by  Scheele,  in  the  year 
1775,  by  dissolving  arsenious  oxide  in  aqua  regia,  and  also  by 
acting  with  chlorine  upon  this  oxide  in  the  presence  of  water, 
thus : — 

As4O6  +  10H2O  +  4C12  =  4AsO(OH)3  +  8HC1. 

1  Das  Eisenoxydhydrat,  ein  Gegengift  der  arsenigen  Saure.     Gottingen.     1834. 


628  THE  NON-METALLIC  ELEMENTS 

Arsenic  acid  is  also  formed  very  readily  when  the  lower  oxide 
is  warmed  with  nitric  acid,  and  this  is  the  process  which  is  em- 
ployed for  the  manufacture  of  the  substance  on  the  large 
scale.  Nitrous  fumes  escape,  and  pass,  together  with  air,  up  a 
tower  filled  with  coke  where  they  meet  a  current  of  water; 
they  are  then  oxidized  to  nitric  acid  which  is  condensed. 

The  arsenic  acid  as  it  appears  in  commerce  is  a  thick  very 
acid  liquid,  having  a  specific  gravity  of  2'0.  It  deposits,  when 
cooled,  transparent  crystals  having  the  formula  2AsO(OH)3+H2O. 
These  crystals  melt  at  100°,  and  give  off  water,  the  anhy- 
drous ortho-arsenic  acid,  H3AsO4,  remaining  behind  as  a 
crystalline  powder.  Arsenic  acid  possesses  a  very  acid  and 
unpleasantly  metallic  taste,  and  acts  as  a  poison,  though  not  so 
powerful  a  one  as  the  trioxide.1  The  concentrated  solution  of 
the  acid  acts  on  the  skin  as  a  strong  cautery.  Heated  to  a 
temperature  of  180°,  it  loses  water,  and  hard  glittering  crystals 
of  pyro-arsenic  acid,  H4As2O7,  separate  out.  This  on  heating 
to  200°  again  loses  water,  a  white  crystalline  mass  of  metarsenic 
acid,  HAsO3,  being  left.  The  pyro-  and  meta- acids  both  dis- 
solve in  water  with  evolution  of  heat,  and  are  transformed  at 
once  into  the  ortho-acid.  This  property  serves  to  distinguish 
these  two  varieties  from  the  corresponding  forms  of  phosphoric 
acid,  both  of  which  can  be  obtained  in  solution. 

Crystals  of  the  formula  As2O5  -f  4H2O,  melting  at  35—36°, 
may  be  obtained  from  a  syrup  of  the  same  composition,  and 
these  when  heated  in  a  sealed  tube  give  crystals  of  As2O5  -f 
3H20.2 

The  specific  gravity  of  aqueous  solutions  of  arsenic  acid  of 
known  strength  is  according  to  the  experiments  of  Schiff,3  as 
follows  : — 

Percentage  of 
H3As04. 
22-5 
15-0 

7-5 

Arsenic  acid  is  completely  reduced  to  arsenious  acid  by  warm- 
ing with  an  aqueous  solution  of  sulphurous  acid. 

The  Arsenates. — The  salts  of  arsenic  acid,  or  the  arsenates,  are 
isomorphous  with  the  corresponding  phosphates.  Indeed,  it  was 
by  the  comparison  of  these  two  series  of  salts  that  Mitscherlich 

1  Wohler  and  Frerichs,  Annalen,  65,  335. 

2  Joly,  Compt.  Rend.  101,  1262.  3  Annalen,  H3,  183. 


Specific 
Gravity. 

1-7346 

Percentage  of 
H3As04. 
67-4 

Specific 
Gravity. 

1-1606 

1-3973 

45-0 

1-1052 

1-2350 

30-0 

1-0495 

SULPHIDES  OF  ARSENIC  629 

in  the  year  1819  was  led  to  the  discovery  of  the  law  of  isomor- 
phism. In  their  reactions,  as  well  as  in  their  general  properties, 
both  classes  of  salts  exhibit  great  analogy.  Thus  the  soluble 
arsenates  give  with  ammonia,  ammonium  chloride  and  a  mag- 
nesium salt,  a  crystalline  precipitate  of  MgNH4As04  +  6H2O, 
corresponding  exactly  to  the  phosphorus  compound,  and  when 
gently  warmed  with  a  solution  of  ammonium  molybdate  in 
nitric  acid,  the  arsenates  give  a  yellow  precipitate  similar  to 
that  obtained  with  a  phosphate.  They  may,  however,  be  dis- 
tinguished from  the  latter  class  of  salts,  inasmuch  as  a  neutral 
solution  produces  with  silver  nitrate  a  dark  reddish-brown 
precipitate,  and  when  acetate  or  nitrate  of  lead  is  added  to  a 
neutral  solution,  a  white  precipitate  of  lead  arsenate  Pb3(As04)2 
is  obtained,  and  this  when  heated  on  charcoal  before  the  blow- 
pipe is  reduced  to  arsenic,  which  emits  its  peculiar  garlic-like 
smell. 

Being  tribasic,  arsenic  acid  forms  three  series  of  salts.  Of 
the  normal  salts  only  those  of  the  alkali-metals  are  soluble  in 
water.  Two  series  of  acid-  salts  or  hydrogen-salts  also  exist,  such 
as  Na2HAsO4,  and  NaH2AsO4.  The  former  of  these  salts  when 
carefully  heated  yields  sodium  pyro-arsenate,  Na4As207,  and 
the  latter  sodium  metarsenate,  NaAs03.  These  salts  can,  how- 
ever, only  exist  in  the  solid  state,  as  when  they  are  dissolved 
in  water  they  are  at  once  transformed  into  the  ortho-compound. 

Arsenic  acid  is  used  largely  in  commerce  in  the  manufacture 
of  magenta ;  and  its  salts,  especially  sodium  arsenate,  are 
employed  to  a  great  extent  in  the  processes  of  calico-printing. 


ARSENIC  AND  SULPHUR. 

372  Arsenic  forms  three  definite  compounds  with  sulphur, 
viz.: — 

Arsenic  disulphide  or  realgar,  As2S2 ; 
Arsenic  trisulphide  or  orpiment,  As2S3; 
Arsenic  pentasulphide,  As2S5. 

Of  these,  the  last  two  correspond  to  the  oxides,  and  act  as 
acid-forming  sulphides,  giving  rise  to  well-defined  series  of 
salts  termed  the  thio-arsenites  and  the  thio-arsenates. 


€30  THE  NON-METALLIC  ELEMENTS 

ARSENIC  BISULPHIDE,  As.2S2. 

This  compound  occurs  native  as  realgar,  crystallizing  in  ob- 
lique prisms  belonging  to  the  inonosymmetric  system.  These 
possess  an  orange-yellow  colour  and  resinous  lustre,  and  are 
more  or  less  translucent ;  the  streak  varies  from  orange  yellow  to 
red.  The  specific  gravity  is  3'4  to  3*6,  and  hardness  1*5  to  2'0. 
It  occurs  together  with  silver  and  lead  ores  at  Andreasberg  in 
the  Harz  and  other  localities,  and  imbedded  in  dolomite  on 
the  St.  Gothard,  and  has  been  found  in  minute  crystals  in 
Vesuvian  lavas.  Strabo  mentions  the  occurrence  of  "  Sandaraca  " 
in  a  mine  in  Paphlagonia. 

When  it  is  heated  to  150°  with  a  solution  of  sodium  bicar- 
bonate it  dissolves  in  the  liquid  and  afterwards  separates  out  in 
the  crystalline  form  (Senarmont). 

The  red  arsenic  glass  or  ruby  sulphur  which  occurs  in 
commerce  is  an  artificial  disulphide  of  arsenic  prepared  in 
various  arsenic  works.  In  Freiberg,  arsenical  pyrites  and 
common  pyrites  are  used  for  this  purpose,  mixed  in  such  pro- 
portions that  the  mixture  contains  about  15  per  cent,  of  arsenic 
and  27  per  cent,  of  sulphur.  Such  a  mixture  is  then  sublimed 
in  a  furnace  in  which  are  placed  twelve  iron  tubes.  Each  tube 
holds  about  thirty  kilograms  of  the  ore,  and  the  charge  is  re- 
newed every  twelve  hours.  In  order  to  give  the  product  the 
right  degree  of  colour  it  is  again  melted  with  sulphur. 

Ruby  sulphur  is  a  red  glassy  mass,  translucent  at  the  edges. 
It  does  not  possess  a  constant  composition ;  the  material 
manufactured  at  Freiberg  contains  generally  75  per  cent,  of 
arsenic  and  25  per  cent,  of  sulphur,  and  that  made  at 
Reichenstein  in  Silesia  is  a  mixture  of  ninety-five  parts  of  disul- 
phide with  five  parts  of  sulphur.  This  body  was  formerly  much 
used  as  a  pigment,  and  is  still  employed  in  the  manufacture  of  the 
so-called  Indian-  or  white-fire,  which  is  a  mixture  of  two  parts  of 
the  disulphide  with  twenty-four  parts  of  nitre,  and  burns  with 
a  splendid  white  light  when  ignited.  The  disulphide  is  also 
employed  in  tanning,  being  mixed  with  lime  and  employed  for 
removing  the  hair  from  the  skins. 


ARSENIC  TRISULPHIDE,  As2S3. 

373  This  substance  occurs  in  nature  and  is  known  under  the 
name  of  orpiment   (auri  pigmentum)  or  the  yellow  sulphide  of 


ORPIMENT  631 


arsenic,  and  was  known  to  Pliny  as  arsenicum.  It  crystallizes 
in  translucent  lemon-coloured  prisms  belonging  to  the  mono- 
symmetric  system,  and  has  a  specific  gravity  of  3*46. 

When  sulphuretted  hydrogen  is  passed  through  an  aqueous 
solution  of  arsenious  oxide  the  liquid  becomes  of  a  yellow  colour, 
but  no  precipitate  is  formed,  the  liquid  containing  arsenic 
trisulphide  in  the  colloidal  form.1  If  a  small  quantity  of 
hydrochloric  acid  be  present,  a  beautiful  yellow  precipitate  of 
arsenic  trisulphide  is  at  once  thrown  down.  On  heating  this  sub- 
stance it  melts  to  a  yellowish  red  liquid  which  volatilizes  without 
decomposition  at  a  temperature  of  about  700°.  Heated  in  the 
air  it  takes  fire  and  burns  with  a  pale  blue  coloured  flame  to 
arsenious  oxide  and  sulphur  dioxide.  It  is  soluble  in  solutions 
of  the  alkalis  and  their  carbonates,  a  metarsenite  and  a  meta- 
thioarsenite  being  formed  : — 

2As2S3  +  4KOH  =  KAsO2  +  3KAsS2  +  2H20. 

When  hydrochloric  acid  is  added  to  the  solution  thus  obtained 
the  whole  of  the  arsenic  is  again  precipitated  as  trisulphide : — 

KAs02  +  3KAsS2  +  4HCU  2As2S3  +  4KC1  +  2H20. 

The  sulphide  of  arsenic  occurring  in  commerce  is  prepared  by 
subliming  a  mixture  of  seven  parts  of  pulverized  arsenious  oxide 
with  one  part  of  sulphur,  and  it  is  really  a  mixture  of  arsenious 
oxide  with  more  or  less  sulphide  of  arsenic.  The  material 
thus  prepared,  which  is  very  poisonous,  from  the  excess  of 
arsenious  oxide  which  it  contains,  was  formerly  much  used  as  a 
pigment  under  the  name  of  King's  yellow,  but  it  is  now  almost 
entirely  superseded  by  the  comparatively  innocuous  chrome 
yellow.  The  yellow  sulphide  of  arsenic  is  also  used  in  the  arts 
and  manufactures,  for  instance  in  the  printing  of  indigo  colours  ; 
and  a  mixture  of  orpiment,  water,  and  slaked  lime,  is  used  in 
the  East  under  the  name  of  Rusma  as  a  depilatory,  its  action 
depending  upon  the  formation  of  a  hydrosulphide  of  calcium. 

The  thio-arsenites. — These  salts,  which  are  frequently  termed 
the  sulpho-arsenites,  stand  in  the  same  relation  to  the  trisulphide 
as  the  arsenites  to  arsenious  oxide.  They  are  formed  by  the  com- 
bination of  the  trisulphide  with  the  sulphide  of  a  metal,  and 
they  may  be  arranged,  like  the  arsenites,  in  different  groups. 

1  Schulze,  /.  Prakt.  Chem.  (2),  25,  431  ;  see  also  Picton,  Journ.  Chem.  Soc. 
1892,  i.  127,  140. 


632  THE  NON-METALLIC  ELEMENTS 

Acids  decompose  them  with  precipitation  of  the  trisulphide. 
The  thio-arsenites  of  the  alkali  metals  are  soluble  in  water 
yielding  yellow  solutions;  those  of  the  other  metals  exist  in 
the  form  of  coloured  precipitates. 

ARSENIC  PENTASULPHIDE,  As.2S5. 

374  When  sulphuretted  hydrogen  is  passed  through  a  solution 
of  arsenic  acid  at  some  temperature  between  4°  and  80°,  the  pre- 
cipitate consists  of  arsenic  pentasulphide  mixed  with  arsenic 
trisulphide  and  sulphur,  a  partial  reduction  of  the  arsenic  acid 
having  taken  place.  In  the  presence  of  hydrochloric  acid,  how- 
ever, and  when  the  gas  is  passed  rapidly  into  the  warm  solution, 
the  precipitate  consists  entirely  of  the  pentasulphide.1  The 
pentasulphide  can  also  be  obtained  by  fusing  the  trisulphide  in 
the  proper  proportions  with  sulphur.  It  forms  a  yellow  fusible 
mass,  which  can  be  sublimed  without  decomposition  in  absence 
of  air.  It  is  more  readily  obtained  by  acidulating  a  dilute 
solution  of  sodium  thio-arsenate  with  hydrochloric  acid 
(Fuchs)  :— 

2Na3AsS4  +  6HC1  =  GNaCl  +  3H2S  4  As2S5. 

Tine  thio-arsenates  or  sulpharsenates  are  formed  by  dissolving  the 
pentasulphide  in  a  solution  of  the  sulphide  of  an  alkali-metal 
or  by  treating  the  trisulphide  with  an  alkaline  poly  sulphide : — 

As2S3  +  K2S3  =  2KAsS3. 

They  may  be  likewise  obtained  by  the  action  of  sulphuretted 
hydrogen  on  a  solution  of  an  arsenate  :— 

K3AsO4  +  4H2S  =  K3AsS4  +  4H2O. 

As  will  be  seen  by  the  above  formula,  both  ortho-  and  meta- 
thio-arsenates  exist,  and  we  are  likewise  acquainted  with  pyro- 
thio-arsenates  such  as  K4As2S7.  The  thio-arsenates  of  the 
alkali  metals  are  soluble  in  water,  yielding  yellow  solutions, 
whereas  the  corresponding  salts  of  the  other  metals  form 
insoluble  precipitates. 

ARSENIC  AND    SELENIUM. 

375  Certain  compounds  of  arsenic  and  selenium  containing 
sulphur  at  the  same  time  have  been  prepared  by  von  Gerichten.2 
1  Brauner  and  Tomitscheck,  Ber.  21,  221c.  2  Ber.  7,  29. 


DETECTION  OF  ARSENIC  633 


These  substances  were  obtained  by  fusing  the  components  in  the 
proper  proportions.  Thus  arsenic  seleno-sulphide  AsSeS2  is  a 
red  translucent  mass  which  dissolves  in  the  state  of  powder  in 
ammonium  hydrosulphide,  yielding  a  brownish-red  coloured 
solution.  Arsenic  thio-selenide  AsSSe2  is  an  opaque  crystalline 
mass  which  can  be  distilled  without  decomposition,  and  dissolves 
in  ammonium  hydrosulphide,  forming  a  deep  yellow  coloured 
solution. 


ARSENIC  AND  PHOSPHORUS. 

ARSENIC  PHOSPHIDE,  AsP. 

376  When  dry  arseniuretted  hydrogen  is  led  into  phosphorus 
trichloride,  the  above  compound  is  precipitated  in  the  form  of  a 
brownish -red    powder.     It  is  slightly  soluble  in  .bisulphide  of 
carbon,  and  is  oxidized  with  ignition  when  acted  upon  by  nitric 
acid.     Heated  in   absence  of  air,  it  decomposes  into  its  con- 
stituents,  and   when   heated  with  an   aqueous  solution  of  an 
alkali  it  gives  off  arseniuretted  hydrogen   and  phosphuretted 
hydrogen,   leaving    behind   a    residue   of    an    arsenite    and    a 
phosphite.1 

THE  DETECTION  OF  ARSENIC  IN  CASES  OF  POISONING. 

377  With  a  few  exceptions,  all  the  compounds  of  arsenic  are 
to   some    extent  poisonous.     Arsenious  oxide  is  a  particularly 
powerful  poison,  and  as  this  substance  is  largely  used  in  the 
arts  and  manufactures,  and  likewise  employed  on  a  large  scale  as 
a  rat-  and  vermin-poison,  it  is  not  difficult,  in  spite  of  legislative 
enactments  respecting  the  sale  of  poisons  (Sale  of  Poisons  Bill), 
to  obtain  this  body  in   quantity,  and  hence  cases  of  accidental 
poisoning  with  this  substance  are  not  uncommon,  and,  in  addi- 
tion, it  is  too  frequently  made  use  of  for  the  express  purpose  of 
destroying  human  life. 

In  medico-legal  investigations,  in  cases  of  poisoning  by 
arsenic,  whether  it  be  in  the  contents  of  the  stomach,  in 
vomited  matter,  or  in  the  several  portions  of  the  body  itself,  it 
is  not  sufficient  for  the  toxicologist  to  ascertain  with  certainty 
the  presence  of  arsenic.  He  is  bound  also  to  determine,  with  as 
great  a  degree  of  accuracy  as  is  attainable,  the  absolute  amount 
1  Janowsky,  Ber.  6,  216. 


634  THE  NON-METALLIC  ELEMENTS 

of  this  poison  found  in  the  body,  in  order  that  he  may  be  able 
to  give  a  distinct  opinion  as  to  whether  the  quantity  is  sufficient 
to  produce  fatal  effects.  Traces  of  arsenic  are,  as  we  have  seen, 
widely  distributed  in  nature.  It  is  liable  to  occur,  although  in 
small  quantities  only,  in  various  pharmaceutical  and  chemical 
preparations.  Hence  it  is  the  first  duty  of  the  chemist  who 
investigates  such  matters,  to  ascertain  that  the  whole  of  the 
reagents  employed  as  well  as  the  apparatus  used  in  his  experi- 
ments are  altogether  free  from  arsenic.  This  is  especially 
necessary,  as  hydrochloric  and  sulphuric  acids,  which  are  in- 
variably employed  for  the  investigation,  are  very  apt  to  con- 
tain traces  of  arsenic ;  and  inasmuch  as  arsenic  is  sometimes 
present  in  the  glaze  of  certain  kinds  of  porcelain.  The  opera- 
tion which  is  conducted  for  the  purpose  of  satisfying  the  ex- 
perimenter as  to  the  freedom  of  his  apparatus  and  chemicals 
from  arsenic  is  termed  a  Hind  experiment,  and  must  be  carried 
on  with  the  same  quantities  of  the  same  materials,  and  with 
the  same  kind  of  apparatus,  side  by  side  with  the  real  ex- 
periment in  which  the  substance  supposed  to  contain  the 
poison  is  examined. 

In  cases  of  poisoning  with  white  arsenic,  the  quantity  of  the 
poison  employed  is  almost  always  more  than  is  necessary  to 
produce  death,  and,  as  arsenious  oxide  is  very  difficultly  soluble 
in  water,  white  particles  may  frequently  be  found  either  adher- 
ing to  the  coatings  of  the  stomach  and  intestines,  or  found  in 
the  vomit.  These  white  particles  must  be  carefully  looked  for 
by  help  of  a  lens,  amongst  the  folds  and  in  the  inflamed  portions 
of  the  stomach  and  intestines  as  well  as  in  the  contents  of  the 
stomach  itself,  and  picked  out  with  pincettes,  washed  with  cold 
water,  dried,  and  then  brought  into  a  tube  made  of  hard  glass 
drawn  out  to  a  point  of  the  form  and  size  shown  in  Fig.  172. 


FIG.  172. 


In  the  tube,  above  these  dried  white  particles,  a  small  splinter 
of  ignited  wood-charcoal  is  placed,  and  then  this  charcoal 
heated  in  the  flame.  As  soon  as  this  is  red-hot,  the  tube, 
which  is  at  first  held  in  a  horizontal  direction,  is  gradually 
slanted,  keeping  the  charcoal  still  heated  in  the  flame,  and 


DETECTION  OF  ARSENIC 


635 


gradually  brought  into  a  nearly  vertical  position,  so  that  at 
last  the  point  of  the  tube  becomes  red-hot.  The  vapour  of  the 
trioxide  then  passes  over  the  red-hot  carbon  and  is  reduced  to 
the  metal  which  is  deposited  in  the  form  of  a  bright  metallic 
mirror,  shown  in  Fig.  173,  on  the  part  of  the  tube  above  the 


FIG.  173. 

carbon.  When  cooled,  the  charcoal  is  shaken  out,  and  the 
metallic  arsenic  is  heated  by  itself  so  as  to  drive  it  up  into  the 
wider  portion  of  the  tube.  If  it  be  not  present  in  large  quantity, 
it  is  wholly  oxidized  in  the  act  of  volatilization  to  the  oxide, 
which  forms  a  white  sublimate  in  the  upper  part  of  the  tube, 
consisting,  as  may  be  seen  with  the  lens,  of  small  glittering 
octahedra,  shown  in  Fig.  174.  This  is  then  dissolved  in  a  small 


FIG.  174. 


quantity  of  boiling  water,  and  when  the  solution  is  cold,  a  solu- 
tion of  silver  nitrate  is  added,  and  then  very  dilute  ammonia 
drop  by  drop  until  the  liquid  is  neutral.  A  yellow  precipitate  of 
silver  arsenite  is  then  formed  :  — 


As4O6  +  12AgNO 


12NH4HO  =  4Ag3AsO3  +  12NH4NO3 
6H0. 


The  arsenious  oxide  may  also  be  dissolved  in  warm  hydro- 
chloric acid  and  then  sulphuretted  hydrogen  passed  through  the 
solution,  when  the  yellow  sulphide  of  arsenic  is  precipitated. 

If  no  particles  of  undissolved  arsenious  oxide  can  be  found,  the 
organic  matter  under  investigation,  which  if  it  consists  of  the 
solid  portion  of  the  body  must  be  cut  up  into  small  pieces,  is 
brought  into  a  retort  provided  with  a  well-cooled  receiver  and 
containing  fused  common  salt  or  pure  rock  salt.  This  mixture 
is  then  distilled  with  pure  sulphuric  acid,  whereby  arsenic 
trichloride  is  formed,  and  this  substance  volatilizes  with  the 
vapours  of  water  and  hydrochloric  acid.  In  order  that  this 
operation  should  be  successful,  it  is  necessary  that  a  smaller 


636  THE  NON-METALLIC  ELEMENTS 

quantity  of  sulphuric  acid  should  be  added  than  that  needed 
to  decompose  the  whole  of  the  salt.  The  cooled  distillate 
is  then  treated  with  sulphuretted  hydrogen,  and  the  precipi- 
tate, consisting  of  impure  sulphide  of  arsenic,  is  treated  as 
hereafter  described. 

The  above  method  can  only  be  employed  when  the  arsenic  is 
in  the  state  of  arsenious  oxide,  or  as  an  arsenite.  In  order  to 
ascertain  with  certainty  whether  arsenical  compounds  in  general 
are  present,  the  following  method  must  be  adopted.  The  organic 
matter  contained  in  the  substance  mast  first  be  removed  as 
completely  as  possible.  For  this  purpose  the  best  process  to 
employ  is  that  of  Fresenius  and  Babo.  The  contents  of  the 
stomach,  or  solid  portions  of  the  stomach  or  other  organs, 
obtained  in  as  fine  a  state  of  division  as  possible  by  trituration 
in  a  mortar  or  by  other  means  of  mechanical  division,  are 
brought  into  a  large  porcelain  dish  and  diluted  with  a  quantity 
of  water  sufficient  to  make  it  into  a  thin  paste.  A  volume  of 
pure  hydrochloric  acid  of  specific  gravity  T12  is  then  added 
equal  to  that  of  the  solid  substance  taken,  and  the  mixture 
warmed  on  a  water-bath,  whilst  every  five  to  ten  minutes  from 
one  to  two  grams  of  pure  potassium  chlorate  is  added,  the  water, 
as  it  evaporates,  being  from  time  to  time  renewed.  This  opera- 
tion is  continued  until  the  whole  assumes  the  form  of  a  thin 
homogeneous  yellow  liquid.  A  few  more  grams  of  potassium 
chlorate  are  then  added  and  the  mass  is  heated  until  the  smell 
of  chlorine  has  completely  disappeared.  The  solution  is  next 
filtered,  and  into  the  clear  solution,  heated  to  about  70°,  a 
current  of  pure  sulphuretted  hydrogen  is  passed  until  it  smells 
strongly  of  the  gas.  The  saturated  solution  is  then  loosely 
covered,  and  allowed  to  stand  in  a  warm  place  for  twenty-four 
hours.  If  after  this  time  the  smell  of  sulphuretted  hydrogen 
has  disappeared,  it  must  be  treated  a  second  time  with  the 
gas.  As  soon  as  the  liquid,  after  standing,  smells  distinctly  of 
sulphuretted  hydrogen,  the  precipitate  of  impure  arsenic  tri- 
sulphide  is  collected  on  a  filter,  and  the  filtrate  again  treated 
with  sulphuretted  hydrogen  in  order  to  ensure  the  complete 
precipitation  of  the  arsenic.  Great  care  must  of  course  be  taken 
that  the  sulphuretted  hydrogen  is  prepared  from  pure  materials 
free  from  arsenic. 

The  precipitate,  obtained  according  to  one  or  other  of  these 
methods,  invariably  contains  organic  matter,  and  that  obtained 
according  to  the  last  process  may  contain,  in  addition,  the  sul- 


DETECTION  OF  ARSENIC  637 

phides  of  other  poisonous  metals  such  as  antimony,  tin,  lead, 
mercury,  &c.  The  precipitate  is  now  washed,  first  with  water 
•containing  sulphuretted  hydrogen,  and  afterwards  with  pure 
water,  and  then  treated  with  dilute  ammonia,  which  dissolves 
the  sulphide  of  arsenic,  the  organic  matter,  and  traces  of  any 
sulphide  of  antimony  which  may  be  present.  The  filtrate 
is  evaporated  to  dryness,  and  the  residue  placed  in  a  small 
porcelain  crucible,  repeatedly  moistened  with  pure  concentrated 
nitric  acid,  and  the  solution  evaporated  to  dryness.  The  pale  yel- 
low residue  is  then  neutralized  with  a  few'  drops  of  pure  caustic 
soda,  and  the  liquid  obtained  evaporated  to  dryness.  The  residue 
is  next  mixed  with  the  requisite  quantity  of  a  finely  powdered 
mixture  of  one  part  of  sodium  carbonate  and  two  parts  of  sodium 
nitrate,  and  the  mixture  gently  heated  in  a  porcelain  crucible 
until  the  mass  fuses.  The  fused  mass  is  treated  with  water,  the 
soluble  portion  filtered  off,  and  the  residue  washed  with  a  mixture 
of  equal  parts  of  water  and  alcohol,  when  any  antimony  present 
remains  behind  as  an  insoluble  sodium  antimoniate.  The  mixed 
aqueous  and  alcoholic  filtrates  containing  the  arsenic  are  then 
evaporated,  having  been  acidified  with  pure  hydrochloric  acid 
for  the  purpose  of  removing  all  traces  of  nitric  and  nitrous  acids. 
The  residue  is  boiled  with  water,  diluted,  and  the  solution  then 
heated  to  70°,  and  treated  with  sulphuretted  hydrogen  in  the 
manner  before  described.  The  precipitate  thus  formed,  after 
having  been  washed,  is  dissolved  in  dilute  ammonia,  the  solution 
evaporated,  and  the  residual  sulphide  of  arsenic  weighed.  A  very 
necessary  point  to  attend  to  in  these  operations  is  that  all  the 
chemicals  employed  shall  be  free  from  chlorine,  as  otherwise  some 
arsenic  trichloride  may  be  formed  and  volatilized  (Wohler). 

The  next  operation  is  to  convert  the  sulphide  into  metallic 
arsenic,  in  order  to  be  positively  certain  that  this  substance  is 
present.  A  portion  of  the  weighed  precipitate  is  heated 
with  pure  nitric  acid,  the  solution  obtained  evaporated  on  the 
water-bath,  and  the  dry  mass  dissolved  in  water.  Metallic 
arsenic  is  best  obtained  from  this  solution,  which  of  course 
contains  arsenic  acid,  by  employing  the  well-known  reaction  of 
Marsh.1  Two  forms  of  the  apparatus  employed  for  this  purpose 
are  shown  in  Figs.  175  and  177.  The  first  of  these  is  a  simple 
form  usually  employed  in  most  laboratories,  the  second  is 
one  recommended  by  Otto.  The  first  form  requires  no  ex- 
planation, the  second  consists  of  a  gas-generating  flask  (A) 

1  New  Edin.  Phil.  Joum.  1836,  p.  229. 


638  THE  NON-METALLIC  ELEMENTS 

of  100-200  cc.  capacity,  provided  with  a  tube-funnel  (b) 
and  a  drying-tube  (a),  containing  in  the  nearer  part  some 
plugs  of  cotton  wool  for  the  purpose  of  retaining  any  liquid 
which  may  be  mechanically  carried  over,  and  in  the  further 


FIG.  175. 

portion  some  pieces  of  solid  caustic  potash  for  the  purpose  of 
drying  the  gas  and  withdrawing  from  it  any  traces  of  acid  which 
may  be  carried  over.  The  end  of  this  tube  is  connected  with 
a  reduction-tube  (d),  consisting  of  hard  glass,  of  the  diameter 
and  thickness  shown  in  Fig.  176,  which  may  be 
drawn  out  at  various  points  as  shown  in  Fig.  177. 
The  flask  contains  pure  granulated  zinc,  together 
with  some  water.  A  cold  mixture  of  one  part  of 

rlG.   17o.  .  A 

sulphuric  acid  and  three  parts  of  water  is  now 
gradually  added  by  the  funnel  tube  when  the  hydrogen  is 
quickly  evolved.  As  soon  as  the  whole  of  the  air  has  been 
driven  out  of  the  apparatus,  the  portion  of  the  reduction  tube 
nearest  the  flask  is  strongly  heated  by  means  of  the  flame  for 
fifteen  minutes,  whilst  the  gas  which  escapes  may  be  allowed 
to  pass,  by  means  of  the  tube  bent  at  right  angles,  through  a 
dilute  solution  of  nitrate  of  silver.  If,  after  the  expiration  of 
this  time,  the  tube  does  not  exhibit  any  dark  mirror-like 
deposit,  and  if  the  silver  solution  remains  clear,  the  materials 
are  free  from  arsenic.  The  solution  under  examination  is  then 
gradually  added  to  the  contents  of  the  flask  by  means  of  the 
tube-funnel,  and  the  reduction-tube  is  heated  again,  a  little  in 


DETECTION  OF  ARSENIC 


639 


advance  of  the  first  drawn-out  portion,  so  that  the  arsenic  is 
deposited  in  the  narrow  portion  of  the  tube.  As  soon  as  a 
sufficient  quantity  has  been  formed  a  second  portion  of  the 
tube  is  heated  in  the  same  way.  In  spite  of  the  ignition  of 
the  tube,  a  certain  quantity  of  undecomposed  gas  often  escapes, 
the  flame  of  the  gas  burning  at  the  end  assumes  a  pale 
lavender  tint,  and  pieces  of  white  porcelain  when  placed  in  the 
flame  become  covered  with  a  dark  coating  of  metallic  arsenic. 
This  stain  is  at  once  dissolved  by  sodium  hypochlorite  solution, 
the  soluble  sodium  arsenate  being  formed  by  oxidation. 


FIG.  177. 


The  gas  escaping  during  the  operation  may  preferably  be  passed 
through  a  dilute  solution  of  silver  nitrate,  when  the  whole  of 
the  arseniuretted  hydrogen  is  decomposed,  silver  being  precipi- 
tated, and  arsenious  acid  remaining  in  solution.  In  order 
that  this  reaction  should  be  carried  on  satisfactorily  it  is 
necessary  that  the  hydrogen  gas  be  evolved  at  a  low  tempera- 
ture, for  when  the  liquid  becomes  hot,  sulphuretted  hydrogen 
is  formed,  and  this  precipitates  the  arsenic  as  the  insoluble 
sulphide  of  arsenic,  thus  withdrawing  it  from  the  reducing 
action  of  the  hydrogen.  In  addition  to  this  neither  nitrates, 
nitrites,  chlorides,  nor  free  chlorine  must  be  present.1 

1  For  further  particulars  respecting  the  detection  of  arsenic,  Fresenius's  Quan- 
titative Analysis  may  be  consulted. 


640  THE  NON-METALLIC  ELEMENTS 

If  care  be  taken  to  dilute  the  arsenic  solution  sufficiently  and 
to  have  a  slow  evolution  of  hydrogen,  the  whole  of  the  arsenic 
may  be  obtained  in  the  form  of  a  mirror  and  its  amount 
estimated  by  a  comparison  of  the  mirror  with  a  set  of  standard 
mirrors  prepared  from  known  amounts  of  arsenic.1 

It  has  already  been  remarked  that  arsenious  oxide  is  em- 
ployed largely  for  the  manufacture  of  Scheele's  green  and 
emerald  green,  and  that  arsenic  acid  is  employed  on  a  very 
large  scale  in  the  manufacture  of  the  aniline  colours.  The 
employment  of  arsenical  wall-papers  is  much  to  be  deprecated ; 
still  more  is  the  employment  of  the  insoluble  arsenical  green 
for  colouring  light  cotton  fabrics,  such  as  gauze,  muslin,  or 
calico,  to  be  condemned.  The  colour  is  merely  pasted  on  with 
size,  and  rubs  off  with  the  slightest  friction. 

For  the  purpose  of  detecting  arsenic  in  wall-papers  or  cotton 
fabrics  a  very  convenient  test  is  that  of  Reinsch.2  The  colour 
is  dissolved  by  hydrochloric  acid,  into  this  arsenical  solution  a 
small  piece  of  bright  copper  foil  or  wire  is  brought,  and  this 
soon  becomes  dark  coloured  from  the  deposition  on  its  surface 
of  metallic  arsenic.  In  order  still  further  to  prove  the  presence 
of  arsenic  the  metallic  copper  is  well  washed  with  water,  dried, 
and  then  strongly  heated  in  a  dry  test  tube.  A  portion  of  the 
arsenic  is  thus  oxidized  and  volatilized,  and  the  arsenious  oxide 
deposited  as  a  white  sublimate  on  the  cold  portions  of  the  tube. 
This  is  then  fully  examined  by  the  methods  already  described. 
This  process,  however,  can  only  be  employed  when  the  quantity 
of  arsenic  present  is  considerable,  inasmuch  as  the  greater  part 
of  the  arsenic  remains  behind  in  combination  with  the  copper. 


BORON.     B  =  1074. 

378  Boron  does  not  occur  in  nature  in  the  free  state,  but  is 
found  combined  with  hydrogen  and  oxygen,  forming  boric  or 
boracic  acid,  B(OH)3,  and  its  salts.  Of  these  latter  the  most 
important  are  tincal  or  native  borax,  Na2B4O7  +  10H2O,  boracite, 
2Mg3B8015  +  MgCl2,  and  borocalcite,  CaB4O7  +  4H2O~ 

The  name  borax  is  found  in  the  writings  of  Geber  and  other 
alchemists.  It  is,  however,  doubtful  whether  they  understood 

1  Selmi,  Gazzetta,  10,  435  ;  Sanger,  Proc.  Amer.  Academy  of  Arts  and  Sciences, 
26,  24.  2  Schw.  Journ.  53,  377. 


PREPARATION  OF  BORON  641 

by  the  word  the  substance  which  we  now  denote  by  it.  Nothing 
satisfactory  was  known  concerning  the  chemical  nature  of  this 
salt  for  a  long  time.  Homberg 1  first  prepared  boric  acid  from 
borax  in  the  year  1702,  u,nd  he  termed  it  sal  sedativum,  for  he 
was  unacquainted  with  the  composition  of  the  acid.  It  was  not 
till  1747-8  that  Baron  showed  in  two  memoirs  read  before  the 
French  Academy  that  borax  was  a  compound  of  sal  sedativum 
and  soda.  After  the  establishment  of  the  Lavoisierian  system, 
the  name  boracic  acid  was  given  to  sal  sedativum,  and  it  was 
then  assumed  that  this  acid  contained  an  unknown  element,  for 
the  isolation  of  which  we  are  indebted  to  Gay-Lussac  and 
Thenard,2  as  well  as  to  Sir  Humphry  Davy,3  who  about  the  year 
1808  obtained  elementary  boron. 

Boron  occurs  in  two  allotropic  modifications :  amorphous  and 
crystallized. 

Amorphous  Boron. — This  modification  was  obtained  by  Gay- 
Lussac  by  heating  boron  trioxide,  B2O3  (obtained  by  the  ignition 
of  boric  acid),  with  potassium  in  an  iron  tube.  It  may  also  be 
obtained  by  mixing  ten  parts  of  coarsely-powdered  boron  tri- 
oxide with  six  parts  of  sodium,  bringing  the  mixture  into  a 
crucible  already  heated  to  redness,  and  covering  it  with  a  layer 
of  powdered  chloride  of  sodium  previously  well  dried.  As  soon 
as  the  reaction,  which  is  very  violent,  has  subsided,  the  mass  is 
stirred  with  an  iron  rod  until  all  the  sodium  has  been  oxidized, 
and  then  carefully  poured  into  water  acidified  with  hydrochloric 
acid.  The  soluble  salts  dissolve  in  the  water,  whilst  the  boron 
remains  behind  as  an  insoluble  brown  powder.  This  is  then 
collected  on  a  filter,  and  it  must  be  very  carefully  dried,  as  it  is 
easily  oxidized  and  may  take  fire. 

The  boron  obtained  in  this  way  is  impure,  and  a  similar 
product  is  got  when  boron  trioxide  or  borax  is  heated  with 
magnesium  powder.  In  this  case  a  boride  of  magnesium  is 
probably  formed  of  the  formula  Mg2B5,  which  is  decomposed  by 
acids.  The  product  left  after  this  treatment,  however,  still 
contains  about  5  per  cent,  of  magnesium  and  about  1  per  cent, 
of  hydrogen,  which  is  also  found  in  boron  prepared  by  means 
of  sodium,  and  is  probably  present  in  the  form  of  a  solid 
hydride.4 

In  order  to  prepare  pure  boron,  70  gr.  of  magnesium  powder 

1  Crell.  Chem.  Archiv.  2,  265.  2  Recherches,  i,  269. 

3  Decomposition  of  boracic  acid,  Phil.  Trans.  1809,  i   75. 

4  Gattermann,  Ber.  22,  195  ;  Winkler,  Ber.  23,  772  ;  Lorenz,  Annalen,  247, 
226. 

42 


642  THE  NON-METALLIC  ELEMENTS 

is  heated  with  210  gr.  of  boron  trioxide.  The  product,  which 
consists  of  boron,  accompanied  by  magnesium  boride  and 
borate,  is  treated  with  dilute  acid  which  dissolves  the  borate 
and  the  greater  part  of  the  boride.  In  order  to  remove 
the  last  portions  of  the  boride  the  residue  is  then  fused  with 
borax  and  again  treated  with  hydrochloric  acid,  which  leaves 
the  boron  containing  only  traces  of  silicon,  iron,  and  magnesium. 
The  presence  of  nitride  of  boron  in  the  final  product  can  only 
be  avoided  by  carrying  out  the  heating  in  an  atmosphere  of 
hydrogen  or  by  placing  the  mixture  in  a  crucible  lined  with 
titanic  acid. 

Amorphous  boron  is  a  chestnut-brown  powder  of  specific 
gravity  2'45.  It  is  infusible  even  at  the  temperature  of  the 
electric  arc,  but  partially  volatilizes,  and  burns  when  heated  to 
700°  in  the  air.  It  is  a  more  powerful  reducing  agent  than 
carbon  or  silicon,  inasmuch  as  it  is  oxidized  by  carbonic 
oxide  and  by  silica.  It  combines  with  bromine  at  700°  but 
not  with  iodine,  and  forms  compounds  with  many  metals, 
among  which  are  silver  and  platinum,  on  heating.  It  is 
acted  on  by  the  oxy-acids,  is  oxidized  by  water  vapour,  and 
combines  with  nitrogen  at  a  high  temperature.1  It  is  a  non- 
conductor of  electricity,  and  when  freshly  prepared  and  not 
strongly  ignited  is  slightly  soluble  in  water,  imparting  to  it  a 
yellow  colour  and  being  precipitated  unchanged  from  its  aqueous 
solution  on  the  addition  of  acids  or  salts. 

379  Crystallized  or  Adamantine  Boron. — This  substance  was 
first  obtained  in  the  year  1856  by  Wohler  and  Deville.2  It  can 
be  prepared  by  several  processes.  Thus,  if  amorphous  boron  be 
pressed  down  tightly  in  a  crucible,  a  hole  bored  in  the  centre  of 
the  pressed  mass,  and  a  rod  of  aluminium  dropped  into  the  hole, 
and  the  crucible  then  heated  to  whiteness,  the  boron  dissolves 
in  the  molten  aluminium  and  separates  out  in  the  crystalline 
form  when  the  metal  cools.  The  aluminium  is  then  dissolved 
in  caustic  soda,  and  thus  the  insoluble  boron  is  left  in  large 
transparent  yellow  or  brownish-yellow  crystals.  The  same 
modification  may  be  obtained  in  smaller  crystals,  which  are 
often  joined  together  in  the  form  of  long  prismatic  needles,  by 
melting  together  boron  trioxide  and  aluminium.  In  order  to 
prevent  the  action  of  the  oxygen  of  the  air  upon  the  fused  mass, 
the  crucible  in  which  the  operation  is  conducted  must  be  placed 
inside  a  larger  one  and  the  space  between  them  filled  up  with 

1  Moissan,  Compt.  Rend.  114,  392,  617.  2  Annalen,  101,  113. 


CRYSTALLINE  BORON  643 

powdered  charcoal.  In  this  process,  however,  the  boron  takes 
up  carbon  to  the  amount  of  from  2  to  4  per  cent.  This  carbon 
must  be  in  the  form  of  diamond  carbon,  inasmuch  as  the  boron 
crystals  containing  this  impurity  are  transparent,  and  more 
transparent  the  larger  the  percentage  of  carbon.  In  addition  to 
carbon  the  boron  thus  prepared  is  found  to  contain  a  certain 
quantity  of  iron  and  silicon  from  the  crucible  used.  These  im- 
purities can  be  removed  by  treatment  with  hydrochloric  acid, 
and  afterwards  with  a  mixture  of  nitric  and  hydrofluoric  acids. 
According  to  the  experiments  of  Hampe,1  the  crystals  of 
adamantine  boron  contain  aluminium  as  well  as  carbon,  and 
possess  a  constant  composition  which  is  represented  by  the 
formula  B48C2A13. 

In  the  preparation  of  crystalline  boron  the  occurrence  of  cer- 
tain graphite-like  laminae   has  been 
observed.     These  were  at  one  time 
supposed  to  be  a  third  modification 
of  boron  until  Wohler  and  Deville 2 
showed  that  they  consist  of  a  com- 
pound   of    boron    and    aluminium,  FIGS.  178  AND  179. 
having  the  formula  A1B2. 

In  addition  to  these  substances,  large  black  plates  of  an 
aluminium  boride,  A1B12,  and  smaller  black  crystals  of  carbon 
boride,  CB6,  have  been  observed.3 

According  to  W.  H.  Miller,  boron  crystallizes  in  monosym- 
metric  pyramids  or  prisms,  shown  in  Figs.  178  and  179,  which 
have  a  lustre  and  hardness  exceeded  only  by  that  of  the  diamond,, 
as  they  scratch  both  ruby  and  corundum.  The  specific  gravity 
of  this  form  of  boron  is  2'68.  When  heated  in  the  air  or  in 
oxygen  it  ignites  at  the  same  temperature  as  the  diamond  does, 
and  then  does  not  oxidize  throughout  the  mass,  but  becomes 
covered  with  a  coating  of  the  melted  trioxide  (Wohler).  Con- 
centrated nitric  acid  exerts  no  action  upon  it,  and  even  aqua 
regia  attacks  it  but  slowly.  Boiling  caustic  soda  solution  like- 
wise does  not  act  upon  it,  but  if  it  is  fused  with  the  solid  alkali 
it  dissolves  slowly  with  formation  of  sodium  borate  and  with 
evolution  of  hydrogen. 

380  The  Atomic  Weight  of  Boron. — The  composition  of  borax, 
Na2B4O7  +  10H20,  has  been  made  the  basis  of  the  determina- 
tion of  the  atomic  weight  of  boron  by  many  chemists,  although, 

1  Annalen,  183,  75.  2  Ibid.,  141,  268. 

3  Joly,  Cvm.pt.  Rend.  97,  456. 


644  THE  NON-METALLIC  ELEMENTS 

owing  to  the  volatility  of  boric  acid  and  the  presence  of  water  of 
crystallization  in  the  salt,  it  is  difficult  to  attain  great  accuracy 
in  its  analysis.  By  determining  the  water  of  crystallization, 
Berzelius1  obtained  the  number  10'98,  Laurent2  10*84,  and 
Ramsay  and  Aston3  10*85.  The  last-named  chemists  moreover 
obtained  the  number  10*89  'by  converting  anhydrous  borax  into 
sodium  chloride  by  distillation  with  methyl  alcohol  and  hydro- 
chloric acid,  whilst  Rimbach,4  by  the  titration  of  borax  with 
hydrochloric  acid  in  presence  of  methyl  orange  as  an  indicator, 
found  10'86.  Deville  5  analysed  the  chloride  and  bromide  of 
boron,  obtaining  the  numbers  10*73  and  10*88,  and  Abrahall6 
by  the  analysis  of  the  bromide  found  10*74. 


BORON  AND  HYDROGEN. 

BORON  HYDRIDE,  BH3  =  13*74. 

381  Davy  showed  that  when  amorphous  boron  is  heated  with 
potassium,  and  the  product  treated  with  water,  a  gas  is  evolved 
possessing  a  peculiar  odour,  and  supposed  by  him  to  be  a 
hydride  of  boron.  On  dissolving  in  hydrochloric  acid  the  mass 
obtained  by  heating  iron  filings  and  boron  trioxide  to  whiteness, 
hydrogen  is  evolved,  and  this  on  ignition  burns  with  a  green 
flame.  Gmelin  at  first  supposed  that  this  gas  contained  boron, 
but  afterwards  believed  that  its  properties  were  due  to  other 
impurities.  Deville  and  Wohler  doubted  the  existence  of  this 
hydride,  and  hence  it  was  generally  assumed  that  boron  is  the 
only  non-metallic  element  which  does  not  combine  with  hydro- 
gen, until  Francis  Jones 7  succeeded  in  preparing  the  compound. 
He  heated  boron  trioxide  with  magnesium  dust  in  the  following 
proportions : — 

6Mg  +  B203  =  3MgO  +  B2Mg3. 

The  grey  friable  mass  thus  obtained  contains,  in  addition  to 
magnesia  and  magnesium  boride,  boron  nitride,  free  boron, 
and  magnesium.  On  the  addition  of  hydrochloric  acid,  boron 
hydride  is  evolved,  but  it  is  mixed  with  so  much  hydrogen  that, 

1  Pogg.  Ann.  8,  19. 

2  Compt.  Send.  29,  7,  and  Journ.  PraU.  Chem.  47,  415. 

3  Journ.  Chem.  Soc.  1893,  207.  4  Ber.  26,  164. 

6  Ann.  Chim.  Phys.  [3],  55,  181.  6  Journ.  Chem.  Soc.  1892,  i.  650. 

7  Ibid.  1879,  i.  41. 


BORON  HYDRIDE  645 


so  far,  its  preparation  in  the  pure  state  has  not  been  possible. 
The  gas  is  colourless,  possesses  an  extremely  unpleasant  and 
very  characteristic  odour,  and  when  inhaled  even  in  small 
quantities  produces  sickness  and  headache.  It  burns  with  a 
bright  green  flame,  which  deposits  a  brown  spot  of  boron  on 
porcelain.  It  dissolves  slightly  in  water,  and  its  solution  does 
not  undergo  change  on  standing.  At  a  red  heat  it  decomposes 
into  its  constituent  elements,  and  on  passing  it  into  a  solution 
of  silver  nitrate  a  black  precipitate  is  thrown  down,  although 
only  in  small  quantities,  this  being  probably  owing  to  the  forma- 
tion of  free  nitric  acid.  This  precipitate  contains  boron  and 
silver,  and  is  decomposed  by  hot  water  with  evolution  of  boron 
hydride.  When  this  gas  is  passed  into  ammonia  the  peculiar 
smell  of  boron  hydride  disappears,  but  the  gas  has  a  foetid  odour, 
and  burns  with  a  yellow-green  flame.  The  characteristic  smell 
of  the  gas  is  again  observed  when  the  ammoniacal  solution 
is  acidified.  An  accurate  analysis  of  the  gas  was  not  found  to 
be  possible,  but  the  results  obtained  by  combustion  with  copper 
oxide  agreed  as  nearly  as  could  be  expected  with  the  formula 


BORON  AND  FLUORINE. 

BORON  TRIFLUORIDE,  BF3  =  67'4. 

382  This  gas  was  discovered  in  the  year  1808  by  Gay-Lussac 
and  Thenard.  They  obtained  it  by  heating  a  mixture  of  one 
part  of  boron  trioxide  with  two  parts  of  fluor-spar  to  whiteness 
in  a  gun-barrel.  It  may  be  more  easily  prepared  by  heating 
the  above  mixture  with  twelve  parts  of  strong  sulphuric  acid  in 
a  glass  flask  (J.  Davy): — 

B2O3  +  3CaF2  +  3H2SO4  =  2BF3  +  3CaSO4  +  3H2O. 

Another  method  is  to  heat  a  mixture  of  potassium  fluoborate 
and  the  trioxide  with  sulphuric  acid  (Schiff)  : — 

6KFBF3  +  B2O3  +  6H2SO4  =  8BF3  +  6KHSO4  +  3H2O. 

The  colourless  gas  evolved  by  any  of  these  processes  must  be 
collected  over  mercury  or  by  displacement,  as  it  is  dissolved 
when  brought  into  contact  with  water.  Both  amorphous  and 
crystalline  boron  become  ignited  in  fluorine  gas,  the  trifluoride 
1  Jones  and  Taylor,  Journ.  Chem.  Soc.  1881,  i.  213. 


646  THE  NON-METALLIC  ELEMENTS 

being  formed.1  It  fumes  strongly  in  the  air,  does  not  attack 
glass,  possesses  an  intensely  pungent  odour,  and  has  a  specific 
gravity  of  2*37  (J.  Davy).  It  acts  upon  certain  organic  bodies 
by  withdrawing  from  them  the  elements  of  water,  and  carbonizes 
them  like  sulphuric  acid.  Potassium  and  sodium  burn  very 
brilliantly  when  heated  in  the  gas. 

When  equal  volumes  of  ammonia  and  boron  trifluoride  are 
brought  together,  a  compound  BF3NH3  is  produced.  This  is  a 
white  opaque  solid  which  can  be  sublimed  without  alteration. 
If  one  volume  of  the  fluoride  be  brought  in  contact  with  two  or 
three  volumes  of  ammonia,  the  gases  condense  and  form  colour- 
less liquids  having  the  composition  BF3(NH3)2  and  BF3(NH3)3. 
These  substances  lose  ammonia  on  heating,  and  are  converted 
into  the  solid  compound  (J.  Davy). 

Hydrated  Boron  Trifluoride. — No  less  than  700  volumes  of 
boron  trifluoride  are  absorbed  by  one  volume  of  water.  Great 
heat  is  evolved  during  the  act  of  absorption,  and  the  solution 
when  saturated  is  an  oily  fuming  liquid,  having  a  specific 
gravity  of  1*77,  and  acting  as  a  powerful  cautery  like  strong 
sulphuric  acid.  When  the  saturated  solution  is  heated,  about 
one-fifth  of  the  absorbed  gas  is  evolved,  and  a  liquid  which 
possesses  approximately  the  composition  represented  by  the 
formula  BF3-f  H2O  remains  behind.  This  boils  between  165° 
and  200°,  and  undergoes  partial  decomposition,  boric  acid 
being  formed.  The  specific  gravity  of  the  vapour  shows 
that  this  substance  is  wholly  dissociated  in  the  gaseous 
condition.2 

FLUOBORIC  ACID,  HBF4. 

383  When  fluoride  of  boron  is  brought  in  contact  with  water, 
boric  and  fluoboric  acids  remain  in  solution  ;  thus  : — 

8BF3  +  6H20  =  6HBF4  +  2B(OH)3. 

This  acid  is  also  formed  when  aqueous  hydrofluoric  acid  is 
saturated  with  boric  acid  (Berzelius).  When  the  solution  is 
allowed  to  evaporate,  the  foregoing  hydrate  is  formed,  and  this 
may  be  considered  to  consist  of  a  solution  of  boric  acid  in 
fluoboric  acid.3 

Fluoboric  is  a  monobasic  acid,  and  when  brought  into  contact 
with  bases,  a  series  of  salts  termed  the  fluoborates  is  obtained 

1  Moissan,  Ann.  CMm.  Phys.  [6],  24,  244. 

2  Basarow,  Ber.  7,  824  and  1121.  3  Basarow,  loc.  cit. 


BORON  TRICHLORIDE  647 

•(Berzelius).  The  same  salts  are  formed  by  bringing  together 
the  acid  fluoride  of  an  alkali  with  boric  acid.  In  this  case 
the  peculiar  phenomenon  presents  itself  of  solutions  which,  to 
begin  with,  have  a  neutral  reaction,  becoming  alkaline  when 
they  are  mixed  together.  This  is  explained  by  the  following 
equation : — 

B(OH)3 + 2NaHF2  =  NaBF4 + NaOH  +  2H2O. 

Most  of  the  fluoborates  are  soluble  in  water,  and  crystalline. 
When  these  salts  are  heated  trifluoride  of  boron  is  given  off, 
and  a  fluoride  remains  behind. 


BORON  AND  CHLORINE. 

BORON  TRICHLORIDE,  BC13  =  116*3. 

384  Amorphous  boron  takes  fire  spontaneously  when  brought 
into  chlorine  gas  with  formation  of  boron  trichloride,1  and  this 
same  body  is  formed,  also  with  the  evolution  of  light  and  heat, 
when  hydrochloric  acid  gas  is  passed  over  amorphous  boron 
(Wohler  and  Deville).  In  order  to  prepare  boron  trichloride  a 
current  of  dry  chlorine  gas  is  passed  over  a  strongly  heated 
mixture  of  boron  trioxide  and  charcoal : — 2 

B2O3  +  3C  +  3C12  =  2BC13  +  3CO. 

For  the  purpose  of  preparing  boron  trichloride  by  this  method 
the  arrangement  represented  in  Fig.  180  is  employed.  It  consists 
of  a  porcelain  tube,  a  b,  placed  in  a  furnace,  and  containing  an 
intimate  mixture  of  fused  boric  acid  and  charcoal.  The  vapour 
of  the  volatile  trichloride  is  admitted  into  a  Y-shaped  tube,  e,  the 
lower  limb  of  which  is  placed  in  a  freezing  mixture ;  this  con- 
denses the  chloride,  and  the  excess  of  chlorine  passes  away  by 
the  other  limb.  The  temperature  of  the  furnace  must  be  high, 
as  the  reduction  of  the  oxide  in  presence  of  chlorine  only  takes 
place  at  a  bright  red  heat.  In  order  to  free  the  product  from  the 
excess  of  chlorine,  the  liquid  is  shaken  with  mercury,  and  the 
pure  trichloride  distilled  off. 

Boron  trichloride  is  also  obtained  when  the  finely  powdered 

1  Berzelius,  Pogg.  Ann.  2,  147. 

2  Dumas,  Ann.  Chim.  Phys.  [2].  31,  436,  and  33,  376. 


648 


THE  NON-METALLIC  ELEMENTS 


trioxide  is  mixed  with  double  its  weight  of  phosphorus  penta- 
chloride,  and  heated  in  a  sealed  tube  for  three  days  at  a  tempera- 
ture of  150°.  The  tube  is  well  cooled,  opened,  first  warmed  in  a 
water  bath,  and  afterwards  heated  to  redness,  in  order  to  drive 
off  the  last  portions  of  the  chloride  which  remain  in  combination 
with  boron  trioxide  (Gustavson). 

The  chloride  can  also  be  readily  obtained  by  passing  chlorine 
over  the  crude  boron  obtained  by  heating  borax  with  half  its 
weight  of  magnesium  powder.1 

Boron    trichloride    is    a   colourless    liquid   boiling   at    18*2° 


FIG.  180. 

(Regnault).  At  17°  it  has  a  specific  gravity  of  1'35.  On  heat- 
ing in  closed  tubes  it  expands  very  rapidly,  and  yields  a  colourless 
vapour,  which  has  a  specific  gravity  of  4'065.  Boron  trichloride 
fumes  strongly  in  the  air,  and  is  decomposed  in  contact  with 
water  into  hydrochloric  and  boric  acids.  When  brought  together 
with  small  quantities  of  cold  water  it  forms  a  solid  hydrate,  and 
this  when  ignited  in  a  current  of  hydrogen  decomposes  into 
hydrochloric  acid  and  amorphous  boron.  Boron  trichloride  can 
be  distilled  over  sodium  without  undergoing  decomposition,  and 
zinc  dust  does  not  act  upon  it  at  temperatures  below  200°. 

1  Gattennann,  Ber.  22,  195. 


BORON  IODIDE  649 


When  the  chloride  and  trioxide  are  brought  together  in  the  pro- 
portion of  two  molecules  of  the  former  to  one  of  the  latter,  and 
the  mixture  heated  in  a  closed  tube  to  150°,  a  white  gelatinous 
mass  is  produced,  from  which  half  of  the  chloride  is  driven  off  at 
100°,  whilst  the  other  half  is  not  volatilized  below  a  red  heat.  It 
would  thus  appear  that  an  oxychloride  is  formed  (Gustavson) ; 
thus : — 

3BOC1  =  B203  +  BC13. 

When  boron  trichloride  is  heated  with  sulphuric  anhydride 
in  a  closed  tube  to  120°,  sulphuryl  chloride  is  formed 
(Gustavson)  : — 

3S03  +  2BC13  =  3S02C12  +  B203. 

Ammonia-Chloride  of  Boron,  2BC13  +  3NH3. — This  substance 
is  obtained  as  a  white  crystalline  body,  when  ammonia  gas  is 
led  into  very  well-cooled  boron  trichloride.  The  compound 
can  be  sublimed  without  decomposition,  but  in  contact  with 
water  it  splits  up  into  sal-ammoniac  and  ammonium  borate 
(Berzelius). 

BORON  AND  BROMINE. 

BORON  TRIBROMIDE,  BBr3  ==  2487. 

385  This  compound  can  be  obtained  by  the  direct  union  of  the 
two  elements  at  a  red  heat,  but  it  is  best  prepared  by  passing 
the  vapour  of  bromine  over  a  mixture  of  charcoal  and  boron 
trioxide  and  rectifying  the  product  over  mercury.  It  is  a 
colourless,  strongly  fuming  liquid,  having  a  specific  gravity  of 
2'69  and  boiling  at  90°'5  (Wb'hler  and  Deville).  Its  vapour 
is  colourless,  and  has  the  normal  specific  gravity  of  -8'78.  It 
behaves  towards  water  and  ammonia  exactly  in  the  same  way 
as  the  chloride. 


BORON  AND  IODINE. 

BORON  TRIIODIDE,  BI3  =  388-47. 

386  Pure  boron  does  not  combine  directly  with  iodine,  but  the 
iodide  may  be  prepared  by  passing  boron  trichloride  and 
hydrogen  iodide  through  a  hot  porcelain  tube,  or  by  passing 
dry  hydrogen  iodide  over  impure  amorphous  boron  heated  to 
the  softening  point  of  potash  glass.  It  forms  white  crystalline 


650  THE  NON-METALLIC  ELEMENTS 

plates,  melts  at  43°,  boils  at  210°,  is  very  hygroscopic,  and  is 
decomposed  by  water  in  a  similar  manner  to  the  chloride.  It 
dissolves  in  carbon  bisulphide,  benzene,  &c.,  and  has  a  density 
of  3*3  at  50°.  It  very  readily  parts  with  its  iodine  in  chemical 
reactions,  and  has  therefore  been  used  in  the  preparation  of 
iodine  derivatives  of  other  elements.1 


OXIDES  AND  OXY-ACIDS  OF  BORON. 

BORON  TRIOXIDE,  B2O3,  AND  BORIC  ACID,  H3BO3. 

387  Boron  trioxide,  the  only  known  oxide  of  the  element, 
is  obtained  when  boron  burns  in  the  air,  or  in  oxygen.  It  is, 
however,  best  prepared  by  heating  boric  acid  to  redness ;  thus : — 

2B(OH)3  =  B2O3  +  3H20. 

The  fused  mass  thus  obtained  solidifies  to  a  brittle  glassy 
solid,  having  a  specific  gravity  at  4°  of  1*83.  It  is  a  very 
hygroscopic  substance,  uniting  easily  with  water  to  form  boric 
acid.  It  is  not  volatile  at  a  red  heat,  but  volatilizes  when 
heated  to  whiteness.  In  consequence  of  its  non-volatility, 
boron  trioxide  decomposes,  at  a  red  heat,  all  salts  whose  acids 
or  corresponding  oxides  are  volatile  at  a  lower  tempera- 
ture, and  thus  carbonates,  nitrates,  sulphates,  and  other  salts 
are  converted  into  borates.  Most  metallic  oxides  dissolve  in 
fused  boron  trioxide  at  a  red  heat,  many  of  them  imparting  to 
the  mass  characteristic  colours;  hence  this  substance  is  much 
used  in  blowpipe  analysis. 

Orthoboric  Acid,  B(OH)3. — This  compound  is  formed  by  the 
union  of  the  oxide  with  water.  It  was  first  prepared  by  the 
decomposition  of  borax  by  means  of  a  mineral  acid,  and  known 
under  the  name  of  Homberg's  sal  sedativum.  In  the  year 
1774  Hofer,  a  Florentine  apothecary,  observed  the  occurrence 
of  this  compound  in  the  water  of  the  lagoons  of  Monte  Rotondo 
in  Tuscany,  and  in  1815  a  manufactory  was  erected  on  the 
spot  for  the  purpose  of  obtaining  boric  acid  from  the  water. 
The  undertaking  did  not  flourish  until  the  year  1828,  owing 
to  the  cost  of  fuel  needed  for  the  evaporation  of  the  water 
containing  the  acid  in  solution.  In  that  year  Larderel  gave  a 
new  impetus  to  the  manufacture  by  using  the  natural  heat  of 

1  Moissan,  Cmn.pt.  Rend.  112,  717  ;  114,  617. 


BORIC  ACID  651 


the  volcanic  jets  of  steam,  termed  suffioni,  to  evaporate  the  water 
charged  with  the  acid. 

Almost  the  whole  of  the  boric  acid  brought  into  the  European 
market  is  derived  from  these  Tuscan  lagoons.  Large  volumes 
of  steam  issue  from  volcanic  vents  near  Monte  Rotondo,  Lago 
Zolforeo,  Sasso,  and  Larderello,  and  this  steam  is  condensed  in 
the  lagoons.  The  vapours  themselves,  the  temperature  of  which 
varies  between  90°  and  120°,  contain  only  traces  of  boric  acid, 
but  when  this  steam  is  allowed  to  pass  into  the  lagoons,  the  water 
soon  becomes  charged  with  the  substance,  and  on  evaporation 
yields  crystals  of  the  acid.  Daring  the  last  fifty  years  many 
borings  have  been  made  through  the  eocene  strata,  and  thus 
.artificial  suffioni  have  been  formed. 

In  order  to  obtain  the  boric  acid,  the  suffioni  are  surrounded 
by  basins,  built  of  bricks  or  of  glazed  masonry,  large  enough 
to  contain  two  or  three  of  the  vents.  Several  of  these  basins 
are  usually  built  on  the  side  of  a  hill,  as  shown  in  Fig.  1 81,  and 
the  water  of  a  spring  or  lagoon  is  allowed  to  run  into  the  upper- 
most one.  The  steam  and  gases  are  then  permitted  to  pass 
through  this  water  for  twenty-four  hours,  after  which  it  is 
conducted  by  a  wooden  pipe  to  the  second  basin,  and  so  on 
until  the  liquor  has  passed  through  from  six  to  eight  basins 
and  cannot  take  up  any  more  boric  acid.  It  then  contains 
about  2  per  cent,  of  this  substance.  After  settling,  the  clear 
liquid  is  run  in  a  thin  stream  on  to  a  large  sheet  of  corrugated 
lead,  125  meters  in  length  and  2  meters  in  breadth,  placed  in  a 
slightly  inclined  position  and  kept  hot  by  the  vapours  from  the 
suffioni,  which  are  allowed  to  pass  underneath.  In  this  way 
20,000  liters  of  water  can  be  evaporated  every  twenty-four 
hours.  The  liquid  running  off  the  end  of  the  plate  is  then 
further  evaporated  in  leaden  pans  until  the  boric  acid  begins 
to  crystallize  out. 

Hot  water  flows  out  from  some  of  the  artificial  suffioni, 
and  this  sometimes  contains  as  much  as  0'4  per  cent,  of 
boric  acid,  and  may  be  directly  brought  on  to  the  pan.  The 
water  of  the  Laofo  Zolforeo  formerly  contained  only  0'05  per 
cent,  of  boric  acid,  but  this  percentage  has  been  considerably 
raised  by  cutting  off  all  ingress  of  fresh  water,  and  dam- 
ming off  that  part  of  the  lake  to  which  the  suffioni  have 
access.  The  temperature  in  this  portion  of  the  lake  was  thus 
raised  to  65°,  and  the  water  contained  from  0'2  to  0'3  per 
cent,  of  the  acid,  whilst  in  the  other  portion  the  water  had  a 


652 


THE  NON-METALLIC  ELEMENTS 


temperature  of  26° 
,and  only  contained 
0'08  per  cent,  of 
boric  acid.  These 
lagoons  produce  no 
less  than  from  1,200 
tol, 500  kilosof  boric 
acid  daily.  In  order 
to  purify  the  com- 
mercial acid,  which 
contains  about  25 
per  cent,  of  foreign 
matter,  it  is  re- 
crystallized  from 
hot  water  and  then 
dried  in  chambers 
heated  by  the  suf- 
fioni. 

It  is  still  a  matter 
of  doubt  in  what 
form  the  boric  acid, 
thus  obtained,  oc- 
curs in  the  earth. 
The  occurrence  of 
ammoniacal  salts 
and  sulphide  of  am- 
monium, together 
with  the  boric  acid, 
is  very  remarkable. 
The  most  probable 
hypothesis  appears 
to  be  that  of  Wohler 
and  Deville,1  ac- 
cording to  which 
the  acid  is  derived 
from  the  decompo- 
sition of  a  nitride 
of  boron,  BN. 
Boron  is  one  of  the 
elements  which  can 
combine  directly 


Annalen,  74,  72,  and  105,  71. 


BORIC  ACID  653 


with  nitrogen,  and  the  compound  thus  formed  is  decomposed 
by  steam  into  boric  acid  and  ammonia.1 

This  theory  is  rendered  the  more  probable  by  the  observation 
made  by  Warrington,  that  the  boric  acid  and  sal-ammoniac  found 
in  the  crater  of  the  Island  of  Volcano  contain  traces  of  boron 
nitride.2  It  is,  however,  possible  that  boric  acid  may  be  derived 
from  a  sulphide  of  boron,  which  is  decomposed  by  water  into 
sulphuretted  hydrogen  and  boric  acid  (Sartorius  von  Walters- 
hausen). 

Boric  acid  is  also  manufactured  from  certain  minerals,  such  as 
borocalcite,  which  occurs  in  considerable  quantities  in  the  nitre 
beds  of  Peru  and  Chili.  It  is  likewise  prepared  from  the  natural 
borax  or  tincal,  which  was  first  obtained  from 
the  basins  of  dried-up  lagoons  in  Central  Asia, 
and  has  lately  been  found  in  the  borax  lake 
in  California  in  such  quantities  that  the  amount 
there  obtained  is  sufficient  to  supply  the  whole 
demand  of  the  United  States.  For  the  purpose 
of  preparing  boric  acid  from  these  sources,  the 
minerals  are  dissolved  in  hot  hydrochloric  acid,  the  boric  acid, 
which  separates  out  on  cooling,  being  recrystallized  from  hot 
water. 

Boric  acid  crystallizes  from  aqueous  solution  in  shining  six- 
sided  laminaB  unctuous  to  the  touch,  and  belonging  to  the  asym- 
metric system,  having  the  form  shown  in  Fig.  182.  It  has  a 
specific  gravity  of  1*4347  at  15°  (Stolba),  and  is  much  more 
readily  soluble  in  hot  than  in  cold  water,  as  is  shown  by  the 
following  table  (Brandes  and  Firnhaber) : — 

Parts  of  water.       At 

One  part  of  boric  acid  requires  for  solution        25'66  19° 

14-88  25° 

12-66  37°'5 

10-16  50° 

6-12  62°-5 

4-73  75° 

3-55  87°'5 

2-97  100° 

Boric  acid  is  a  weak  acid,  and  its  cold  saturated  solution 
colours  blue  litmus  tincture  of  a  wine-red  colour  like  car- 
bonic acid,  but  is  without  action  on  methyl-orange.  When 

1  Compare  Popp,  "  Ueber  die  Bildungsweise  der  Borsaure  in  den  Fumerolen 
Toscanas."  Annalen,  Suppl.  8,  5.  2  Chemical  Gazette,  1855,  419, 


654  THE  NON-METALLIC  ELEMENTS 

the  solution  is  boiled,  the  acid  volatilizes  with  the  aqueous 
vapour,  and  this  property  explains  the  presence  of  the  acid  in 
the  suffioni.  Boric  acid  is  also  easily  soluble  in  alcohol,  and 
when  this  solution  is  inflamed  it  burns  with  a  characteristic 
green-edged  flame.  The  same  green  tint  is  seen  when  a  small 
bead  of  the  molten  acid  on  the  end  of  a  very  fine  platinum, 
wire  is  brought  into  the  fusion-zone  of  a  non-luminous  flame. 
The  spectrum  of  this  green  flame  consists  of  several  bright 
bands  :  the  brightest  of  these  (a)  is  situated  in  the  yellowish 
green,  and  two  others  (/3  and  7),  equally  characteristic,  occur  in 
the  green.1 

The  action  of  boric  acid  upon  the  colouring  matter  of 
turmeric  is  highly  characteristic.  If  a  piece  of  paper 
coloured  with  turmeric  be  moistened  with  a  solution  of  the 
acid  it  turns  brown,  and  this  coloration  increases  when  the 
paper  is  dried.  Alkalis  give  a  similar  coloration  to  turmeric 
paper,  but  the  colour  thus  produced  disappears  on  the  addition 
of  an  acid,  whilst  the  brown  tint  imparted  to  turmeric  paper  by 
boric  acid  remains  unaltered  in  presence  of  free  hydrochloric 
acid  and  is  converted  into  blue  or  green  by  dilute  alkalis. 

Metdboric  Acid,  BO  (OH),  is  produced  when  boric  acid  is 
heated  to  100°.  It  forms  a  white  powder  which  at  the  above 
temperature  undergoes  a  slow  but  complete  volatilization.2 

Pyroboric  Acid,  B4O5(OH)2. — This  substance  is  a  brittle  glass- 
like  mass  obtained  when  boric  acid  is  heated  for  a  long  time 
to  140°. 

Boric  acid  dissolves  readily  in  fuming  sulphuric  acid,  and 
from  this  solution  tabular  crystals  separate  out  having  the 
composition — 

2S04{gB+S03. 

These  crystals  when  heated  evolve  sulphur  trioxide. 

When  boric  acid  is  evaporated  with  an  excess  of  concentrated 
phosphoric  acid,  and  the  dry  residue  treated  with  water,  in  order 
to  separate  the  phosphoric  acid,  a  white  amorphous  mass  is  left 
which  possesses  the  composition  BPO4.  This  substance  is  in- 
fusible, it  is  not  attacked  by  strong  acids,  but  dissolves  in 
aqueous  potash.  The  existence  of  these  compounds  points  to 
the  conclusion  that  boron  trioxide  possesses  feebly  basic  pro- 
perties, resembling  in  this  respect  alumina,  A1203.  This  oxide 

1  Lecoq  de  Boisbaudran,  Spectres  Lumineux,  193. 

2  Schaffgotsch,  Pogg.  Ann.  107,  427. 


THE  BORATES  655 


also  sometimes  acts  as  a  weak  acid,  and  sometimes  as  a  weak 
base  forming  a  corresponding  phosphate,  A1PO4. 

388  The  Borates. — Boric  acid,  like  phosphoric  acid,  forms 
many  series  of  salts,  several  of  which  are  derived  from  the  above- 
named  modifications  of  the  acid.  Ortho-boric  acid  is  tribasic  ; 
but  its  salts  are  very  unstable,  and  the  only  well-defined  ortho- 
borate  which  is  known  is  Mg3(BO3)2.  The  tribasic  character  of 
boric  acid  is  however  clearly  shown  by  its  volatile  ethereal 
salts,  compounds  in  which  the  hydrogen  of  the  boric  acid  is 
replaced  by  the  organic  radical,  C2H5.  Thus  ethyl  orthoborate, 
B(OC2H6)3,  is  a  colourless  liquid,  which  volatilizes  without 
decomposition,  and  has  a  vapour  density  corresponding  to 
the  above  formula. 

The  metaborates  are  much  more  stable  compounds.  Thus  we 
are  acquainted  with  the  following : — 


Potassium  metaborate KBO2. 

Sodium  metaborate NaBO2. 

Magnesium  metaborate     .    .    .    .    .  Mg(B02)2. 

Calcium  metaborate Ca(B02)2. 


The  pyroborates  are  also  stable  compounds,  and  to  this  class 
belong  the  following  :  — 

Borax,  or  sodium  pyroborate    .    .    .    Na2B407. 
Borocalcite,  or  calcium  pyroborate  .    CaB4O7. 
Boronatrocalcite     ........     Na2B4O7  +  2CaB407. 

In  addition  to  these,  other  salts  are  known  which  have  a  more 
complicated  constitution,  corresponding  in  this  respect  to  cer- 
tain classes  of  phosphates.  The  following  are  examples  of 
such  borates  :  — 

Larderellite  .........    .    .     (NH4)2B8013. 

Lagonite   ............     Fe2B6O12. 

Boracite    ..........  ..    . 


The  crystalline  borates  almost  always  contain  water  of  crystalli- 
zation, and,  with  the  exception  of  those  of  the  alkalis,  are  either 
insoluble  or  only  slightly  soluble  in  water.  They  are  all  easily 
decomposed  by  acids,  and  therefore,  when  they  are  warmed  with 


656  THE  NON-METALLIC  ELEMENTS 

sulphuric  acid  and  alcohol,  and  the  mixture  is  ignited,  the 
characteristic  green  flame  of  boric  acid  is  observed.  The  same 
green  coloration  is  observed  when  a  trace  of  a  borate  is  brought 
on  to  a  platinum  wire  with  a  small  quantity  of  acid  potassium 
sulphate,  and  this  held  in  the  non-luminous  flame. 


BORON  AND  SULPHUR. 

BORON  TRISULPHIDE,  B2S3. 

389  Berzelius  first  obtained  this  compound  by  heating  boron 
in  the  vapour  of  sulphur.  It  can  be  most  readily  prepared 
by  the  action  of  the  vapour  of  carbon  bisulphide  upon  an 
intimate  mixture  of  lamp-black  and  boron  trioxide  (Wohler  and 
Deville)  :— 

3CS2  +  30  +  2B20S  =  2B2S3  +  6CO. 

It  may  also  be  obtained  1  by  heating  boron  to  bright  redness  in 
a  current  of  sulphuretted  hydrogen,  or  by  heating  boron  iodide 
with  sulphur  above  440°. 

Boron  trisulphide  occurs  generally  as  a  white,  glassy,  fusible 
solid,  but  it  is  sometimes  obtained  in  the  form  of  silky  needles. 
It  melts  on  heating,  softening  at  310°,  has  a  density  of  1*55,  and 
can  be  distilled  in  a  current  of  sulphuretted  hydrogen.  It  is 
at  once  decomposed  in  contact  with  water  with  formation  of 
boric  acid  and  sulphuretted  hydrogen  : — 

B,S3  +  6H2O  =  2B(OH)3  +  3H2S. 
It  possesses  a  pungent  smell  and  attacks  the  eyes. 

BORON  PENTASULPHIDE,  B2S5. 

This  compound  is  formed  as  a  white  crystalline  powder  by 
treating  a  solution  of  sulphur  in  carbon  bisulphide  at  60°  with 
boron  iodide.  It  melts  at  390°,  has  a  density  of  1*85,  and 
is  decomposed  by  water  into  sulphuretted  hydrogen,  sulphur, 
and  boric  acid.2 

1  Sabatier,  Compt.  Rend.  112,  862 ;  Moissan,  Compt.  Rend.  115,  203. 

2  Moissan,  Compt.  Rend.  115,  271 


BORON  NITRIDE  657 


BORON  AND  NITROGEN. 

BORON  NITRIDE,  BN. 

390  Amorphous  boron  combines  directly  with  nitrogen  at  a 
white  heat  to  form  the  above  compound,  which  is  also  produced 
when  the  compound  of  chloride  of  boron  and  ammonia  is 
passed  together  with  ammonia  gas  through  a  red-hot  tube 
(Martius).  Boron  nitride  was  first  obtained  by  Balmain,  in 
the  year  1842,  by  heating  boron  trioxide  with  the  cyanide  of 
potassium  or  of  mercury  : — 

B203  +  Hg(CN)2  =  2BN  +  CO  +  G02  +  Hg. 

The  best  mode  of  preparing  the  substance  is  by  heating  an 
intimate  mixture  of  one  part  of  anhydrous  borax  with  two  parts 
of  dry  sal-ammoniac  to  redness  in  a  platinum  crucible  (Wohler). 
The  mass  is  washed  first  with  water  containing  hydrochloric  acid, 
and  afterwards  with  pure  water,  and  lastly,  treated  with  hydro- 
chloric acid  in  order  to  remove  completely  the  boric  acid  which 
is  mixed  with  the  nitride.  The  reaction  which  takes  place  is 
represented  by  the  equation  : — 

Na2B407  +  4NH4C1  =  4BN  +  2NaCl  +  2HC1  +  7H2O. 

The  nitride  thus  obtained  is  a  white,  light,  perfectly  amorphous 
powder  resembling  finely-divided  talc.  When  it  is  heated  in 
the  flame  it  phosphoresces  with  a  bright  greenish-white  light. 
Heated  in  a  current  of  steam  it  yields  ammonia  and  boric 
acid : — 

BN  +  3H20  =  H3B03  +  NH3. 

Hydrofluoric  acid  dissolves  the  nitride  slowly  with  formation  of 
ammonium  fluoboride  ;  thus  : — 

BN  +  4HF  =  NH4BF4. 


BORON  AND  PHOSPHORUS. 

391  Boron  does  not  combine  directly  with  phosphorus,  but  a 
phosphide  can  be  prepared  by  the  use  of  boron  iodide.1  When 
this  substance  and  yellow  phosphorus  are  dissolved  in  carbon  bi- 
sulphide, a  red  insoluble  powder  is  formed,  which  can  be  sublimed 

1  Moissan,  Compt.  Rend.  113,  624,  726. 
43 


658  THE  NON-METALLIC  ELEMENTS 

in  vacuo  at  200°  in  red  crystals.  This  compound  has  the  formula 
PBI2,  is  very  hygroscopic,  and  is  decomposed  by  water.  When  it 
is  heated  at  160°  in  hydrogen,  it  loses  one  atom  of  iodine,  form- 
ing a  volatile  crystalline  substance  of  the  formula  FBI,  which 
on  further  heating  in  hydrogen  is  converted  into  boron  phos- 
phide, PB.  This  compound  is  a  colourless  insoluble  powder, 
which  burns  brilliantly  in  the  air  at  200°,  and  on  strong  ignition 
in  a  current  of  hydrogen  is  converted  into  a  lower  phosphide  of 
the  formula  P3B5.  Phosphide  of  boron  is  also  formed  when  the 
white  solid  compound  of  boron  bromide  with  phosphine,  BBr3 
PH.,  is  heated  at  300°,  hydrobromic  acid  being  evolved.1 


CARBON.     C  =  11-91. 

392  Carbon  occurs  in  the  free  state  in  nature  in  two  distinct 
allotropic  modifications  as  diamond  and  graphite.  It  forms, 
moreover,  an  invariable  constituent  of  all  organized  bodies,  and 
when  any  such  substance  is  heated  in  absence  of  air,  a 
portion  of  the  carbon  remains  behind  in  the  form  of  amorphous 
carbon  or  charcoal. 

Diamond. — On  account  of  its  brilliant  lustre  and  remarkable 
hardness  the  diamond  has  been  valued  for  ages  as  a  precious 
stone.  Manilius  appears  to  be  the  first  to  mention  it  in  his 
Astronomia  :  "  Adamas  punctum  lapidis,  pretiosior  auro."  Up 
to  the  year  1777  the  diamond  was  believed  to  be  a  species  of 
rock-crystal,  but  Bergman  in  that  year  proved,  by  means  of 
blowpipe  experiments,  that  the  diamond  contained  no  silica, 
and  came  to  the  conclusion  that  it  was  composed  of  a  peculiar 
earth  to  which  he  gave  the  name  of  terra  nolilis.  But  as  soon 
as  the  fact  of  its  combustibility  had  been  definitely  ascertained 
it  was  classed  amongst  the  fossil  resins. 

.This  combustibility  of  the  diamond  appears  to  have  been 
observed  at  an  early  period,  although  the  fact  does  not  seem  to 
have  attracted  the  general  attention  of  the  older  chemists,  as 
statements  of  a  contrary  character  are  recorded  by  them. 
Thus,  for  instance,  Kunkel  states  that  his  father,  at  the  com- 
mand of  Duke  Frederick  of  Holstein,  heated  diamonds  in  his 
gold-melting  furnace,  for  nearly  thirty  weeks,  without  their 
undergoing  any  change.  It  is  to  Newton,  however,  that  we 
owe  the  first  argument  which  went  to  prove  that  the  diamond 
1  Besson,  Compt.  Rend.  113,  78. 


DIAMOND  659 


was  capable  of  undergoing  combustion  on  account  of  its  high 
refractive  power,  a  property  characteristic  of  the  class  of  oily 
bodies.  In  the  second  book  of  his  Opticks,  Newton  says  upon 
the  subject,  "  Again  the  refraction  of  camphire,  oyl-olive,  lint- 
seed  oyl,  spirit  of  turpentine  and  amber,  which  are  fat  sulphu- 
reous unctuous  bodies,  and  a  diamond,  which  probably  is  an 
unctuous  substance  coagulated,  have  their  refractive  powers  in 
proportion  to  one  another  as  their  densities  without  any  con- 
siderable variation."  The  conclusion  to  which  Newton  was 
led  by  theoretical  considerations  was  experimentally  proved  to 
be  correct  in  the  year  1694-5  by  Averami  and  Targioni,  members 
of  the  Academia  del  Cimento,  who,  at  the  request  of  the  Grand 
Duke  Cosmo  III.,  of  Tuscany,  placed  a  diamond  in  the  focus 
of  a  large  burning-glass  and  observed  that  it  entirely  disap- 
peared. Francis  I.,  who  is  said  to  have  received  from  an 
alchemist  an  anonymous  receipt  for  melting  diamonds,  exposed, 
in  the  year  1751,  diamonds  and  rubies  of  the  value  of  6,000 
gulden  for  twenty-four  hours  to  the  action  of  a  powerful  fire ; 
the  rubies  were  found  unaltered,  but  the  diamonds  had  altogether 
disappeared.  The  volatilization  of  the  diamond  by  means  of  heat 
was  from  this  time  forward  made  the  subject  of  numerous  expe- 
riments. Thus,  Darcet  observed  in  1766  that  diamonds  dis- 
appear when  they  are  heated  in  a  cupel-furnace,  even  in  closed 
crucibles,  but,  continuing  his  experiments  at  the  request  of  the 
Paris  Academy,  he,  together  with  Rouelle,  found  that  when  heated 
in  perfectly  hermetically-sealed  vessels,  the  diamond  did  not 
disappear.  Macquer,  in  the  year  1771,  was  the  first  to  observe 
that  when  the  diamond  undergoes  volatilization  it  appears  to 
be  surrounded  by  a  flame.  In  conjunction  with  Cadet  and 
Lavoisier,  he  afterwards  found  that  a  true  combustion  takes 
place.  In  continuation  of  these  experiments  Lavoisier,  to- 
gether with  Macquer,  Cadet,  Brisson,  and  Baume,1  placed  a 
diamond  in  a  glass  vessel  containing  air  collected  over  mercury, 
and  on  igniting  the  diamond  by  means  of  a  burning-glass,  they 
found  that  carbonic  acid  gas  was  produced. 

Charcoal  and  diamond  were  now  placed  together  under  the 
head  of  carbon,  and  their  chemical  identity  was  fully  proved. 
Smithson  Tennant,  in  1796,  corroborated  these  results  by  show- 
ing that  equal  weights  of  these  two  substances  yielded  equal 
weights  of  carbon  dioxide  on  burning,  whilst  Mackenzie  in  1800 
added  to  this  the  proof  that  the  same  weight  of  graphite  also 
1  Lavoisier,  (Euvres,  tome  ii.  38,  64. 


660  THE  NON-METALLIC  ELEMENTS 

gives  the  same  weight  of  carbon  dioxide.  Allen  and  Pepys,  in 
1807,  came  to  the  same  conclusion,  and  Davy,  from  experiments 
made  upon  diamonds  with  the  same  lens  which  the  Florentine 
Academicians  had  used  in  1694,  showed,  in  1814,1  that  no  trace 
of  water  is  formed  in  the  combustion  of  diamonds,  thus  proving 
that  this  substance  contains  no  hydrogen,  but  consists  of  chemi- 
cally pure  carbon.  Davy  likewise  reduced  the  carbonate  of  lime 
obtained  from  the  air  in  which  a  diamond  had  burnt,  by  means 
of  potassium,  in  this  way  preparing  a  black  powder  which,  like 
ordinary  carbon,  took  fire  when  thrown  into  a  flame. 

393  The  diamond  came  to  Europe  from  the  East.  The  mines 
in  Purteal,  which  in  former  days  were  famous  as  those  of 
Golconda,  and  where  the  Koh-i-noor  was  found,  are  at  present 
almost  entirely  exhausted.  The  diamond  fields  of  Minas  Geraes 
in  Brazil  which  have  been  worked  since  the  year  1727  are  prob- 
ably the  richest  in  the  world,  and  yield  yearly  about  2,000 
kilogr.  of  stones.  Of  late  years  the  diamond  fields  in  the  Cape 
have  become  celebrated,  and  they  now  supply  nearly  the  whole 
demand  of  the  world.  During  the  year  1888  the  mines  of 
British  South  Africa  produced  diamonds  weighing  in  all 
3,567,744J  carats,  the  value  of  which  amounted  to  £3,608,212.2 
Diamonds  are  also  found  in  Borneo,  in  the  Ural,  in  New 
South  Wales,  in  Bahia,  in  California,  in  Georgia,  as  well  as  in 
other  localities. 

The  substance  termed  carbonado  is  a  porous  and  massive  form 
of  impure  diamond,  and  occurs  in  black  or  brownish  fragments, 
which  sometimes  weigh  as  much  as  1  kilogr.  When  examined 
with  a  lens  it  exhibits  cavities  filled  with  small  octohedra. 

The  diamond  always  occurs  in  alluvial  deposits  in  the  neigh- 
bourhood of  a  certain  kind  of  micaceous  rock  which  was 
first  observed  in  the  Brazils,  and  has  been  termed  itacolumite. 
This  rock  is  distinguished  by  the  fact  that  in  thin  plates  or  bars 
it  is  very  flexible.  It  was  for  a  long  time  doubtful  whether  the 
diamonds  occur  in  situ  in  the  rock ;  small  diamonds  have, 
however,  been  found  embedded  in  the  matrix,  and  Jeremejew  3 
has  observed  the  existence  of  microscopic  diamonds  in  a  talcose 
schist  occurring  in  the  Southern  Ural.  This  rock  contains  a 
hydrated  silicate  termed  xanthophyllite  occurring  in  yellow 
tabular  crystals,  in  the  inside  of  which  the  small  crystals  of 

1  "  Some  Experiments  on  the  Combustion  of  the  Diamond  and  other  Carbona- 
ceous Substances,"  Phil.  Trans.  1814,  p.  557.     Read  June  23,  1814. 

2  Thorpe's  Diet.     Article  "Diamond."  3  Bcr.  4,  903. 


ARTIFICIAL  DIAMOND  661 

diamond  are  embedded  in  a  direction  parallel  to  the  cleavage 
of  the  xanthophyllite. 

"  The  "  blue  earth  "  in  which  the  diamonds  occur  at  the  Cape 
contains,  in  addition  to  the  occasional  diamonds  large  enough  to 
be  picked  out,  about  O'5-O'l  grams  of  crystallized  carbon  per 
cubic  metre,  and  this  can  be  isolated  from  the  whole  mass  by 
first  of  all  boiling  with  sulphuric  acid,  washing  with  water,  and 
treating  with  aqua  regia,  after  which  the  mass  is  again  washed 
with  water  and  then  treated  successively  with  hydrofluoric  acid, 
sulphuric  acid,  and  water  twelve  or  fourteen  times  to  remove 
the  whole  of  the  mineral  matter,  the  residue,  which  amounts 
to  about  0'094  mgr.  from  2  kilogr.  of  earth,  then  consisting 
almost  entirely  of  graphite,  carbonado,  and  microscopic  trans- 
parent diamonds,  the  graphite  being  present  in  greater  propor- 
tion than  the  diamond.1 

All  diamonds  when  burnt  leave  a  residue  consisting  of  a  small 
quantity  of  incombustible  ash,  those  which  are  colourless  leaving 
the  least.  The  ash  amounts  to  from  Q'05-0'2  per  cent,  in  the 
case  of  diamond,  and  to  as  much  as  4r5  per  cent,  in  the  case  of 
carbonado.  It  has  a  reddish  colour,  and  always  contains  iron 
and  silica,  which  are  usually  accompanied  by  lime  and 
magnesia.2 

Diamond  has  also  been  found,  both  in  the  form  of  colourless 
crystals  and  of  carbonado,  accompanied  by  graphite,  in  a  meteo- 
rite found  in  Canon  Diablo,3  and  this  occurrence  is  of  the 
highest  importance,  as  showing  that  the  diamond  has  been 
probably  formed  by  crystallization  from  a  mass  of  iron  heated  to 
a  high  temperature. 

This  probability  has  been  converted  into  a  certainty  by 
Moissan,  who  has  succeeded  in  preparing  both  diamond  and 
carbonado  artificially.  He  has  found  that  when  carbon  is 
allowed  to  crystallize  from  solution  in  molten  iron  or  silver  under 
a  high  pressure  diamond  is  produced,  whilst  under  ordinary 
conditions  the  carbon  separates  out  as  graphite. 

In  order  to  obtain  artificial  diamond,  pure  sugar  charcoal  is 
strongly  compressed  in  a  cylinder  of  soft  iron,  which  is  then 
closed  by  a  plug  of  the  same  metal.  This  is  placed  in  a  crucible 
containing  about  200  gram,  of  molten  iron,  melted  by  means  of 

1  Couttolenc  ;  Moissan,  Compt.  Rend.  116,  292. 

2  Roscoe,  Proc.  Manchester  Lit.  and  Phil.  Soc.  ;  Moissan,  Compt.  Rend.  116, 
458. 

3  Friedel,  Compt.  Rend.  116,  290  ;  Moissan,  Compt.  Rend.  H6,  288. 


662 


THE  NON-METALLIC  ELEMENTS 


an  electric  furnace,  and  the  crucible  at  once  withdrawn  from  the 
furnace  and  cooled  as  rapidly  as  possible.  Water  does  not  cool 
the  mass  quickly  enough,  owing  to  the  formation  of  a  badly 
conducting  layer  of  steam,  and  it  is,  therefore,  better  to  cool 
the  crucible  by  immersion  in  molten  lead.  Iron,  like  water, 
expands  when  it  solidifies,  and  the  solid  crust  first  formed  on 
the  outside  of  the  mass  therefore  exerts  an  enormous  pressure 
on  the  interior  portion  during  the  crystallization  of  the  latter. 
The  mass  is  then  treated  with  hydrochloric  acid,  which  dis- 
solves the  iron,  and  the  residue  is  then  subjected  to  a  treat- 
ment resembling  that  employed  in  isolating  the  crystallized 
carbon  from  the  blue  earth.  The  residue  consists  of  graphite, 
a  maroon-coloured  variety  of  carbon,  carbonado,  and  trans- 
parent colourless  diamond.  The  densest  part  of  this  is  isolated 
by  further  treatment  with  hydrofluoric  and  sulphuric  acids, 


FIG.  183. 

followed  by  the  action  of  fuming  nitric  acid  and  potassium 
chlorate,  to  remove  the  graphite,  after  which  the  remaining 
fragments  are  separated  according  to  their  density  by  placing 
them  in  liquids  of  different  densities.  In  this  way  small 
fragments  having  the  crystalline  form,  density,  and  hardness  of 
diamonds  are  obtained,  and  these  are  found  on  combustion  to 
yield  the  expected  amount  of  carbonic  acid.  Enlarged  sketches 
of  some  of  these  artificial  diamonds  are  shown  in  Fig.  183, 
which  exhibits  their  crystalline  structure  and  octahedral  shape. 
The  size  and  transparency  of  the  diamonds  produced  depends 
largely  upon  the  rapidity  of  the  cooling,  diamonds  of  O5  mm. 
diameter  and  of  the  brilliant  limpidity  of  the  natural  diamond 
having  been  obtained  by  the  use  of  molten  lead,  whilst  in  the 
fusions  which  were  cooled  by  water  the  diamonds  were  much 
smaller  and  less  limpid. 


PROPERTIES  OF  DIAMOND 


663 


One  of  the  crystals  thus  produced  showed  the  interesting 
property,  which  has  been  observed  in  certain  Cape  diamonds,  of 
splitting  into  fragments  when  preserved,  owing  probably  to  a 
state  of  strain  produced  by  the  rapid  cooling.1 

The  diamond  crystallizes  in  hemihedral  forms  belonging  to  the 
regular  system,  the  crystals  being  usually  octohedrai  in  type, 
although  the  simple  form  rarely  occurs  alone.  Combinations  of 
two  tetrahedra  or  two  rhombic  dodecahedra,  the  hexakistetra- 
hedron  (a),  and  the  hexakisoctahedron,  a  forty-eight  sided 
figure  (b  and  c),  or  combinations  and  twins  (d)  of  this  form  as 
well  as  combinations  of  the  hexakistetrahedron  with  two  tetra- 


FIG.  184. 

hedra  (e),  are  those  which  usually  occur  (Fig.  184).  The  faces 
of  the  crystals  are  not  unfrequently  curved,  and  the  form 
of  the  crystal  distorted,  whilst  twin  crystals  are  also  found. 
All  diamonds  cleave  easily  in  directions  parallel  to  the  faces 
of  the  regular  tetrahedron,  showing  this  to  be  the  primary 
form.  The  fracture  is  conchoidal.  The  crystals  are  usually 
colourless  and  transparent,  though  sometimes  they  are  green, 
brown,  and  yellow.  Blue  and  black  crystals  rarely  occur. 
The  specific  gravity  of  diamond  varies  from  3'5  to  3'6,  that  of 

1  Moissan,  Compt.  Rend.  116,  218  ;  118,  320  ;  see  also  Friedel,  Coinpt.  Rend. 
116,  224. 


664  THE  NON-METALLIC  ELEMENTS 

the  purest  specimens  being,  according  to  Baumhauer,  3'518  at 
4°,  whilst  carbonado  is  found  to  vary  in  density  from  3  to  3'5. 
The  diamond  possesses  a  peculiar  and  characteristic  lustre,  and 
refracts  light  very  powerfully,  its  index  of  refraction  being  2'439. 
It  is  also  the  hardest  of  known  substances.  The  lustre  (termed 
adamantine)  of  the  natural  faces  of  the  diamond  is  greatly  in- 
creased by  cutting  and  polishing  and  by  giving  it  numerous 
facets  which  render  it  capable  of  reflecting  and  dispersing  light 
in  all  directions  and  thus  add  greatly  to  its  lustre,  which 
depends  on  the  fact  that  it  reflects  all  light  which  falls  on 
its  posterior  surface  at  an  angle  of  incidence  greater  than 
24°  13'.  Diamonds  are  cut  or  polished  by  pressing  the  surface 
of  the  gem  against  a  revolving  metal  wheel  covered  with  a 
mixture  of  diamond  dust  and  oil,  no  other  substance  except 
boride,  and  perhaps  silicide  of  carbon  being  hard  enough  to 
abrade  the  diamond. 

Most  diamonds  when  examined  under  the  microscope  exhibit 
cloud-like  darker  portions.  Dark  spots  are  also  frequently  seen 
in  them,  which  Brewster  considers  to  be  cavities,  but  Sorby 
has  shown  that  they  consist  of  small  crystals  of  much  lower 
refractive  power  than  the  diamond  itself. 

Crystals  have  been  found  within  which  impressions  of  other 
diamonds  are  seen.  Kenngott  observed  a  yellow  octohedron 
which  was  enclosed  in  a  colourless  diamond,  whilst  Goeppert 
has  noticed  in  certain  diamonds  the  occurrence  of  a  cell-like 
structure  resembling  that  obtained  when  a  jelly  undergoes 
solidification. 

The  specific  heat  of  diamond  at  ordinary  temperatures  is 
0-1469  (Regnault),  whilst  at  985°  it  is  0'459  (Weber). 

394  The  diamond  burns  with  tolerable  ease  when  heated  in 
the  air  or  in  oxygen.  Thus  if  a  diamond  is  placed  on  a  piece  of 
platinum  foil  it  may  be  ignited  by  the  flame  of  the  mouth  blow- 
pipe. To  demonstrate  the  combustibility  of  the  diamond  in 
oxygen  and  the  production  of  carbonic  acid,  the  apparatus 
shown  in  Fig.  185  may  be  employed.  Two  thick  copper  wires 
(c)  pass  through  the  caoutchouc  cork  fitting  into  the  cylinder 
containing  oxygen.  These  are  connected  together  by  a  spiral 
of  thin  platinum  wire  (b  b)  wrapped  round  the  copper.  Into 
this  spiral  a  splinter  or  small  diamond  (a)  is  placed,  and  on 
allowing  a  current  from  6  to  8  Grove's  cells  to  pass  through  the 
platinum  it  is  heated  to  whiteness.  The  diamond  then  takes 
fire,  and  on  breaking  the  circuit  it  is  seen  to  burn  brilliantly 


PROPERTIES  OF  DIAMOND 


665 


until  it  is  completely  consumed.  A  small  quantity  of  clear 
lime-water  may  be  poured  into  the  cylinder  before  the  experi- 
ment ;  the  liquid  remains  clear  until  after  the  diamond  is 
burnt. 

The  exact  identity  of  the  gas  produced  by  burning  the 
diamond  in  oxygen  with  carbon  dioxide  has  been  proved  by 
combining  it  with  caustic  soda.  The  resulting  substance  was 
found  to  be  in  every  respect  identical  with  ordinary  sodium 
carbonate.1  The  spectrum  given  by  the  gas  is  also  identical 
with  that  given  by  carbon  dioxide  prepared  in  the  usual  manner.2 

The  temperature  of  ignition  of  the  diamond  varies  from 
760°— 8750.3 


FIG.  185. 

When  heated  in  hydrogen,  diamond  undergoes  no  change 
even  when  heated  to  whiteness,  and  a  crystal  which  was  heated 
embedded  in  charcoal  powder  to  the  melting  point  of  cast 
iron  remained  unchanged,  while  a  cut  brilliant,  on  the  other 
hand,  when  thus  heated  became  black,  owing  to  a  thin 
coating  of  graphite  being  formed  on  its  surface. 4  When 
the  diamond  is  heated  between  the  carbon  poles  of  a  powerful 
electric  battery  it  swells  up  and  becomes  converted  into  a 
black  mass  of  graphite. 

1  Krause,  Ber.  23,  2409. 

2  Roscoe  and  Schuster,  Proc.  Manchester  Lit.  and  Phil.  Soc.  7,  80- 

3  Moissan,  Compt.  Rend.  116,  460. 

4  G.  Rose,  Berlin  Acad.  Ber.  1872,  p.  516. 


666  THE  NON-METALLIC  ELEMENTS 

The  diamond  is  by  no  means  easily  attacked  by  most 
chemical  reagents.  It  is  unaffected  by  chlorine  or  hydrochloric 
acid  at  1200°,  and  is  not  acted  on  when  heated  with  potassium 
chlorate  or  iodic  acid.  It  is  moreover  riot  attacked  by  boiling 
sulphuric  acid,  hydrofluoric  acid  or  a  mixture  of  nitric  acid  and 
potassium  chlorate,  and  these  reagents  are  therefore  used,  as 
already  described,  for  separating  the  diamond  from  the  other 
forms  of  carbon  and  all  mineral  substances.  When  fused  with 
carbonate  of  sodium  or  potassium  the  diamond  gradually 
disappears,  being  converted  into  pure  carbon  monoxide.  The 
diamond  is  also  attacked  by  sulphur  at  10000.1 

The  diamond  is  the  most  valuable  of  precious  stones,  those 
which  are  colourless  and  have  the  purest  water  being  espe- 
cially prized.  The  most  beautiful  of  this  kind  is  the  Pitt  or 
Regent  diamond,  which  weighs  136'25  carats2  or  431  grains, 
and  is  worth  £125,000.  More  rarely  some  of  the  transparent 
but  coloured  stones  are  highly  prized.  Thus  the  celebrated 
blue  Hope  diamond,  weighing  only  4J  carats,  but  of  peculiar 
beauty  and  brilliancy,  is  valued  at  £25,000. 

Among  other  celebrated  diamonds  that  in  the  possession  of 
the  Nizam  of  Hyderabad  must  be  mentioned.  It  was  found 
about  half  a  century  ago,  having  been  used  by  a  child  as  a  toy. 
During  the  Indian  mutiny  a  portion  of  this  diamond  was  broken 
off,  and  the  remainder,  of  which  there  is  a  model  in  the 
collection  of  the  British  Museum,  weighs  about  277  carats 
(Maskelyne).  The  largest  diamond  in  Europe  is  set  on  one 
end  of  the  Russian  sceptre;  it  has  a  yellow  colour,  and  weighs 
194J  carats.  The  yellow  Tuscan  diamond  of  the  Emperor  of 
Austria  weighs  139 \  carats.  The  Koh-i-noor,  one  of  the  British 
crown  diamonds,  originally  came  from  India,  and  when  brought 
to  this  country  in  its  rough  state,  weighed  186  carats.  Owing  to 
the  imperfect  character  of  its  original  cutting  it  had  to  be  recut, 
and  was  thus  reduced  to  106  carats.  The  largest  diamond  ever 
found  in  the  Brazils,  termed  the  Star  of  the  South,  originally 
weighed  254 \  carats,  but  was  reduced  by  cutting  to  127  carats. 
A  Cape  diamond  of  288J  carats  has  been  found,  but  has  since 
been  cut,  and  in  consequence  has  lost  a  considerable  amount 
of  its  weight.  The  diamonds  in  the  possession  of  the  Shah  of 

1  Moissan,  Compt.  Rend.  116,  460. 

2  The  word  "carat"  (Arabic,   "  qirat ")  is  derived  from  Kepdnov,  St.  John's 
bread,  or  karob,  the  seeds  of  this  plant  having  been  formerly  used  as  weights. 
1  diamond  carat  =  3 '17  grains  or  0'2054  grm. 


GRAPHITE  667 


Persia  were  undoubtedly  derived  from  the  plunder  of  Delhi  by 
Nadir  Shah.  Their  weight  is  unknown.  According  to  Tavernier, 
the  Great  Mogul  possessed  a  diamond,  which  before  cutting 
weighed  900  carats,  whilst  afterwards  it  only  weighed  279*6 
carats. 

Rough  or  small  diamonds,  which  cannot  be  used  as  brilliants, 
are  termed  "  Boart "  and  are  employed  for  a  number  of  other 
purposes.  Their  powder  is  largely  used  for  the  purpose  of 
cutting  diamonds  and  other  precious  stones,  whilst  the  splinters 
are  used  for  the  purpose  of  writing  upon  glass,  although  they 
will  not  cut  that  substance.  For  that  purpose  we  require  a 
naturally-curved  edge  of  the  crystal ;  the  curved  edge  pro- 
ducing a  deep  slit  determining  the  fracture  of  the  glass  with 
certainty,  whilst  the  straight  edges  merely  scratch  the  surface. 
Diamonds  are  also  largely  used  in  the  construction  of  rock-boring 
tools. 

395  Graphite  was  also  known  to  the  ancients,  but  up  to  the 
time  of  Scheele  no  distinction  was  made  between  it  and  the 
closely  analogous  substance,  sulphide  of  molybdenum,  MoS2,  and, 
at  that  period,  both  these  metal-like  minerals  which  leave  a  mark 
on  paper,  were  termed  indiscriminately  plumbago  or  molybdcena. 
Graphite  appears  to  have  been  first  distinguished  by  Conrad 
Gessner  in  his  work  De  rerum  fossilium  figuris,  in  1565.  A 
picture  of  a  black-lead  pencil  occurs  there,  and  underneath 
is  written  "  Stylus  inferius  depictus  ad  scribendum  factus  est, 
plumbi  cujusdam  (factitii  puto,  quod  aliquos  stimmi  Anglicum 
vocare  audio)  genere,  in  mucronem  derasi,  in  manubrium  ligneum 
inserti."  l 

For  many  years  graphite  was  supposed  to  contain  lead,  whence 
the  name  plumbago,  or  black-lead.  The  former  name  seems  to 
be  derived  from  the  Italian  grafio  piombino,  which  also,  like  the 
other  name  graphite,  from  ypd(f>co,  I  write,  indicates  its  use. 

In  the  year  1779  Scheele  showed  that  molybdenum-glance 
is  totally  different  from  graphite,  and  that  this  latter  body 
when  treated  with  nitric  acid  is  converted  into  carbonic  acid,  so 
that  it  must  be  looked  upon  as  a  kind  of  mineral  carbon. 

Graphite  occurs  in  nature  tolerably  widely  distributed.  It 
is  usually  found  in  lumps  or  nodules  in  granite,  gneiss,  and 
other  crystalline  rocks.  The  best  graphite  for  the  purpose  of 

1  "  The  pencil  represented  below,  is  made,  for  writing,  of  a  certain  kind  of  lead 
(which  I  am  told  is  an  artificial  substance  termed  by  some,  English  antimony,) 
sharpened  to  a  point  and  inserted  in  a  wooden  handle." 


668  THE  NON-METALLIC  ELEMENTS 

making  black-lead  pencils,  was  that  formerly  found  exclusively 
at  Borrowdale  in  Cumberland,  in  green  slate.  These  mines  are 
however,  now  almost  exhausted,  not  having  been  worked  for  the 
last  forty  years.  In  the  sixteenth  and  seventeenth  centuries 
they  were  so  productive  as  to  yield  an  annual  revenue  of 
£40,000,  although  they  were  only  worked  a  few  weeks  in  the 
year  for  fear  of  exhausting  the  mine.  Graphite  is  also  found  at 
Passau  in  Germany,  in  Bohemia  and  in  Styria.  It  likewise 
occurs  in  many  places  in  the  United  States,  the  deposits  at 
Sturbridge,  Mass.,  and  at  several  localities  in  New  York  being 
large  enough  to  yield  a  considerable  supply.  By  far  the  largest 
mine  in  the  United  States  is  the  "  Eureka  Black-Lead  Mine  "  at 
Sonora  in  California.  The  graphite  here  forms  a  layer  of  some 
twenty  to  thirty  feet  in  thickness.  It  is  so  pure  that  it  may  be 
obtained  in  large  blocks.  In  the  year  1868  not  less  than  one 
million  of  kilos,  were  raised  each  month.1  Large  quantities  of 
graphite  are  also  found  in  Ceylon.  In  Southern  Siberia  this 
substance  occurs  in  considerable  quantities  in  the  Batougal 
mountains,  and  is  largely  exported  to  Europe. 

Graphite  commonly  occurs  in  compact  foliated  or  granular 
masses,  but  occasionally  in  small  six-sided  tables,  which  accord- 
ing to  Kenngott  belong  to  the  hexagonal,  but  according  to 
Nordenskjold  to  the  monosymmetric  system.  It  has  a  steel-gray 
colour,  an  unctuous  touch,  and  it  is  so  soft  that  it  gives  a  black 
streak  on  paper.  Its  specific  gravity  varies  from  2'015  to  2'583r 
and  this  considerable  variation  is  due  to  the  fact  that  almost 
all  natural  graphite  contains  more  or  less  impurity  which, 
when  the  graphite  is  burnt,  remains  behind  as  ash  and  consists 
of  alumina,  silica,  and  ferric  oxide,  with  small  traces  of  lime 
and  magnesia.2  It  usually  contains  0'5  to  T3  per  cent,  of 
hydrogen,3  a  fact  which  seems  to  point  to  its  organic  origin. 
Graphite  is  a  good  conductor  of  heat  and  electricity,  whilst  the 
diamond  is  a  non-conductor.  The  mtumescent  graphite  obtained 
by  Moissan  commences  to  burn  in  oxygen  at  575°,  but  the 
temperature  of  ignition  varies  for  the  different  specimens  of 
natural  graphite.  The  specific  heat  at  ordinary  temperatures 
is  0-202  (Regnault),  whilst  at  978°  it  is  0'467  (Weber). 

The  artificial  production   of   graphite  by  melting  cast  iron 
containing  a  large  proportion  of  carbon,  and  allowing  it  to  cool 

1  Chem.  News,  1868,  299. 

2  Mene,  Compt.  Rend.  64,  1019  and  1867. 

3  Regnault,  Ann.  Chim.  Phys.  [2],  1,  202. 


GRAPHITE  669 


slowly,  was  first  observed  by  Scheele  in  1778.  Cast  iron 
contains  a  certain  amount  of  carbon  combined  with  iron;  in  the 
molten  condition,  however,  it  can  dissolve  a  much  larger 
quantity  of  carbon,  up  to  as  much  as  4  per  cent,  of  its  weight. 
This  excess  crystallizes  out  as  graphite  when  the  metal  cools. 
The  coarsely  crystalline  gray  pig-iron  owes  its  peculiar  pro- 
perties as  well  as  its  appearance  to  the  presence  of  graphite,  and 
when  this  form  of  iron  is  dissolved  in  acid,  scales  of  graphite 
remain  as  an  insoluble  residue. 

Graphite  also  occurs  in  many  meteoric  masses,  as,  for 
instance,  in  the  meteorite  which  fell  in  1861  at  Cranbourne 
near  Melbourne,  and  this  meteoric  graphite  is,  according  to 
Berthelot,  identical  in  properties  with  iron-graphite.  We  may 
thus  conclude  that  the  meteoric  mass  in  which  it  has  been  found 
has  been  exposed  to  a  very  high  temperature. 

According  to  Wagner,  the  black  deposit  which  is  formed 
by  the  spontaneous  decomposition  of  hydrocyanic  acid,  CNH, 
•contains  graphite,  as  may  be  seen  by  washing  the  deposit 
with  strong  nitric  acid,  when  the  insoluble  scales  of  graphite 
are  left  behind.1  Another  remarkable  mode  of  production 
of  graphite,  most  likely  from  cyanogen  compounds,  was  first 
observed  by  Pauli 2  in  the  manufacture  of  caustic  soda  from  the 
black-ash  liquors.  These  liquors  are  evaporated  to  a  certain 
degree  of  consistency,  and  Chili  saltpetre  is  added  to  oxidise 
the  sulphur  and  cyanogen  compounds  present.  Torrents  of 
ammonia  are  thus  evolved,  and  a  black  scum  of  graphite  is 
observed  to  rise  to  the  surface. 

When  the  vapour  of  chloride  of  carbon  is  led  over  melted  cast 
iron,  ferric  chloride  is  evolved,  and  carbon  dissolves  in  the  iron 
until  it  becomes  saturated,  after  which  hexagonal  plates  of 
graphite  separate  out.3 

The  carbon  which  occurs  in  crystalline  boron  remains  as 
amorphous  carbon  when  the  diamond  boron  is  heated  to  red- 
ness. When  it  is  heated  to  whiteness  it  takes  the  form  of 
graphite.  Both  diamond  and  amorphous  carbon  are  converted  into 
graphite  at  the  temperature  of  the  electric  arc,  and  when  the 
arc  is  maintained  between  two  poles  of  amorphous  carbon  the 
negative  pole  is  found  to  be  largely  converted  into  graphite. 
Many  specimens  of  graphite  possess  the  remarkable  property 

1  Wagner's  Jahresb.  1869,  p.  230. 

2  Phil.  Mag.  [4],  21,  541. 

3  H.  Deville,  Ann.  Ghim,  Phys.  [3],  49,  72. 


670  THE  NON-METALLIC  ELEMENTS 

of  swelling  up  and  leaving  a  very  voluminous  residue  of  finely- 
divided  graphite,  in  the  same  manner  as  mercuric  sulphocyanide 
(Pharaoh's  serpents)  when  moistened  with  strong  nitric  acid  and 
then  strongly  heated.  This  property,  however,  is  not  common 
to  all  specimens  of  graphite,  and  it  has  even  been  proposed 
to  divide  the  known  graphites  into  two  classes,  those  which 
give  this  nitric  acid  reaction  being  recognized  as  graphite, 
whilst  those  which  do  not,  such  as  the  graphite  of  the  electric 
arc  and  that  obtained  from  cast  iron,  are  distinguished  as 
graphitite.1 

The  intumescent  variety  may  be  prepared  by  fusing  platinum 
in  a  carbon  crucible  in  an  electric  furnace.  When  the  ingot  is 
treated  with  aqua  regia  a  residue  of  graphite  of  this  kind  is 
left.  It  is  also  found  in  the  interior  of  an  ingot  of  cast  iron 
which  has  been  cooled  by  water,  whilst  the  external  layers 
only  contain  the  non-intumescing  variety.  When  intumescence 
takes  place  oxides  of  nitrogen  and  a  little  carbon  dioxide  are 
given  off  and  a  residue  of  pure  graphite  left,  which  is  scarcely 
altered  by  further  treatment  with  nitric  acid.  The  intume- 
scence is,  therefore,  probably  due  to  the  sudden  evolution  of 
gas  produced  by  the  action  of  traces  of  nitric  acid  remaining 
in  the  graphite  upon  a  little  amorphous  carbon  entangled 
among  the  plates  of  graphite.2 

Graphite  in  its  chemical  relations  occupies  a  position  totally 
distinct  from  that  of  all  other  forms  of  carbon.  When  graphite 
is  repeatedly  treated  with  fuming  nitric  acid  and  potassium 
chlorate  it  is  converted  into  a  compound  which  contains 
both  hydrogen  and  oxygen,  and  is  known  as  graphitic  acid 
or  graphitic  oxide  (p.  729),  whereas  under  the  same  treat- 
ment diamond  is  unaltered  and  amorphous  carbon  completely 
dissolved.  This  reaction,  as  has  been  pointed  out,  provides  a 
means  of  distinguishing  graphite  from  the  other  allotropic 
forms  of  carbon,  and  is  the  basis  of  Berthelot's  method  for 
estimating  the  proportions  of  the  three  varieties  in  a  mixture. 

When  the  electric  arc  is  maintained  between  carbon  poles,  it 
is  observed  that,  however  powerful  the  current,  a  maximum 
degree  of  brightness  is  attained.  This  seems  to  indicate  that 
the  carbon  of  the  pole  volatilises  at  the  temperature  corre- 
sponding to  this  brightness.  By  heating  a  portion  of  the  pole 
to  this  maximum  temperature  and  then  shaking  it  off  into  a 

1  Luzi,  Ber.  24,  4085  ;  25,  216,  1378  ;  26,  890. 

2  Moissan,  Compt.  Rend.  H6,  608. 


GRAPHITE  671 


calorimeter  it  has  been  found  that  the  temperature  of  volatili- 
sation of  carbon  is  about  3,500°,  and  this  is,  therefore,  the 
maximum  temperature  which  can  be  attained  in  an  electric 
furnace  in  which  carbon  poles  are  employed.1 

396  As  already  mentioned,  many  specimens  of  graphite  in- 
tumesce  when  moistened  with  nitric  acid  and  strongly  heated.  It 
is  also  found  that  when  finely-powdered  graphite  is  heated  with  a 
mixture  of  one  part  of  nitric  and  four  parts  of  strong  sulphuric 
acid,  or  when  a  mixture  of  fourteen  parts  of  graphite  and  one 
part  of  potassium  chlorate  is  warmed  with  twenty-eight  parts  of 
strong  sulphuric  acid,  the  graphite  assumes  a  purple  tint,  but 
on  subsequent  washing  returns  to  its  original  colour.  It 
contains  now  oxygen,  hydrogen,  and  sulphuric  acid,  and  when 
it  is  heated  to  redness  swells  up  with  a  copious  evolution  of 
gas,  and  then  falls  to  an  extremely  finely-divided  powder  of 
pure  graphite,  which  has  a  specific  gravity  of  2'25.2  This 
process  is  employed  for  the  purpose  of  purifying  natural  gra- 
phite. With  this  object  it  is  first  ground,  and  the  powder  well 
washed  in  long  troughs  in  order  to  remove  as  much  as  possible 
of  the  earthy  matrix  with  which  it  is  mixed.  The  graphite  thus 
obtained  is  pure  enough  for  many  uses ;  but  if  it  is  required  in 
the  pure  state,  the  powder  must  be  treated  with  potassium 
chlorate  and  sulphuric  acid  as  above  described.  The  fine  powder 
is  then  thrown  upon  water,  on  the  surface  of  which  it  swims, 
whilst  the  earthy  matters  sink  to  the  bottom.  The  foliated 
graphite  answers  best  for  this  purpose,  amorphous  graphite 
being  more  difficult  to  purify.  This  variety  may,  however,  also 
be  rendered  pure  if  a  small  quantity  of  fluoride  of  sodium  be 
added  to  the  mixture  as  soon  as  the  evolution  of  chlorine 
and  its  oxides  has  ceased,  the  object  of  this  addition  being  to 
remove  the  silica  as  silicon  tetrafluoride. 

Graphite  is  employed  for  a  great  variety  of  purposes.  The 
Cumberland  black-lead  pencils,  the  first  of  their  kind,  were 
originally  manufactured  by  cutting  slips  of  graphite  out  of  the 
solid  block.  Experiments  were  afterwards  made  for  the  purpose 
of  making  use  of  the  graphite  powder,  which  was  fused  with 
sulphur  or  antimony.  The  pencils  thus  prepared  were,  however, 
hard  and  gritty.  A  remarkable  improvement  in  the  manufac- 
ture, used  up  to  the  present  day,  was  introduced  by  Comte.  The 
powdered  graphite  is  mixed  with  carefully-washed  clay,  and  the 

1  Violle,  Cmn.pt.  Rend.  115,  1273. 

2  Brodie,  Ann.  Chim.  Phys.  [3],  45,  351. 


672  THE  NON-METALLIC  ELEMENTS 

mixture  placed  in  short  iron  cylinders  having  an  opening  at  the 
bottom.  The  semi-solid  mass  is  then  pressed  through  the  hole 
and  assumes  the  form  of  a  fine  thread,  which  can  be  cut  up  into 
the  required  lengths  and  used  for  pencil-making. 

Another  important  application  of  graphite  is  for  the  prepara- 
tion of  the  black-lead  crucibles  largely  used  in  metallurigical 
operations,  especially  in  the  manufacture  of  cast  steel.  The 
Patent  Plumbago  Crucible  Company  at  Battersea  employ  Ceylon 
graphite  for  the  manufacture  of  their  crucibles,  whilst  the  graphite 
employed  by  Krupp  of  Essen  in  the  manufacture  of  the  cruci- 
bles in  which  his  celebrated  cast  steel  is  melted  is  obtained 
from  Bohemia.  The  finely-ground  graphite  is  well  mixed  with 
Stourbridge  fire-clay  and  water,  so  as  to  obtain  a  homogeneous 
mass  ;  the  water  is  then  pressed  out  and  the  mass  formed  into 
blocks,  which  have  to  lie  for  many  weeks.  The  plasticity  of  the 
mass  is  thus  much  increased.  The  crucible  is  next  moulded  by 
hand  on  a  potter's  wheel,  or  sometimes  formed  in  a  mould ; 
it  is  then  slowly  dried,  and  afterwards  ignited  in  a  pottery 
furnace,  being  placed  in  saggers  in  order  to  prevent  the  com- 
bustion of  the  graphite.  These  crucibles  are  good  conductors 
of  heat,  they  do  not  crack  readily  on  change  of  temperature, 
and  they  likewise  possess  a  clean  surface,  so  that  the  metal  can 
be  poured  out  completely. 

Finely-divided  graphite  purified  according  to  Brodie's  process 
is  largely  used  for  polishing  gunpowder,  especially  the  large- 
grain,  blasting,  and  heavy  ordnance  powder.  This  coating 
of  black  lead  gives  a  varnish  to  the  corn  and  prevents  it 
from  absorbing  moisture.  The  explosive  force  of  the  powder  is, 
however,  somewhat  diminished  by  this  coating  of  graphite,  for 
Abel  has  shown  that  the  explosive  force  of  unpolished  powder 
being  107'6,  the  same  powder  polished  with  common  graphite 
has  an  explosive  force  of  89 '9,  and  when  varnished  with  Brodie's 
graphite  of  99*7. 

The  particles  of  the  finely-divided  and  purified  graphite 
adhere  together  when  they  are  brought  into  close  contact,  as  by 
great  pressure,  and  the  mass  thus  obtained  may  be  used  for 
pencil-making  and  for  other  purposes.  The  pressed  graphite  is 
found  to  conduct  electricity  very  much  more  readily  than  the 
ordinary  graphite,  and  better  than  gas-coke.  Thus,  according  to 
Matthiesen,  its  conducting  power  is  eighteen  times  greater  than 
that  of  natural  graphite,  and  twenty-nine  times  as  great  as  that 
of  the  dense  coke  used  for  the  Bunsen's  batteries.  From  this 


AMORPHOUS  CARBON  673 

property,  graphite  powder  is  used  largely  in  electro  typing,  the 
moulds  upon  which  it  is  desired  to  deposit  the  metal  being 
covered  with  a  fine  coating  of  powdered  graphite.  This  acts  as 
a  conductor  of  electricity,  and  a  uniform  coating  of  the  metal  is 
deposited  upon  it.  Graphite  is  used  not  only  for  the  purpose  of 
preventing  the  rusting  of  iron  objects,  but  in  some  cases  also 
instead  of  oil  for  lessening  the  friction  in  running  machinery. 

397  Amorphous  Carbon. — In  early  ages  the  attention  of 
chemists  was  attracted  to  charcoal  from  the  fact  that  it  is  a  body 
which  cannot  be  acted  upon  by  any  solvent.  The  supporters  of 
the  phlogistic  theory  made  this  substance  a  special  study,  because 
they  believed  that  it  contained  more  phlogiston  than  any 
other  known  body.  This  modification  of  carbon  is  produced 
when  substances  which  contain  that  element  are  strongly 
heated  in  the  absence  of  air.  The  amorphous  carbon  thus 
obtained  has  received  various  names  which  indicate  its  origin 
or  mode  of  production,  but  the  different  varieties  are  identical 
in  chemical  properties.  The  chief  forms  which  are  thus  dis- 
tinguished are  (1)  lamp-black,  (2)  gas-carbon,  (3)  charcoal,  (4) 
animal  charcoal  and  (5)  coke.  The  last  three  of  these  usually 
contain  mineral  matter  which  was  originally  present  in  the 
wood,  bone  or  coal  from  which  they  are  derived.  The  sub- 
stances known  by  these  names  also  generally  contain  small 
amounts  of  both  hydrogen  and  oxygen. 

(1)  Lamp-black.  The  luminosity  of  flame  is  probably  due  to 
the  presence  in  it  of  intensely  heated  particles  of  carbon,  which 
are  deposited  as  soot  on  any  cold  surface  held  in  the  flame. 
When  the  burning  substance  is  rich  in  carbon,  the  flame  smokes 
even  without  its  being  cooled,  and  it  does  so  the  more  strongly 
the  smaller  the  supply  of  air.  This  fact  is  made  use  of  for  the 
purpose  of  preparing  finely-divided  amorphous  carbon  or  lamp- 
black. 

In  the  manufacture  of  lamp-black,  tar,  resin,  turpentine  or 
petroleum  is  burnt  in  a  supply  of  air  insufficient  to  burn  it 
completely,  the  smoky  products  of  this  imperfect  combustion 
being  allowed  to  pass  into  large  chambers  hung  with  coarse 
cloths,  on  which  the  lamp-black  is  deposited.  The  finest  kind 
of  lamp-black  is  obtained  by  suspending  metallic  plates  over 
oil-lamps,  or  revolving  over  them  metallic  cylinders,  on  which 
the  soot  is  deposited.  It  is  purified  by  heating  it  in  closed 
vessels,  and  is  used  for  preparing  Indian  ink  and  in  calico- 
printing  for  producing  gray  shades ;  while  common  lamp-black 

44 


674  THE  NON-METALLIC  ELEMENTS 

is  employed  as  a  black  paint  and  for  manufacturing  printer's 
ink.  Soot  or  lamp-black  is,  however,  not  pure  carbon,  but 
contains  about  80  per  cent,  of  that  element  along  with  oily  and 
fatty  matters  and  a  small  amount  of  mineral  matter,  for  it 
always  contains  appreciable  quantities  of  hydrocarbons  arising 
from  the  incomplete  combustion  of  the  tar.  In  order  to 
remove  these  impurities  it  is  not  sufficient  to  ignite  the  lamp- 
black strongly.  It  must  be  heated  to  redness  in  a  current  of 
chlorine  for  a  considerable  length  of  time,  the  hydrogen  then 
combining  with  the  chlorine,  and  the  carbon  remaining  unacted 
upon. 

(2)  Gas  carbon,  which,  next  to  lamp-black,  is  the  purest  form 
of  amorphous  carbon,  is  formed  in  the  preparation  of  coal-gas, 
and  probably  owes  its  origin  to  the  decomposition  of  the  gaseous 
compounds   of  hydrogen  and   carbon,    which    are    evolved,   by 
the  intensely   heated   walls    of  the  retort.     It   is  found   as   a 
deposit  in  the  upper  portion   of  the   retort  in  the  form  of  an 
iron-gray  mass,  so  hard  that  it  strikes  fire  like  a  flint.     The 
portions  of   this  carbon  deposited   on  the  sides  of  the  retort 
contain    no   hydrogen,   and  have   a   specific  gravity  of   2*356, 
whereas  those    lying   further  from   the    surface  of  the   retort 
contain  some  hydrogen.     Gas  carbon  is  also  obtained  by  passing 
olefiant   gas,    C2H4    (which   is   one   of   the   chief   constituents 
of  coal-gas),  through  a  red   hot    porcelain   tube.      This   form 
of  carbon  conducts  heat  and  electricity  well,  and  is  used  for 
the   preparation  of  the  carbon  cylinders   or   plates   employed 
in    Bunsen's    battery,   and    the   carbon-poles   for    the   electric 
light. 

(3)  Charcoal.     This  substance  is  obtained  in  the  pure  state 
by  heating  pure  white  sugar  in  a  platinum  basin.     The  carbon 
thus  obtained   is  purified  by  ignition    in    a    current    of  pure 
chlorine.     It  is  tasteless  and  possesses  no  smell.     It  is  a  good 
conductor  of   electricity,   and   has  a   specific   gravity  of    T57, 
Like  the  other  modifications  of  carbon  it  is  infusible,  and  is  in- 
soluble  in  every  solvent.      Pure  sugar-charcoal   is   used   as  a 
reducing  agent,  especially  in  the  preparation  of  volatile  metallic 
chlorides,  and  it  is  peculiarly  valuable  inasmuch  as  its  freedom 
from  silica  prevents  the  formation  of  volatile  tetrachloride  of 
silicon. 

Amorphous  carbon  is  converted  by  many  oxidising  agents 
into  complex  soluble  compounds,  among  which  the  most 
important  are  oxalic  acid,  C2H2O4,  and  mellitic  acid,  C12H6O12 


CHARCOAL  675 


(Vol.  III.  Part  v.  p.  374).  Both  of  these  acids  are  formed  by  the 
action  of  alkaline  potassium  permanganate  solution  on  amor- 
phous carbon,1  whilst  mellitic  acid  is  also  formed  by  the  oxida- 
tion of  wood  charcoal  with  concentrated  sulphuric  acid,2  carbon 
dioxide,  and  sulphur  dioxide  being  evolved  at  the  same  time 
(p.  367),  and  when  a  current  of  electricity  is  passed  through  a 
solution  of  caustic  potash  between  carbon  poles.3  The  aluminium 
salt  of  mellitic  acid,  known  as  honey-stone,  occurs  in  seams  of 
brown  coal  and  is  possibly  formed  by  oxidation  from  that 
material. 

The  specific  heat  of  amorphous  carbon  (wood  charcoal)  is 
0-241. 

On  the  large  scale,  wood  charcoal  is  prepared  in  the  same  way 
from  wood  as  coke  is  from  coal.  Charcoal-burning  is  a  very 
old  process,  and  the  simple  methods  which  were  originally 
adopted  are  carried  on  up  to  the  present  day.  The  method 
consists  in  allowing  heaps  of  wood  covered  with  earth  or  sods  to 
burn  slowly  with  an  insufficient  supply  of  air.  It  is  usual  to 
build  up  large  conical  heaps  with  billets  of  wood  placed  verti- 
cally and  covered  over  with  turf  or  moistened  soil,  apertures  being 
left  at  the  bottom  for  the  ingress  of  air.  and  a  space  in  the  middle 
serving  as  a  flue  to  carry  off  the  gases  (Fig.  186).  The  pile 
is  lighted  at  the  bottom,  and  the  combustion  proceeds  gradually 
to  the  top.  Much  care  is  needed  in  regulating  the  supply  of 
air,  and  the  consequent  rate  of  the  combustion.  One  hundred 
parts  of  wood  thus  treated  yield,  on  an  average,  sixty-one  to 
sixty-five  parts  by  measure,  or  twenty-five  parts  by  weight, 
of  charcoal.  In  Austria  and  Sweden  charcoal  is  made  from 
long  logs  of  fir-wood,  which  are  placed  horizontally  in  a  rect- 
angular pile  (Fig.  187). 

In  countries  like  our  own  where  wood  is  scarce,  charcoal  is 
obtained  from  small  wood  or  sawdust  by  a  more  modern  process. 
It  consists  in  the  carbonisation  of  the  wood  in  cast-iron  retorts, 
and  in  this  process  not  only  is  charcoal  obtained,  but  the  volatile 
products,  especially  wood  spirit,  and  pyroligneous  acid,  as  well 
as  tar,  are  collected. 

Good  charcoal  possesses  a  pure  black  colour  and  a  bright 
glittering  fracture.  When  struck  with  a  hard  object  it  emits  a 
sonorous  tone,  and  it  burns  without  smoke  or  flame.  The 
specific  gravity  of  charcoal,  when  the  pores  are  filled  with 

1  Schnlze,  Ber.  4,  802,  806.  2  Verneuil,  Compt.  Rend.  118,  195. 

3  Bartoli  and  Papasogli,  Gazetta,  J3,  37. 


676 


THE  NON-METALLIC  ELEMENTS 


air,  varies  between  0*106  (ash  charcoal)  and  0'203  (birch  char- 
coal). Such  charcoal  will  swim  on  water,  but  it  sinks  when  the 
pores  which  were  filled  with  air  become  filled  with  liquid. 
Charcoal  rapidly  absorbs  gases  and  vapours  (Saussure),  and 


FIG.  186. 


possesses  the  remarkable  property  of  precipitating  certain  sub- 
stances from  solution,  and  absorbing  them  in  its  pores  (Lowitz, 
1790).  Animal  charcoal  possesses  this  absorptive  power  in  a 
much  higher  degree  than  common  charcoal. 


FIG.  187. 


The  properties  and  chemical  composition  of  charcoal  vary 
much  according  to  the  temperature  to  which  the  wood  is 
heated.  According  to  Percy,1  wood  becomes  perceptibly  brown 
at  220°,  whilst  at  280°  it  becomes  after  a  time  a  deep  brown- 

1  Metallurgy,  Fuel,  p.  107. 


CHARCOAL 


677 


black,  and  at  310°  it  is  resolved  into  an  easily  pulverisable  black 
mass.  Charcoal  made  at  300°  is  brown,  soft,  and  friable,  taking 
fire  easily  when  heated  to  380° ;  whilst  that  prepared  at  a  high 
temperature  is  a  hard,  brittle  substance,  which  does  not  take 
fire  till  it  is  heated  to  about  700°.  The  proportion  of  carbon 
contained  in  charcoal  prepared  at  different  temperatures  varies 
considerably.  That  prepared  at  the  lowest  point  contains  much 
more  of  the  volatile  constituents  of  the  original  wood  than  the 
charcoal  made  at  a  red-heat,  though,  as  a  matter  of  course,  the 
yield  of  charcoal  is  greater  at  the  low  temperature.  According 
to  Violette1  100  parts  of  buckthorn  wood  yield  the  following 
amounts  of  charcoal : — 

At     250°  50  parts  of  charcoal  containing  65  per  cent,  carbon 

„     300°  33  „  „  „  73 

„     400°  20  „  „  „  80 

„  1500°  15  „  „  „  96 

Thus  charcoal,  like  lamp-black,  as  ordinarily  prepared,  never 
entirely  consists  of  pure  carbon  (Davy).  The  following  table  gives 
the  composition  of  charcoal  obtained  at  different  temperatures. 


270° 

350° 

432° 

1023° 

1100° 

1250° 

1300° 

1500° 

Over 

1500° 

Carbon  .     .     . 

70-45 

76-64 

81-64 

81-97 

83-29 

88-14 

90-81 

94-57 

96-51 

Hydrogen  .     . 

4-64 

4-14 

1-96 

2-30 

1-70 

1-41 

1-58 

074 

0-62 

Oxygen  with  ) 

some  nitro-  > 

24-06 

18-61 

15-24 

1413 

13-79 

9-25 

6-46 

3-84 

0'93 

gen  .     .     .  ] 

Ash    ... 

0-85 

0-61 

1-16 

1-60 

1-22 

1-20 

1-15 

0-66 

1-94 

100-00 

100-00 

100-00 

100-00 

10000 

ioo-oo|ioo-oo 

100  00 

100-00 

I 

The  following  results  were  obtained  by  Faisst.2 


Beech 
charcoal. 

Carbon 85'89 

Hydrogen 2'41 

Oxygen  and  nitrogen     .  1*45 

Ash 3-02 

Water  7'23 


Hard  charcoal,  Light  charcoal, 
made  in  iron  from  wood- 
cylinders,  gas-works. 

85-18  87'43 

2-88  2-26 

3-44  0-54 

2-46  1-56 

6-04  8-21 


100-00    100-00     100-00 

Ann.  Chim.  Phys.  [3],  32,  305.  2  Wagner,  Jahresb.  1855,  p.  457. 


678  THE  NON-METALLIC  ELEMENTS 

(4)  Animal  Charcoal,  which  is  obtained  by  the  carbonisation 
of  animal  materials,  differs  from  that  prepared  from  vegetable 
sources,  inasmuch  as  it  contains  considerable  quantities  of  nitro- 
gen. Bone-Hack  is  obtained  by  charring  bones  in  iron  cylinders 
or  retorts,  and  is  formed  as  a  by-product  in  the  manufacture  of 
animal,  or  Dippel's  oil  (Oleum  animale  Dippelii).  Dry  bones 
contain  about  70  per  cent,  of  inorganic  material,  consisting 
chiefly  of  calcium  phosphate.  This  inorganic  matter  remains 
behind  with  the  charcoal  when  the  bones  are  charred,  and  thus 
the  charcoal  is  deposited  in  an  extremely  finely-divided  con- 
dition. It  is  probably  for  this  reason  that  bone-black  possesses 
a  greater  absorptive  and  decolourising  power  than  wood  charcoal. 
The  presence  of  the  phosphate  of  lime,  however,  prevents  the 
application  of  this  decolourising  power  of  the  charcoal  to  cases  of 
acid  liquids  which  dissolve  the  phosphate.  In  such  cases  it  is 
necessary  to  make  use  of  blood-charcoal.  This  is  obtained  by 
evaporating  four  parts  of  fresh  blood  with  one  part  of  carbonate 
of  potash,  and  heating  the  residue  in  a  cylinder.  The  charred 
residue  is  then  boiled  out  with  water  and  hydrochloric  acid,  and 
again  heated  to  redness  in  a  closed  vessel.  The  addition  of  potash 
serves  the  purpose  of  making  the  charcoal  as  porous  as  possible. 

The  following  is  an  analysis  of  a  good  sample  of  dried  animal 
charcoal.1 

Carbon 10'51 

Calcium  and  magnesium  phosphates,  calcium  fluoride, 

&c 80-21 

Calcium  carbonate 8'30 

Other  mineral  matter      .  0'98 


100-00 

398  The  power  possessed  by  wood  charcoal  and,  in  a  much 
higher  degree,  by  animal  charcoal,  of  withdrawing  many  sub- 
stances from  their  solution,  seems  to  be  almost  entirely  of  a 
physical  nature.  Thus,  if  a  solution  of  iodine  in  iodide  of  potas- 
sium be  shaken  up  with  finely-divided  charcoal,  the  iodine  is 
completely  withdrawn  from  solution.  In  the  same  way  certain 
metallic  salts,  especially  basic  salts,  are  decomposed  when  their 
solutions  are  filtered  through  charcoal.  This  power  of  withdrawing 
bodies  from  solution  is  exerted  most  effectually  upon  colouring 
1  Thorpe's  Dictionary  of  Applied  Chemistry,  Vol.  I.  171. 


ANIMAL  CHARCOAL  679 


matters  and  astringent  principles.  Thus,  if  a  quantity  of 
red  wine  (claret  or  port),  or  a  solution  of  indigo  in  sulphuric 
acid,  be  shaken  up  and  gently  warmed  with  a  quantity  of 
freshly-ignited  bone  charcoal  and  the  mixture  filtered,  the 
liquid  which  comes  through  is  colourless.  In  the  same  way,  if 
alcohol  containing  fusel  oil  be  shaken  up  with  animal  charcoal, 
the  characteristic  smell  of  the  fusel  oil  disappears.  Wood  char- 
coal is  largely  used  for  the  latter  purpose,  whereas  animal 
charcoal  is  employed  for  decolourising  the  juice  of  raw  sugar. 
Animal  charcoal  is  employed  in  a  similar  way  in  the  purifica- 
tion of  many  organic  compounds,  although  it  not  unfrequently 
happens  that  the  compounds  themselves  are  attracted  by  the 
porous  charcoal.  Occasionally  this  property  is  made  use  of 
for  the  separation  of  compounds  which  thus  adhere  to  the 
charcoal,  as,  for  instance,  the  alkaloids,  from  other  bodies,  these 
compounds  being  afterwards  dissolved  from  the  charcoal  by 
the  addition  of  hot  alcohol.  Like  many  other  porous  bodies 
both  of  these  forms  of  charcoal  possess  the  property  of  largely 
absorbing  gaseous  bodies.  This  power  depends  upon  the  fact 
that  all  gases  condense  in  greater  or  less  degree  on  to  the 
surface  of  solid  bodies  with  which  they  come  in  contact,  and  as 
charcoal  is  very  porous,  or  possesses  a  very  large  surface  to  a 
given  mass,  its  absorbent  power  is  proportionately  great.  Char- 
coal, when  exposed  to  the  air,  condenses  large  quantities  of  the 
latter  upon  its  surface.  This  may  be  easily  shown  by  attach- 
ing a  piece  of  metal  to  it  and  sinking  the  mass  in  a  cylinder 
filled  with  water.  If  the  cylinder  be  now  placed  under  the 
receiver  of  an  air-pump  and  the  air  exhausted,  a  rapid  stream 
of  bubbles  will  be  seen  to  rise  in  brisk  effervescence  from  the 
charcoal.  The  remarkable  absorptive  power  of  charcoal  for 
certain  gases  is  also  well  illustrated  by  inserting  a  stick  of 
recently  calcined  charcoal  into  a  tube  filled  over  mercury  with 
dry  ammonia  gas.  The  gas  is  so  quickly  absorbed  by  the 
charcoal  that  the  tube  soon  becomes  filled  with  mercury. 
Another  mode  of  showing  a  similar  absorption  of  sulphuretted 
hydrogen  gas  is  to  plunge  a  small  crucible  filled  with  freshly- 
ignited  and  nearly  cold  powdered  charcoal  into  a  jar  of  sul- 
phuretted hydrogen.  This  gas  is  then  absorbed  by  the  charcoal 
in  such  quantity  that  if  it  be  removed  when  saturated  and 
plunged  into  a  jar  of  oxygen  the  charcoal  will  burst  into  vivid 
combustion,  owing  to  combination  occurring  between  the  ab- 
sorbed sulphuretted  hydrogen  and  oxygen  gases. 


680  THE  NON-METALLIC  ELEMENTS 

The  absorptive  power  of  wood  charcoal  for  gases  was 
first  investigated  by  Saussure.  In  his  experiments  he  made 
use  of  beech-wood  charcoal  which  had  been  recently  heated 
to  redness  and  then  cooled  under  mercury  in  order  to  remove 
the  air  from  its  pores.  The  following  numbers  were  obtained 
by  him : — 

1  volume  of  charcoal  absorbs  at  12°  and  under  724  mm.  the 
following  (Saussure) : 

Vols.  Vols. 

Ammonia 90  Ethylene     ....  35 

Hydrochloric  acid     .     .  85  Carbon  monoxide     .  9'42 

Sulphur  dioxide  ...  65  Oxygen  .....  9'25 

Sulphuretted  hydrogen  55  Nitrogen     ....  6*50 

Nitrogen  monoxide  .     .  40  Hydrogen   ....  1*25 

Carbon  dioxide    ...  35 

Hunter,1  who  also  made  experiments  on  the  same  subject, 
found  that  one  volume  of  charcoal  absorbed  the  following 
quantities  of  gas  at  the  temperature  0°  and  under  a  pressure  of 
760  mm. 

Absorption  of  gases  by  charcoal  (Hunter). 

Vols.  Vols. 

Ammonia    ....  171'7  Phosphine   ....  691 

Cyanogen    ....  107*5  Carbon  dioxide     .     .  677 

Nitrous  oxide  .     .     .  86'3  Carbon  monoxide .     .  21*2 

Methyl  chloride  .     .  76'4      Oxygen 17 '9 

Methyl  ether  .     .     .  76'2      Nitrogen 15'2 

Ethene 74'7  Hydrogen     ....  4'4 

Nitric  oxide    .     .     .  70*5 

The  differences  observed  between  these  two  series  of  experi- 
ments with  the  same  gases  are  to  be  explained  by  the  fact  that 
the  charcoals  employed  were  not  of  equal  porosity.  Both  series, 
however,  show  that  the  more  readily  the  gas  is  condensible  the 
more  it  is  absorbed  by  charcoal,  which  seems  to  show  that  the 
gases  condensed  by  charcoal  undergo  at  any  rate  a  partial  lique- 
faction. This  view  is  rendered  possible  by  the  observation  of 
Melsen,2  that  when  dry  hydrogen  is  brought  into  contact  with 
charcoal  saturated  with  chlorine  a  considerable  quantity  of 

1  Phil.  Mag.  [4],  25,  364 ;  29,  116.  2  Compt.  Rend.,  76,  81,  92. 


ABSORPTIVE  POWER  OF  CHARCOAL  681 

hydrochloric  acid  is  formed,  even  when  the  experiment  is 
carried  on  in  complete  darkness ;  and  that  when  charcoal 
saturated  with  chlorine  is  brought  into  a  Faraday's  tube  and 
the  other  limb  placed  in  a  freezing  mixture,  liquid  chlorine 
is  obtained.  In  a  similar  way  ammonia,  cyanogen,  sulphur 
dioxide,  sulphuretted  hydrogen,  and  hydrobromic  acid  have  been 
liquefied. 

Wood  charcoal,  like  bone  charcoal,  has  the  power  of  absorbing 
the  unpleasant  effluvia  evolved  in  the  processes  of  decay  and 
putrefaction  as  well  as  the  moisture  from  the  air.  Stenhouse,1 
who  has  investigated  this  subject,  has  shown  that  charcoal  not 
only  absorbs  these  gases  and  effluvia,  but  has  the  power,  espe- 
cially in  contact  with  air,  of  oxidising  and  destroying  them, 
inasmuch  as  when  absorbed  by  charcoal  these  substances  are 
brought  into  such  close  contact  with  the  atmospheric  oxygen, 
which  is  also  absorbed  by  the  charcoal,  that  a  rapid  oxidation 
is  set  up,  and  the  odoriferous  products  of  decomposition  are 
instantly  resolved  into  carbon  dioxide  and  water,  and  other 
simple  compounds.  This  property  is  retained  by  the  charcoal 
for  a  long  time,  and  when  it  has  been  lost  it  can  be  renewed  by 
ignition.  Hence  charcoal  niters  are  largely  used  for  preventing 
the  foul  sewer  gases  from  polluting  the  air  of  the  streets  and 
houses,  and  charcoal  respirators  and  ventilators  have  been  pro- 
posed by  Stenhouse  as  protections  against  the  ingress  of  dele- 
terious gases  into  the  lungs.  For  the  same  reason,  trays  filled 
with  heated  wood  charcoal,  placed  in  the  wards  of  hospitals  or 
other  infected  apartments,  have  proved  very  effective  in  absorb- 
ing noxious  emanations.  Charcoal  filters  are  also  largely  em- 
ployed for  filtering  water  for  drinking  purposes,  as  in  its 
passage  through  the  charcoal  the  water  is  decidedly  improved 
in  quality,  not  only  organic  and  soluble  colouring  matters  being 
removed  as  well  as  all  suspended  matter,  but  the  water  under- 
going aeration.  Such  filters  do  not,  however,  necessarily  free  the 
water  from  bacterial  life  ;  but  may,  on  the  other  hand,  increase 
its  amount  unless  the  filter  is  frequently  sterilized.  (See  ante, 
p.  301.) 

(5)  Coke.  This  substance  remains  behind  when  bituminous 
coal  is  heated  to  redness  in  absence  of  air.  Coke  is  obtained 
as  a  by-product  in  the  manufacture  of  coal-gas,  but  it  is  also 
specially  manufactured  in  coke  ovens,  and  sometimes  by  burn- 

1  On  Charcoal  as  a  Disinfectant ;  Proc.  Hoy.  Inst.  2,  53  ;  also  Pharm.  Journ. 
16,  363 


682  THE  NON-METALLIC  ELEMENTS 

ing  coal  in  heaps  and  stopping  the  combustion  at  a  certain 
stage  by  quenching  with  water.  When  prepared  by  heating  in 
•covered  ovens  or  kilns,  the  coke  is  harder,  more  lustrous,  and 
less  combustible  than  that  obtained  by  burning  in  heaps.  It  is 
termed  hard-coke  or  engine-coke,  and  is  largely  used  for  iron 
smelting,  whereas  the  other  variety  is  termed  soft-coke  or 
blacksmith's  coke,  and  is  the  more  combustible.  Coke  takes  fire 
at  a  much  higher  temperature  than  common  coal,  and,  when 
burning,  gives  rise  to  a  very  high  temperature,  but  without  the 
•elimination  of  smoke,  as  it  consists  almost  entirely  of  carbon. 
Coke  not  only  contains  the  inorganic  material,  or  ash,  present  in 
the  coal  from  which  it  is  manufactured,  but  in  addition  small 
quantities  of  hydrogen,  oxygen,  and  nitrogen,  as  the  following 
analyses  show : — 

COKE  ANALYSES. 

First  Sample.  Second  Sample. 

Carbon 91'30  91'59 

Hydrogen O33  0'47 

Nitrogen  and  oxygen     .       217  2*05 

Ash 6-20  5-89 

The  amount  of  coke  produced  in  Great  Britain  during  1885 
was  11,077,375  tons.1 


COAL. 

399  When  vegetable  matter  decays  in  absence  of  air  and  under 
water,  or  in  the  earth,  it  undergoes  a  change  similar  to  that 
which  occurs  when  it  is  heated.  Water,  carbon  dioxide,  and 
marsh  gas  are  given  off,  and  the  residual  material  becomes 
richer  in  carbon.  The  fact  of  the  occurrence  of  marsh  gas  and 
carbon  dioxide  as  products  of  decomposition  is  rendered  evident 
by  their  presence  in  a  highly  compressed  condition  in  the  coal 
measures  at  the  present  day,  from  which  the  former  is  evolved 
as  fire-damp  in  enormous  quantities  which  often  produce  fatal 
accidents.  It  is  in  this  way  that  the  coals  of  various  kinds, 
lignites  and  peats,  have  been  formed.  Their  composition  com- 
pared with  that  of  cellulose  is  shown  in  the  following  table,  the 

1  Thorpe's  Dictionary  of  Applied  Chemistry,  vol.  ii.  162. 


COAL 


683 


amount  of  mineral  matter  which  is  left  behind  as  ash  on  com- 
bustion having  been  subtracted  : — 


Oxygen 



Carbon. 

,  Hydrogen. 

and 

Nitrogen. 

Cellulose     

50-00 

6-00 

44-00 

Irish  peat   

60-02 

5-88 

34-10 

Lignite  from  Cologne   .     . 

66-96 

5-25 

2776 

Earthy  coal  from  Dax  .     . 

74-20 

5-89 

19-90 

Cannel  coal  from  Wigan  . 

85-81 

5-85 

8-34 

Newcastle  Hartley  .     .     . 

88-42 

5-61 

5-97 

Welsh  anthracite      .     .     . 

94-05 

3-38 

2-57 

The  above  table  shows  that  coal  is  a  less  pure  form  of  carbon 
than  wood  charcoal.  It  consists  of  the  more  or  less  altered 
remains  of  a  vegetable  world  which  once  flourished  at  various 
points  on  the  earth's  surface.  The  plants  of  the  coal  formation 
consist  of  Calamites,  the  representatives  of  the  living  Equisetums, 
of  Lepidodendra,  arid  SigillariaB,  which  were  the  principal  forest- 
trees,  and  which,  though  of  gigantic  dimensions,  were  true  cryptd- 
gams,  represented  by  the  living  Lycopods  and  Selaginellse,  and 
an  important  group  of  Conifers  and  Cycads,  as  yet  not  well 
understood,  and  chiefly  known  through  their  seeds,  which  are 
numerous  and  varied.  Besides  these  arborescent  types,  ferns 
.formed  an  abundant  undergrowth,  many  of  these  also  having 
been  representatives  of  the  living  tree-ferns. 

In  the  passage  into  coal  the  original  woody  fibre  has  not  only 
undergone  a  loss  of  hydrogen  and  oxygen  but  it  has  at  the  same 
time  become  bituminised,  so  that  for  the  most  part  all  vegetable 
structure  has  disappeared,  and  the  coal  possesses  a  fatty  lustre 
and  coarse  slaty  fracture.  There  are  many  different  kinds  of 
coal  containing  more  or  less  of  the  hydrogen,  oxygen,  and 
nitrogen  of  the  original  woody  fibre.  Cannel  coal  and  boghead 
coal  contain  the  most  hydrogen,  and  anthracite  contains  the 
least,  whilst  the  various  kinds  of  bituminous  coals  lie  between 
these  extremes. 

Anthracite  Coal  of  all  coals  contains  the  largest  percentage  of 
carbon,  and  has  therefore,  undergone  the  most  complete  change 
from  woody  fibre.  It  is  found  in  the  oldest  deposits  of  the  car- 
boniferous series,  especially  in  Wales,  Pennsylvania,  and  Rhode 


684  THE  NON-METALLIC  ELEMENTS 

Island,  and  in  smaller  quantities  in  France,  Saxony,  and  South- 
ern Russia.  It  has  a  conchoidal  fracture,  possesses  a  bright  lustre, 
often  sub-metallic,  an  iron-black  colour,  and  is  frequently  iri- 
descent. It  burns  with  a  smokeless  flame.  Anthracite  gradu- 
ally passes  into  bituminous  coal,  becoming  less  hard  and 
containing  more  volatile  matter.  Its  specific  gravity  varies 
from  1-26  to  1'8. 

The  Bituminous  Coals  consist  of  a  large  number  of  varieties 
differing  considerably  from  one  another  in  their  chemical  com- 
position, as  also  in  their  products  of  decomposition  by  heat. 
They  have  the  common  property  of  burning  with  a  smoky 
flame  when  placed  in  the  fire,  and  yielding  on  distillation 
volatile  hydrocarbons,  tar  or  bitumen,  whence  their  name  is 
derived. 

The  most  important  kinds  of  bituminous  coals  are  (1)  caking 
coal,  which  softens  and  becomes  pasty  or  semi-solid  in  the  fire, 
and  yields,  when  completely  decomposed,  a  greyish-black  cellu- 
lar mass  of  coke  ;  (2)  non-caking  coal,  agreeing  with  the  last- 
named  variety  in  all  its  external  characters  and  even  in  its 
chemical  composition,  but  burning  freely  without  softening,  and 
without  any  appearance  of  incipient  fusion  ;  the  residue  which 
it  yields  is  not  a  proper  coke,  being  either  in  powder  or  in  the 
form  of  the  original  coal. 

Cannel  Coal,  sometimes  called  parrot  coal.  This  is  a  variety 
of  coal  differing  from  the  preceding  in  texture,  and  yielding 
usually  more  volatile  matters  and  being  therefore  specially 
employed  for  the  purpose  of  gas-making.  Cannel  coal  is  more 
compact  than  bituminous,  possesses  little  or  no  lustre,  does  not 
show  any  banded  structure,  breaks  with  a  conchoidal  fracture 
and  smooth  surface;  and  has  a  dull  black  or  grayish-black 
colour.  Its  name  is  derived  from  the  fact  that  small  fragments, 
when  lighted,  will  burn  with  flame,  and  hence  it  was  termed 
candle-  or  cannel-coal. 

400  The  tables  on  pages  685  to  687,  give  examples  of  the 
composition  of  the  different  forms  of  coal. 

Connected  with  the  true  coals  is  a  peculiar  form,  termed 
Boghead  coal  or  Torbane  Hill  mineral,  which  was  first  found 
near  Bathgate  in  Linlithgowshire,  and  has  since  been  observed 
in  other  places,  especially  in  New  South  Wales.  This  deposit 
occurs  in  the  carboniferous  formation,  but  it  is  not  pro- 
perly speaking  a  coal,  but  belongs  to  the  class  of  bituminous 
shales.  Its  specific  gravity  is  lower  than  that  of  true  coal,  and 


COMPOSITION  OF  COALS 


685 


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THE  NON-METALLIC  ELEMENTS 


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Charleroi  .  .  . 
Pas  de  Calais  . 
Hungary  .  .  . 

COMPOSITION  OF  COALS 


687 


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§    «  3        «                  -r  if         5 

•    •  i  ^  ^                   8  ^      '6.  »   « 

S                       Q      *S            «                      *                                        ^r£3                       ? 

.5   s  5  i  2     ,  ^     ?        §  -5        g 

00       >-s  L^           "^     O                            ^       O         ~      QJ 

P^                      <t^^Hg                      QQ                                        J^^                      Q^ 

688  THE  NON-METALLIC  ELEMENTS 

it  possesses  a  brown  instead  of  a  black  colour.  When  com- 
pletely burnt  it  leaves  a  residue  of  about  20  per  cent,  of  ash, 
and  when  heated  in  a  retort,  about  70  per  cent,  volatilises  partly 
as  gas,  partly  as  more  or  less  liquid  or  solid  hydrocarbons. 
These  are  known  under  the  name  of  paraffin  oils,  and  are 
largely  used  for  illuminating  as  well  as  for  lubricating  purposes, 
and  for  the  preparation  of  solid  paraffin. 

Brown  Coal  or  Lignite  belongs  to  a  different  and  more  recent 
geological  period  than  coal  proper,  being  found  in  the  tertiary 
formation.  It  consists  of  the  remains  of  trees  and  shrubs,  as  ash, 
poplar,  and  others,  which  now  exist  on  the  surface  of  the  earth. 
It  possesses  a  brown  colour  and  often  exhibits  a  characteristic 
woody  structure.  Its  specific  gravity  varies  from  I'lo  to  T30. 

Jet  is  a  black  variety  of  brown  coal,  compact  in  texture,  and 
taking  a  good  polish.  Hence  it  is  largely  used  in  jewellery. 

Earthy  Brown  Coal  is  another  brown  friable  material,  some- 
times forming  layers  in  beds  of  lignite,  but  it  is  not  a  true  coal, 
inasmuch  as  a  considerable  portion  of  it  is  soluble  in  ether  and 
benzene,  and  often  even  in  alcohol,  whereas  true  coal  is  nearly, 
if  not  quite,  insoluble  in  these  liquids. 

Turf  or  peat  is  a  material  which  is  being  constantly  formed 
by  the  decomposition  of  marsh  plants,  chiefly  mosses,  &c.  It 
always  contains  nitrogenous  compounds,  which  are  the  cause  of 
the  peculiar  smell  which  it  gives  off  on  heating,  is  very  rich 
in  ash,  and  after  drying  in  the  air  still  contains  15 — 20  per 
cent,  of  water. 

Yield  of  Coal  in  Great  Britain. — The  following  table,  taken 
from  the  final  report  of  the  Royal  Commission  on  Mining 
Royalties  (1893),  shows  the  production  of  coal  in  each  of  the 
districts  named,  giving  the  total  produce  in  the  year  1889.1 

1  For  further  information  on  the  subject  of  coal,  lying  beyond  the  scope  of  this 
work,  we  refer  our  readers  to  the  following  standard  works  : — Percy's  Metallurgy : 
Fuel.  London,  1875  ;  Hull  On  the  Coal  Fields  of  Great  Britain;  Statistics  of 
Coal,  by  R.  C.  Taylor.  1855  ;  Ronalds  and  Richardson's  Chemical  Technology, 
vol.  i. ;  Ronalds  On  Fuel  and  its  Applications  ;  Jevons,  The  Coal  Question;  Report 
of  the  Royal  Commission  on  Coal ;  Thorpe's  Dictionary  of  Applied  Chemistry. 


OUTPUT  OF  COAL 


OUTPUT  OF  COAL  IN  1889. 


Northumberland 8,794,005 

Durham 30,307,177 

Cumberland ) 

Westmoreland j 

Lancashire 1         22,327,968 

Cheshire j 

Yorkshire 21,976,027 

North  Staffordshire )  4  712  010 

Cannock  Chase j 

South  Staffordshire \ 

Warwickshire I        11,819,766 

Worcestershire j 

Derbyshire ^ 

Nottinghamshire I        18,012,378 

Leicestershire j 

Shropshire 710,490 

Somersetshire 876,254 

Gloucestershire : — 

Bristol  Field .     .  504,847 

Forest  of  Dean 854,967 

Monmouth  and  South  Wales     ....  28,064,235 

North  Wales 2,895,499 

Total 153,596,360 

England  and  Wales 153,596,360 

Scotland 23,217,163 

Ireland  .  103,201 


Total  for  United  Kingdom 


Tons. 


176,916,724 


45 


690  THE  NON-METALLIC  ELEMENTS 


CARBON  AND  HYDROGEN. 

401  All  the  elements  hitherto  described  combine  with 
hydrogen,  but  each  of  these  elements  possesses  the  power 
of  forming  only  a  small  number  of  hydrides.  Carbon,  on  the 
other  hand,  is  distinguished  from  the  foregoing  elements,  as 
well  as  from  all  the  others,  by  the  fact  that  it  is  capable 
of  forming  an  extremely  large  number  of  hydrogen  compounds. 
These  substances  are  called  hydrocarbons,  and  most  of  them  are 
volatile  bodies. 

This  peculiarity  of  carbon  depends  upon  the  fact  that  one 
atom  possesses  the  property  of  combining  with  another  atom 
of  carbon,  one  or  more  of  its  four  combining  units  being  thus 
saturated,  whilst  the  remaining  combining  units  are  capable  of 
being  saturated  with  hydrogen. 

It  has  already  been  shown  that  other  elements,  such  as 
oxygen  and  sulphur,  possess  the  same  property,  though  in 
a  much  smaller  degree.  Thus  we  know  of 'no  volatile  com- 
pound which  contains  more  than  seven  atoms  of  sulphur  or 
of  oxygen  linked  together  in  the  same  molecule.  Disulphuryl 
chloride,  S2O5C12,  is  the  compound  which  contains  the  largest 
number  of  atoms  belonging  to  this  group. 

In  the  case  of  carbon  such  a  limit  has  not,  as  yet,  been  found, 
derivatives  containing  as  many  as  sixty  carbon  atoms  directly 
united  together,  having  been  already  obtained.  The  number  of 
hydrocarbons  does  not,  however,  depend  merely  upon  the  number 
of  carbon  atoms  which  can  occur  combined  with  one  another, 
inasmuch  as  the  atoms  may  saturate  one  another  reciprocally 
by  the  union  of  one,  two,  or  three  of  their  own  combining 
units.  Notwithstanding  this  complexity  of  construction,  the 
hydrocarbons  may  be  classed  in  certain  groups,  each  one  of 
which  can  be  represented  by  a  general  formula.  Of  these  the 
three  simplest  are  : — 

GROUP  I  GROUP  II.  GROUP  III. 


Methane,  CH4. 
Ethane,     C2H6. 
Propane,  C3H8. 
Butane,    C4H10. 
Pentane,  C5H12. 

Ethene,    C2H4. 
Propene,   C3H6. 
Butene,    C4H8. 
Pentene,  C5H10. 

Ethine,  C2H2. 
Propine,  C3H4. 
Butine,    C4H6. 
Pentine,  C5H8. 

CARBON  AND  HYDROGEN  691 


The  constitution  of  these  compounds  of  carbon  and  hydrogen, 
or  hydrocarbons  as  they  are  usually  termed,  is  best  understood 
by  starting  with  the  simplest,  in  which  the  four  combining  units 
of  a  single  carbon  atom  are  saturated  by  hydrogen.  This  is  the 
well-known  substance  marsh  gas,  and  its  constitution  is  repre- 
sented by  the  graphic  formula 


H 
H. 


H-i- 
i 


If  one  atom  of  hydrogen  in  this  compound  be  replaced  by  a 
carbon  atom,  and  the  remaining  valencies  of  the  latter  saturated 
by  hydrogen,  we  obtain  a  hydrocarbon  having  the  composition 
C2H6,  the  constitution  of  which  is  as  follows : 

H     H 
H— C— C— H    orCH3— CH3. 


If,  now,  one  of  the  hydrogen  atoms  in  this  hydrocarbon  be  re- 
placed by  a  third  carbon  atom,  and  its  remaining  combining 
units  saturated  with  hydrogen,  we  obtain  a  new  substance,  C3Hg, 
its  constitution  being 

H     H    H 
H— C-  C-C— H    or  CH3 .  CH2 .  CH3. 


The  same  process  may  again  be  carried  out,  but  here  it  is  not, 
as  in  the  previous  cases,  immaterial  which  hydrogen  atom  is 
replaced  by  the  added  carbon  atom,  for  these  have  not  all  an 
equal  value.  An  examination  of  the  graphic  formula  given 
above  for  C3H8  shows  that  two  of  the  hydrogen  atoms  are  com- 
bined with  a  carbon  atom  which  is  itself  attached  to  two  other 
carbon  atoms,  whilst  the  remaining  six  hydrogen  atoms  are 
combined  with  a  carbon  atom  attached  to  only  one  other  carbon 


THE  NON-METALLIC  ELEMENTS 


atom.      Hence  according  to  the  formula  we  can  get  the  two 
following  compounds  of  the  composition  C4H10 : — 

H   ^    H 

HTT  TT  TT  TT     \./"1/      TT 

±1          ±1  Jtl  -H.  V>  ±1 


— C— H 
H 


H— C— C— C— H 
J,    H    i 


or 


CH3 .  CH2 .  CH2 .  CH3 


CH€ 


or 
CH3 

.CH. 


CH3 


and  in  fact  two  and  only  two  compounds  of  this  composition  have 
been  experimentally  obtained.  Substances  of  this  kind,  which 
have  the  same  percentage  composition  but  different  physical 
and  chemical  properties,  are  known  as  isomerides.  Such  cases 
are  occasionally  met  with  in  the  compounds  of  the  other  ele- 
ments, as  for  example  in  the  case  of  the  potassium  sodium 
sulphites  (p.  373),  but  they  are  of  far  more  frequent  occurrence 
among  the  carbon  compounds.  Thus  three  different  hydro- 
carbons having  the  formula  C5H12  are  known,  their  formulae 
being 

(1) 
CH3.CH2.CH2.CH2.CH3 


(2) 

CH3.CH2.CH 


(3) 

CH, 


\ 


CH, 


CH3— C— CH 


and  the  number  of  possible  isomerides  rapidly  increases  with 
the  rise  in  the  number  of  carbon  atoms,  no  less  than  799  different 
isomerides  having  the  composition  C13H26  being  theoretically 
possible. 

All  the  hydrocarbons  derived  in  this  manner  from  marsh  gas 
have  the  general  formula  CnH2n+2,  and  are  usually  termed  the 
paraffins. 


CONSTITUTION  OF  HYDROCARBONS 


A  different  series  of  hydrocarbons,  known  as  the  olefmes,  and 
having  the  general  formula  CnH2n,  contains  two  carbon  atoms 
united  together  by  two  combining  units,  the  simplest  represen- 
tative being  ethylene,  C2H4,  the  constitution  of  which  is 
represented  by  the  formula 


HTT 
\  / 

H/  \H 


orCH2  =  CH2. 


The  hydrogen  atoms  of  this  hydrocarbon  may  be  replaced  by 
other  carbon  atoms  in  exactly  the  same  manner  as  described 
under  methane,  giving  rise  to  new  hydrocarbons,  of  which  the 
following  may  be  taken  as  examples  : — 

Propylene.  o-Butylene.  £-Butylene. 

CH3.CH  =  CH2        CH3.CH.2.CH  =  CH2        CH3.CH  =  CH.CH3 

Another  series,  somewhat  similar  to  the  foregoing,  having  the 
general  formula  CnH2n_9,  contains  two  carbon  atoms  united 
together  by  three  combining  units.  The  first  member  of  this 
series,  known  as  acetylene,  has  the  constitution 

CH  =  CH. 

In  a  fourth  series  of  hydrocarbons  the  carbon  atoms  are  united 
together  in  such  a  manner  that  they  form  a  closed  chain.  The 
most  important  member  benzene,  contains  a  closed  chain  of  six 
carbon  atoms,  its  constitution  being  represented  as  follows  : — 

H 


H— C     C— H 
H— C     C— H 


C 
H 

It  will  be  observed  that  in  this  formula  only  three  valencies 
of  each  carbon  atom  are  represented  as  saturated ;  the  manner 
in  which  the  remaining  valencies  are  disposed  is  still  a  matter 
of  discussion. 


694  THE  NON-METALLIC  ELEMENTS 

Almost  all  the  other  compounds  of  carbon  may  be  regarded 
as  derived  from  one  or  other  of  these  numerous  hydrocarbons  ; 
thus,  for  example,  chloroform,  CHC13,  is  marsh  gas  in  which  three 
of  the  four  hydrogen  atoms  have  been  replaced  by  chlorine,  and 
alcohol,  C.2H6O,  is  ethane,  C2H6,  in  which  one  atom  of  hydrogen 
has  been  replaced  by  the  compound  radical  hydroxyl : — 

CH3.CH3  CH3.CH2.OH 

Ethane.  Alcohol. 

Most  of  the  substances  occurring  in  the  animal  and  vegetable 
kingdoms  belong  to  the  group  of  carbon  compounds,  and  in 
addition  to  these  an  immense  number  of  other  derivatives  con- 
taining this  element  have  been  prepared  artificially,  so  that  the 
total  number  of  carbon  compounds  known  is  greater  than  that 
of  all  the  other  elements  put  together.  For  this  reason  they 
are  separately  treated  of  under  the  head  of  Organic  Chemistry, 
which  is  now  usually  defined  as  the  Chemistry  of  the  Hydrocarbons 
and  their  derivatives.  In  this  portion  of  the  work  only  the 
simpler  hydrocarbons,  containing  one  or  two  atoms  of  carbon, 
will  be  considered. 


METHANE,  METHYL  HYDRIDE,  MARSH  GAS,  OR  FIRE-DAMP. 
CH4  =  15-91. 

402  This  gas  is  found  in  the  free  state  in  nature,  and  its 
occurrence  was  observed  in  early  times.  Thus  Pliny  mentions 
the  combustible  gaseous  emanations  which  occur  in  several  dis- 
tricts, and  Basil  Valentine  remarks  upon  the  outbreaks  of  flame 
which  occur  in  mines,  and  which  are  preceded  by  a  suffocating 
damp  or  vapour.  He  does  not  consider  that  this  vapour  is  com- 
bustible, but  rather  believes  that  the  flame  was  emitted  by  the 
rocks  for  the  purpose  of  destroying  this  poisonous  vapour. 

Marsh  gas,  like  some  other  combustible  gases,  was  not  dis- 
tinguished from  inflammable  air,  or  hydrogen,  until  Volta  in  the 
year  1776  showed  that  inflammable  air,  when  burnt,  required 
only  one-fourth  of  the  volume  of  oxygen  which  was  needed  for 
the  complete  combustion  of  marsh  gas,  and  that  in  this  latter 
case  alone  was  carbonic  acid  formed.  In  the  year  1785  Ber- 
thollet  proved  that  this  gas  contains  both  carbon  and  hydrogen, 
but  it  was  at  that  time  not  distinguished  from  olefiant  gas 


METHANE  695 


(or  ethene,  C2H4).  In  1805  William  Henry  clearly  pointed  out 
the  difference  between  these  two  gases. 

We  have  already  seen  that  marsh  gas  occurs  free  in  nature. 
It  is  evolved,  together  with  other  hydrocarbons,  in  large  quantities 
in  petroleum  springs.  The  holy  fire  at  Baku  on  the  Caspian 
Sea,  which  has  been  burning  from  the  earliest  historical  times, 
is  due  to  marsh  gas,  mixed,  according  to  Hess,  with  small 
quantities  of  nitrogen,  carbon  dioxide,  and  the  vapours  of  petro- 
leum. The  gas  which  is  evolved  from  the  mud  volcanoes  of 
Bulganak  in  the  Crimea  has  been  shown  by  Bunsen l  to  consist 
of  pure  marsh  gas.  The  gases  which  escape  in  large  quantities  from 
the  oil  springs  in  Butler  County,  Pennsylvania,  contain,  according 
to  Sadtler's  analyses,2  marsh  gas  and  its  homologues,  together  with 
hydrogen.  These  gases  are  collected  and  carried  by  pipes  to  the 
rolling  mills  at  Pittsburg,  a  distance  of  fifteen  miles,  where  the 
gas  is  employed  as  a  fuel. 

Enormous  quantities  of  marsh  gas,  or  fire-damp,  as  it  is 
termed  by  the  miners,  are  evolved  in  coal-pits,  due  in  all  proba- 
bility to  a  slow  decomposition  of  the  coal.  Reservoirs  of  this 
gas  in  a  highly  compressed  state  are  often  met  with  pent  up  in 
the  crevices  and  cavities  of  the  coal  measures.  Some  beds  of 
coal  are  so  saturated  with  gas  that  when  they  are  cut  it  may  be 
heard  oozing  from  every  pore  of  the  rock,  and  the  coal  is  called 
by  the  colliers  singing  coal ;  in  other  cases  the  gas  escapes  by 
what  are  termed  blowers,  and  the  mixture  of  gases  frequently 
collects  in  the  old  workings  or  un ventilated  portions  of  the  pit. 
.Not  unfrequently  fire-damp  bursts  forth  in  large  quantities 
from  the  seams  of  coal,  or  from  the  strata  of  clay  which  divide 
them.  This  is  the  frequent  cause  of  the  terrible  accidents  which 
sometimes,  in  spite  of  all  care,  will  occur.  The  Lundhill  colliery 
explosion  in  1857  was  one  of  the  most  calamitous  on  record. 
The  sudden  escape  of  gas  from  a  blower  in  a  neighbouring  col- 
liery is  thus  described  :  "  The  fire-clay  of  the  floor  of  the  seam 
was  seen  to  heave  at  different  points  along  the  face,  and  presently 
large  fractures  were  made  in  it,  through  which  gas  was  ejected 
with  great  violence,  and  with  a  sound  very  similar  to  the  rushing 
of  steam  at  a  high  pressure  from  a  boiler.  After  the  explosion 
at  Lundhill,  the  pent-up  gas  still  issued  within  the  mine  under 
such  pressure  as  to  support  a  column  of  water  thirty  feet 

1  Gasometry,  p.  147. 

2  Amer.  Philos.  Soc.  1876  ;  see  also  Laurence  Smith,  Ann.  Chim.  Phys.  [5], 
8,  566. 


696  THE  NON-METALLIC  ELEMENTS 

high."  The  outburst  of  gas  appears,  sometimes  at  least,  to 
be  connected  with  a  rapid  fall  in  the  barometer,  the  reduced 
atmospheric  pressure  enabling  the  gas  to  force  its  way  out.1 
If  he  should  escape  from  the  effect  of  the  explosion,  the  miner 
has  still  to  fear  its  result,  inasmuch  as  the  gas  in  exploding 
renders  ten  times  its  own  bulk  of  air  unfit  for  respiration,  and 
the  after- damp  or  vitiated  atmosphere  produced  by  the  explo- 
sion contains  carbon  dioxide  sufficient  to  render  it  irrespirable. 
Hence  the  difficulty  of  descending  into  the  pit  after  the  explo- 
sion without  proper  precautions,  or  until  a  sufficient  amount  of 
ventilation  has  been  re-established.  The  only  satisfactory  means 
of  guarding  against  these  sudden  outbreaks  in  fiery  pits  is  the 
establishment  of  a  thorough  and  perfect  system  of  ventilation, 
by  means  of  which  such  an  amount  of  air  is  brought  into  all 
parts  of  the  workings  as  to  render  the  formation  of  the  in- 
flammable mixture  difficult  or  impossible,  even  when  a  sudden 
outbreak  of  gas  occurs. 

The  escape  of  marsh  gas  at  the  surface  of  the  ground  in  the 
neighbourhood  of  the  coal  measures  is  frequently  observed.  This 
was  first  noticed  by  Thomson,  at  Bedley,  near  Glasgow,  where  a 
flame,  once  lighted,  burnt  for  many  weeks  in  succession.  A  similar 
case  has  been  noticed  by  Pauli  near  St.  Helen's.  The  following 
analyses  by  Graham  show  the  composition  of  fire-damp  : — 2 

Sp.  Gr.     Marsh  gas.    Nitrogen.  Oxygen. 

Five-quarter  seam,     j          fl        g  2 

Gateshead  colliery  ) 
Bensham  seam,  )  n-^97  n-fi 

Hebburn  colliery  f 
Killingworth  colliery  (V6306  82'5  16'5  I'O 

Another  instance  of  the  formation  of  methane  by  the  slow 
decomposition  of  vegetable  matter,  somewhat  similar  to  that 
taking  place  in  the  coal  seams,  occurs  in  ponds  or  marshes, 
whence  one  of  the  names  of  the  gas  is  derived.  The  gas- 
bubbles  which  rise  when  a  stagnant  pool  containing  decom- 
posing leaves  and  vegetable  matter  is  stirred,  consist  essen- 
tially of  marsh  gas,  which  is  mixed  with  carbon  dioxide  and 
nitrogen.  The  gas  collected  by  Bunsen,  in  July  1848,  from 
a  pond  in  the  botanical  gardens  of  Marburg,  contained,  after  the 
absorption  of  carbon  dioxide  by  caustic  potash,  the  following  : — 

Methane 48'5 

Nitrogen 51*5 

1  Scott  and  Galloway,  Proc.  Roy.  Soc.  20,  292.  2  Phil.  Mag.  28,  437. 


PREPARATION  OF  METHANE  697 

Methane  also  invariably  occurs  amongst  the  products  of  the 
dry  distillation  of  organic  bodies,  and  hence  it  is  present  in 
very  considerable  quantities  in  coal  gas. 

403  Preparation.  —  (1)  In  order  to  prepare  marsh  gas  an 
intimate  mixture  of  one  part  of  dried  acetate  of  soda  with  four 
parts  of  soda-lime  (a  mixture  of  caustic  soda  and  lime)  is  heated. 
This  is  best  accomplished  in  a  tube  of  hard  glass  closed  at  one 
end,  and  fitted  with  a  delivery-tube  at  the  other,  or,  in  place  of 
this,  an  iron  tube  or  a  copper  flask  may  be  employed.  In  order 
to  prepare  the  gas  as  pure  as  possible,  the  mixture  must  only 
be  heated  to  the  point  at  which  the  gas  begins  to  be  evolved, 
but  even  with  all  care  it  is  impossible  to  avoid  the  presence  of 
some  free  hydrogen  and  some  ethylene.  This  latter  impurity 
may,  however,  be  removed  by  passing  the  gas  through  a  U-tube 
containing  pumice-stone  soaked  in  strong  sulphuric  acid.  In  a 
sample  of  the  gas  thus  prepared  and  purified,  Kolbe  l  found  8 
per  cent,  of  hydrogen.  The  formation  of  methane  from  acetic 
acid  is  shown  by  the  following  equation  :  — 

C2H8O2Na  +  NaOH  =  Na2CO3  +  CH4. 

(2)  Chemically  pure  methane  is  obtained  from  zinc  methyl, 
Zn(CH3)2,  which  is  decomposed  by  water  as  follows  :  —  2 

Zn(CH3)2  +  2H20  =  Zn(OH)2  +  2CH4. 

(3)  The  pure  gas  is  most  readily  obtained  by  the  action  of 
a  zinc  copper  couple  on  a  mixture  of  equal  volumes  of  methyl 
iodide   and    alcohol,   the    reaction    being    assisted    by    gentle 
warming.3 

(4)  Marsh  gas  can  be    obtained  synthetically  by  passing  a 
mixture  of   sulphuretted  hydrogen  and  the  vapour  of  carbon 
bisulphide  over  red-hot  copper  :  —  4 

2SH2  +  CS2  +  8Cu  =  CH4  +  4Cu2S. 

(5)  The   same   gas   is   likewise    formed  when  a  mixture  of 
carbon  monoxide  and  hydrogen  is  exposed  to  the  action  of  the 
electric  induction  spark  :  —  5 


1  Ausfuhrl.  Lehrb.  Org.  Chemie,  i.  275. 

2  Frankland,  Phil.  Trans.  1852  [2],  417. 

3  Gladstone  and  Tribe,  Journ.  Chem.  Soc.  1884,  i.  154. 

4  Berthelot,  Compt.  Rend.  43,  454. 
6  Brodie,  Proc.  Roy.  Soc.  21,  245. 


THE  NON-METALLIC  ELEMENTS 


Properties. — Methane  is  a  colourless  odourless  gas,  which 
has  a  specific  gravity  of  0'559  ;  it  is  very  sparingly  soluble  in 
water,  its  solubility  from  6°  to  20°  being  obtained  from  the 
equation  : 

c  =  0-05449  -  0-001 1807*  +  0-000010278*2. 

It  dissolves  more  readily  in  alcohol,  the  solubility  between  2° 
and  24°  being  found  from  the  equation  : 

c  =  0-522586 -0-0028655^  + 0-0000142^. 

The  gas  was  first  liquefied  by  Cailletet 1  in  1877  ;  the  liquid 
boils  at  — 155°  to  —  156°under  the  ordinary  pressure,  and  at  —  73*5° 
under  a  pressure  of  56'8  atmospheres,2  and  has  at  — 164°  a 
specific  gravity  of  0'415.3  It  burns  with  a  slightly  luminous 
flame,  the  illuminating  power,  when  tested  under  the  condi- 
tions usually  observed  in  testing  coal-gas,  being  equivalent 
to  five  candles  as  compared  with  an  illuminating  power  of  15 — 
20  candles  given  by  ordinary  coal  gas.4  When  burnt  in  such  a 
manner  that  the  temperature  of  the  flame  is  very  high,  as  in  a 
regenerative  burner,  the  illuminating  power  is  much  greater. 
When  mixed  with  twice  its  volume  of  oxygen  it  explodes  in 
contact  with  a  flame,  the  detonation  being  even  more  violent  than 
in  the  case  of  a  mixture  of  hydrogen  and  oxygen.  Its  heat  of 
combustion  per  molecule  in  grams  at  18°  is  211,930  cal. 

Methane  is  an  extremely  stable  compound  and  when  exposed 
to  a  temperature  at  which  hard  glass  softens  is  only  de- 
composed to  a  very  slight  extent.  When  the  sparks  from  a 
strong  induction  coil  are  passed  through  the  gas  it  is  partially 
dissociated  into  carbon  and  hydrogen,  acetylene  being  also 
formed. 

ETHYL  HYDRIDE  OR  ETHANE,  C2H6  =  29-82. 

404  This  gas  is  invariably  present  in  the  gaseous  discharge 
accompanying  petroleum  in  the  oil  springs  of  Pennsylvania, 
and  is  dissolved  in  considerable  quantities  in  the  liquid  hydro- 
carbons.5 

Preparation. — (1 )  Ethane  is  readily  obtained  by  treating  ethyl 

1  Jahresb.  1877,  221. 

2  Wroblewski,  Jahresb.  1884,  197. 
8  Olszewski,  Jahresb.  1887,  72. 

4  Wright,  Journ.  Chem.  Soc.  1885,  i.  200. 
6  Ronalds,  Journ.  Chem.  Soc.  1851,  54. 


ETHANE  AND  ETHYLENE  699 

i 

iodide  with  zinc  and  water  under  pressure  at  a  temperature  of 
150°  ;x  thus:— 

Zn  +  C2H5I  +  H2O  =  ZnO  +  HI  +  C2H6. 

(2)  The  same  gas  is  produced  when  a  galvanic  current  is  passed 
through  a  concentrated  solution  of  potassium  acetate  : — 2 

2C2H302K  +  H20  =  C2H6 + K2CO3 + CO2 + H2. 

Carbon  dioxide  and  ethane  are  evolved  at  the  negative  pole, 
whilst  hydrogen  is  set  free  at  the  positive  pole. 

Properties. — Ethane  is  a  colourless  and  odourless  gas,  slightly 
soluble  in  water,  but  more  soluble  in  alcohol.  According  to 
Schickendantz,  its  coefficient  of  absorption  in  water  is  : 

c  =  0-094556  -  0-0035324*  +  0'00006278£.2 

It  has  a  specific  gravity  of  1*036,  and  condenses  to  a  liquid 
under  a  pressure  of  40  atmospheres  at  4°.  It  burns  with  a 
luminous  flame,  the  illuminating  power  being  about  half  that  of 
ethylene  burnt  under  similar  conditions.3  The  heat  of  combus- 
tion (for  1  mol.  in  grams)  is  370,440  cal. 


ETHYLENE,  ETHENE,  OR  OLEFIANT  GAS,  C2H4. 
Density  =27'82. 

405  This  gas  appears  to  have  been  discovered  by  Becher,  who 
obtained  it  by  heating  alcohol  with  sulphuric  acid.  His  obser- 
vations were,  however,  considered  to  be  erroneous  up  to  the  time 
of  Priestley,  who,  in  his  Experiments  and  Observations  on  Air, 
mentions  that  Ingenhouss  had  seen  such  a  gas  prepared  by  a 
certain  Enee  in  Amsterdam.  The  properties  of  olefiant  gas 
were  accurately  studied  in  the  year  1795  by  Deimann,  Paets 
van  Troostwyk,  Bondt,  and  Lauwerenburgh.4  These  Dutch 
chemists  found  that  the  gas  obtained  from  alcohol  and  sulphuric 
acid  was  totally  different  from  ordinary  inflammable  air,  and  that 
it  contained  both  hydrogen  and  carbon.  The  difference  between 
marsh  gas  and  olefiant  gas  was  first  pointed  out  by  William 
Henry5  in  1805,  and  his  view  of  the  composition  of  the  gas  was 
borne  out  by  the  subsequent  experiments  of  Dalton,  Davy,  and 
Berzelius.  In  those  days,  marsh  gas  and  olefiant  gas  were  the 

1  Frankland,  Journ.  Chem.  Soc.  1850,  263.          2  Kolbe,  Annalen,  69,  257. 
3  Frankland,  Journ.  Chem.  Soc.  1885,  i.  23;.      4  Crell.  Ann.  1795. 
5  Nicholson's  Journal,  1805. 


700 


THE  NON-METALLIC  ELEMENTS 


only  hydrocarbons  known.  s»  that  their  specific  gravities  being- 
very  different,  they  were  termed  the  light-  and  the  heavy- 
carburetted  hydrogen  respectively.  In  his  History  of 
Chemistry 1  Thomas  Thomson  states  that  it  was  by  the  inves- 
tigation of  the  chemical  composition  of  these  two  gases  that 
Dalton  was  led  to  the  recognition  of  the  laws  of  combination  in 
multiple  proportions,  and  to  the  conception  of  his  atomic  theory,, 
from  noticing  that  for  the  same  quantity  of  hydrogen  heavy 
carburetted  hydrogen  contains  twice  the  quantity  of  carbon 
that  is  contained  in  marsh  gas. 

Preparation. — In    order   to   prepare   olefiant   gas,  alcohol  i& 
heated  with  strong  sulphuric  acid.     The  method  proposed  by 


FIG.  188. 

Erlenmeyer  and  Bunte2  is  the  best.  Twenty-five  grams  of 
alcohol  and  150  grams  of  sulphuric  acid  are  brought  into  a  flask 
of  from  two  to  three  liters  in  capacity  (see  Fig.  188),  and  the 
mixture  heated  till  the  evolution  of  gas  begins.  By  means  of 
a  funnel-tube,  furnished  with  a  stopcock,  ft,  a  mixture  of  equal 
volumes  of  alcohol  and  sulphuric  acid  is  then  allowed  to 
drop  into  the  flask.  To  purify  the  gas  thus  obtained  it  must  be 
first  washed  through  concentrated  sulphuric  acid,  and  afterwards 
through  caustic  soda,  contained  in  the  Woulff  s  bottles,  c  and  d. 
It  may  then  be  collected  over  water  and  preserved  in  a  gas  holder. 
Properties. — Ethylene  is  a  colourless  gas,  possessing  a  peculiar 
1  Vol.  ii.  p.  291.  2  Annalen,  168,  64. 


ACETYLENE  701 


ethereal  smell ;  it  is  only  slightly  soluble  in  water,  but  is  more 
so  in  alcohol;  its  solubility  in  water  between  5°  and  21°  is 
•expressed  by  the  following  equation  (Pauli)  : — 

c  =  0-25629  -  0-00913631*  +  0'000188108/!2. 

Its  specific  gravity  is  0'9784.  It  condenses  under  a  pressure 
of  45  atmospheres  to  a  colourless  liquid,  which  solidifies  to  a 
crystalline  mass  at  —181°,  melts  again  at  —169°  and  boils  at 
-  105°.  The  liquid  has  a  specific  gravity  of  0'335  at  8°. 

Ethylene  is  readily  inflammable,  and  burns  with  a  brightly 
luminous  flame,  the  illuminating  power  being  about  70  candles.1 
The  heat  of  combustion  (for  1  mol.  in  grams  at  18°)  is  333,350 
cal.  When  mixed  with  three  times  its  volume  of  oxygen  and 
ignited  it  explodes  with  extreme  violence. 

One  very  characteristic  property  of  ethylene  is  its  power  of 
uniting  with  an  equal  volume  of  chlorine  to  form  a  heavy 
colourless  liquid  termed  ethylene  dichloride,  C2H4C12,  or,  Dutch 
liquid,  it  having  been  first  observed  by  the  four  Dutch  chemists 
already  named.  From  this  property,  indeed,  the  gas  derives  its 
name.  It  was  originally  termed  gaz  hitileux,  but  this  name 
was  afterwards  changed  by  Fourcroy  to  gaz  oUfiant.  When 
brought  in  contact  with  strongly  ozonised  oxygen,  ethylene  de- 
tonates very  powerfully.  In  order  safely  to  exhibit  this  property, 
a  current  of  the  gas  is  allowed  to  pass  through  a  wide  tube 
about  10  mm.  in  diameter,  whilst  ozonised  oxygen  is  allowed  to 
enter  into  this  by  means  of  a  narrow  tube,  which  is  inserted  into 
the  wider  tube  to  a  distance  of  one  centimeter.  As  soon  as  the  ozo- 
nised oxygen  comes  in  contact  with  the  olefiant  gas  a  detonation 
occurs,  usually  accompanied  with  the  formation  of  white  fumes.2 

ACETYLENE  OR  ETHINE,  C2H2  =  25-82 

406  This  gas  was  discovered  and  its  composition  determined 
in  1836,  by  Edmund  Davy,3  who  prepared  it  by  treating  with 
water  the  black  mass  obtained  in  the  manufacture  of  potassium. 
The  existence  of  this  gas  was  afterwards  observed  by  some  other 
chemists,  but  it  was  not  until  the  year  1859  that  Berthelot  4 
investigated  it  completely.  Acetylene  is  produced  by  the 
incomplete  combustion  of  many  volatile  organic  substances, 
especially  of  ethylene,  coal-gas,  and  other  hydrocarbons,  as  well 

1  Frankland,  Journ.  Chem.  Soe.  1885,  i.  237. 

2  Houzeau  and  Renard,  Compt.  Rend.  76,  572. 

3  Reports  of  British  Association,  1836,  p.  62. 

4  Ann.  Chim.  Phys.  [3],  57,  82. 


702  THE  NON-METALLIC  ELEMENTS 

as  the  vapours  of  alcohol,  ether,  &c.  Acetylene  is  also  formed 
when  the  vapours  of  these  organic  liquids  are  passed  through 
red-hot  tubes. 

Acetylene  is  remarkable  as  being  the  only  hydrocarbon 
which  has  been  obtained  by  the  direct  union  of  its  elements. 
These  only  combine  together  at  the  highest  temperature  which 
can  be  artificially  produced.  In  order  to  prepare  acetylene  in 
this  way  the  electric  arc,  obtained  by  the  passage  of  a  powerful 
current  between  two  poles  of  gas  carbon  is  employed.  These 
carbon  poles  are  fitted  through  apertures  in  a  globular  glass 
vessel,  through  which  a  slow  current  of  pure  hydrogen  is  allowed 
to  pass. 

Acetylene  can  also  be  prepared  in  any  wished -for  quantity 
from  ethylene  dibromide.  If  this  substance  be  heated  with  an 
alcoholic  solution  of  caustic  potash,  the  following  reactions  take 
place : — 

(1)  C2H4Br9  +  KOH  =  C2H3Br  +  KBr  +  H20. 

(2)  C2H3Br+KOH  =  C2H2+KBr  +  H20. 

To  remove  any  vapours  of  the  very  volatile  bromethylene 
which  may  be  carried  over  from  this  operation,  the  gases  evolved 
from  the  boiling  liquid  are  allowed  to  pass  through  a  second 
flask  containing  a  boiling  alcoholic  solution  of  caustic  potash. 

Another  method  of  preparation  consists  in  allowing  water  to 
drop  on  to  barium  carbide,  which  is  prepared  by  heating  to 
redness  a  mixture  of  precipitated  barium  carbonate,  magnesium 
powder  and  gas  carbon.  The  gas  thus  obtained  only  contains 
2 — 3  per  cent,  of  hydrogen  and  no  appreciable  quantities  of  other 
hydrocarbons.1  Calcium  carbide  (p.  703)  is  also  decomposed 
by  water  with  formation  of  pure  acetylene.2 

Properties. — Acetylene  is  a  colourless  difficultly  condensible 
gas,  having  a  specific  gravity  of  O92,  with  a  very  unpleasant 
penetrating  smell.  It  is  usually  stated  that  the  smell  observed 
when  a  Bunsen  burner  "  burns  down  "  is  due  to  the  presence 
of  acetylene,  but  according  to  V.  Meyer  the  odour  of  pure 
acetylene  is  quite  distinct  from  this. 

At  18°  it  dissolves  in  its  own  volume  of  water,  whilst  alcohol 
dissolves  six  times  its  volume  of  the  gas.  Acetylene  burns  with 
a  strongly  luminous  and  smoky  flame.  It  acts  as  a  poison  when 
it  comes  in  contact  with  the  blood,  combining  with  the  haemo- 
globin.3 Like  ethylene  it  combines  directly  with  chlorine  and 

1  Maqueime,  Compt.  Rend.  115,  558.        -  Moissan,  Compt.  Rend.  118,  501. 
3  Bistrow  and  Liebreieh,  Bcr.  1,  220. 


ACETYLENE  703 


bromine  forming  with  the  latter  element  acetylene  dibromide, 
C2H2Br2,  as  well  as  the  tetrabromide,  C2H2Br4. 

Very  characteristic  derivatives  of  acetylene  are  the  explosive 
compounds  which  it  forms  with  certain  metals  or  metallic 
salts.  Thus,  if  the  gas  is  allowed  to  pass  over  fused  potassium, 
hydrogen  is  evolved,  and  bodies  having  the  composition  C2HK 
and  C2K2  are  formed.  Both  of  these  substances  are  black 
powders,  which  are  decomposed  in  contact  with  water  with 
explosive  violence,  acetylene  being  reproduced.  A  similar 
calcium  compound,  C2Ca,  is  obtained  by  heating  a  mixture  of 
lime  and  sugar  charcoal  in  the  electric  furnace  : — 1 

CaO  +  3C  =  C2Ca  +  CO. 

This  substance,  when  treated  with  water,  yields  lime  and  acety- 
lene. When  the  gas  is  passed  through  an  ammoniacal  solution  of 
a  cuprous  salt,  a  dark  blood-red  precipitate  is  formed,  having 
the  composition  Cu2C2  or  Cu2C2,H.,0.2  This  reaction  is  a  very 
characteristic  one.  It  serves  as  a  test  for  the  presence  of 
acetylene,  and  is  so  delicate,  that  by  its  means  ^^  part  of  a 
milligram  of  acetylene  can  be  with  certainty  detected  (Berthelot). 

For  the  purpose  of  exhibiting  its  action  with  cuprous  chloride 
the  flame  of  a  large  Bunsen  burner  is  allowed  to  burn  down,  the 
air-openings  at  the  bottom  are  then  nearly  closed,  and  when  the 
maximum  amount  of  gas  is  burning,  a  large  dry  balloon  is  brought 
over  the  tube  of  the  burner,  so  that  the  incompletely  burnt  gas 
is  brought  into  the  middle  of  the  vessel.  After  some  minutes 
the  balloon  is  removed  and  a  few  drops  of  ammoniacal  solution 
of  cuprous  chloride,  Cu2Cl2,  are  poured  in,  the  balloon  being 
shaken  so  as  to  bring  a  thin  film  of  the  liquid  over  the  whole 
of  the  interior  surface.  This  is  then  seen  to  be  covered  with  a 
coating  of  the  red  acetylene-copper  compound. 

If  a  current  of  acetylene  be  led  into  an  ammoniacal  solution 
of  a  silver  salt,  C2Ag2  or  C2Ag2.H00  separates  out  in  the  form  of 
a  white  precipitate.  This  body,  like  the  copper  compound, 
explodes  on  heating  or  by  percussion.  On  the  addition  of 
acids  to  these  compounds,  acetylene  is  evolved.  Hence  they 
may  be  employed  for  the  separation  of  the  gas  from  a  gaseous 
mixture,  as  well  as  for  the  purpose  of  obtaining  it  in  a  chemically 
pure  state  ;  thus  : — 

C2H2Cu2O  +  2HC1  =  C2H2  +  Cu2Cl2  +  H,0. 

1  Moissan,  Compt.  Rend.  118,  501. 

2  Blochmann,  Ber.  7,  274  ;  Keiser,  Amer.   Chem.  Journ.  14,  285  ;  Plimpton, 
Proc.  Chem.  Soc.  1892,  109. 


704  THE  NON-METALLIC  ELEMENTS 

To  determine  the  quantity  of  acetylene  in  coal-gas,  or  other 
mixture  of  gases,  these  are  passed  through  ammoniacal  silver 
nitrate,  which  gives  a  precipitate  of  silver  acetylide  mixed  with 
metallic  silver.  The  former  is  converted  by  hydrochloric  acid 
into  silver  chloride,  which  may  be  separated  by  treatment  with 
ammonia,  the  filtrate  precipitated  with  nitric  acid,  and  the 
silver  chloride  weighed ;  1  gram,  of  silver  chloride  corresponds 
to  0'09  gram,  or  87  03  cc.  of  acetylene.1 

A  simple  method  for  preparing  acetylene  is  as  follows : — A 
bent  funnel  (see  Fig.  189)  is  placed  over  a  burner,  in  which  the 


FIG.  189. 

flame  is  burning  down.  Connected  with  the  neck  of  the  funnel 
are  two  cylinders,  a  and  b,  containing  an  ammoniacal  solution 
of  nitrate  of  silver,  and  the  products  of  the  combustion  are 
drawn  through  the  liquids  contained  in  the  cylinders  by  means 
of  an  aspirator,  c.  By  the  addition  of  hydrochloric  acid  to  the 
precipitate,  pure  acetylene  can  be  obtained,  and  the  chloride 
of  silver  which  is  precipitated  only  requires  to  be  again  dis- 
solved in  ammonia  in  order  to  fit  it  to  be  employed  a  second 
time  for  the  same  purpose. 

If  the  compound  C2Cu2  be  allowed  to  remain  in  contact  with 
zinc  and  aqueous    ammonia,    the    nascent    hydrogen    evolved 

1  Lewes,  Journ.  Chem.  Soc.  1892,  i.  324. 


CARBON  MONOXIDE  705 


by  the  action  of  the  zinc  combines  with  the  acetylene  and  forms 
ethylene.  Platinum-black  when  brought  in  contact  with  a 
mixture  of  acetylene  and  hydrogen  also  brings  about  the  same 
combination,  both  ethylene  and  ethane  being  formed.1 

Acetylene  is  formed  from  its  elements  with  absorption  of  heat, 
and,  like  many  other  similar  endothermic  compounds,  can  be 
made  to  undergo  explosive  decomposition  when  subjected  to 
the  shock  given  by  the  explosion  of  fulminate  of  mercury. 
(Of.  p.  734.) 


CARBON  AND  THE    HALOGENS. 

407  Carbon  combines  with  fluorine  directly  to  form  carbon 
tetrafiuoride,  CF4,  the  reaction  taking  place  spontaneously  at 
the  ordinary  temperature  with  the  less  dense  forms,  whilst  the 
denser  varieties  must  be  heated  to  50 — 100°.  The  remaining 
halogens,  however,  do  not  combine  directly  with  carbon,  but 
compounds  of  carbon  with  these  elements  can  be  prepared  by 
indirect  methods.  Thus  the  ultimate  product  of  the  action  of 
chlorine  on  methane  is  carbon  tetrachloride,  CC14,  the  four 
atoms  of  hydrogen  being  replaced  by  chlorine,  which  also  com- 
bines with  the  hydrogen  set  free,  forming  hydrochloric  acid. 

CH4  +  4C12  =  CC14  +  4HC1. 

These  compounds  are  described  in  the  volumes  relating  to 
Organic  Chemistry. 


CARBON    AND    OXYGEN. 

408  Carbon  forms  two  oxides,  both  of  which  are  gaseous  : — 
Carbon  monoxide,  CO. 
Carbon  dioxide,  CO2. 

CARBON  MONOXIDE  OR  CARBONIC  OXIDE,  CO  =  2779 

This  compound,  which  is  commonly  known  as  carbonic 
oxide,  was  first  obtained  by  Lassone  by  heating  zinc  oxide 
with  charcoal.2  He  found  that  a  combustible  gas  was  thus  given 
off  which  burned  with  a  blue  flame,  and  when  mixed  with  air 
did  not  explode,  as  was  usually  the  case  with  inflammable  air. 
Lavoisier  (1777)  obtained  the  same  gas  by  heating  alum  and 

1  V.  Wilde,  Ber.  7,  353.  2  Mem.  Paris  Acad.  1776. 

46 


706  THE  NON-METALLIC  ELEMENTS 

charcoal  together,  and  found  that  on  combustion  it  yielded 
carbonic  acid.  In  spite  of  these  observations,  the  gas  was  for 
a  long  time  mistaken  for  hydrogen,  and  Priestley  in  1796 
showed  that  iron  scale  (oxide  of  iron),  when  heated  with  well- 
calcined  charcoal,  gives  out  an  inflammable  air,  whereas  according 
to  Lavoisier's  theory  it  ought  only  to  give  carbonic  acid.  This 
fact  was,  in  Priestley's  opinion,  opposed  to  the  antiphlogistic 
system,  whilst  it  supported  the  view  that  the  oxides  contain 
water,  and  that  inflammable  air  is  phlogisticated  water,  and  this 
was  corroborated  by  the  fact  that  when  steam  is  led  over  red- 
hot  charcoal  it  is  phlogisticated  to  inflammable  air.  This  con- 
clusion was  one  which  upholders  of  the  Lavoisierian  system 
found  difficult  to  disprove,  and  they  were  driven  to  assume  that 
hydrogen  was  still  contained  even  in  the  most  strongly  heated 
charcoal  ;  this  fact,  however,  Priestley  most  satisfactorily  proved 
to  be  incorrect  in  his  last  work,  The  Doctrine  of  Phlogiston 
Established,  published  in  1800. 

In  the  same  year  Cruikshank  was  engaged  with  the  exami- 
nation of  this  same  gas,  which  he  obtained  by  heating  carbon 
with  different  metallic  oxides.  From  its  comparatively  high 
specific  gravity  he  concluded  that  this  gas  was  not  a  hydro- 
carbon, as  others  had  assumed  it  to  be.  When  burnt  with 
oxygen  it  yielded  no  water,  and  nothing  but  an  almost  equal 
volume  of  carbon  dioxide,  whilst  the  oxygen  which  was  needed 
for  its  combustion  was  less  in  volume  than  that  contained  in 
the  carbonic  acid  gas  formed.  Hence  he  concluded  that  it 
must  be  an  oxygen  compound,  and  therefore  gave  to  it  the 
name  of  "  gaseous  oxyde  of  carbone."  Clement  and  De*sormes 
soon  after  confirmed  Cruikshank's  results  ;  they  determined  the 
composition  of  carbonic  oxide  more  accurately  than  he  had  done, 
and  found  that  it  is  likewise  formed  \vhen  carbon  dioxide  is  led 
over  red-hot  charcoal. 

409  Preparation.  —  Carbon  monoxide  can  be  prepared  in 
various  ways.  (1)  It  is  formed  when  zinc  oxide,  ferric  oxide 
manganese  dioxide,  and  many  other  oxides  are  heated  with 
charcoal  ;  it  is  also  formed  when  chalk  (calcium  carbonate) 
magnesite  (magnesium  carbonate)  and  other  carbonates  are 
heated  with  metallic  zinc  or  iron  filings;  the  decomposition 
which  takes  place  in  these  cases  is  represented  by  the  follow- 
ing equations  :— 


CaCO.  +  Zn  =  CaO  +  ZnO  +  CO. 


CARBON   MONOXIDE  707 

(2)  When  carbon  dioxide  (carbonic  acid  gas)  is  passed  through 
a  long  tube  filled  with  charcoal  and  heated  to  redness  :  — 

C  +  CO2  =  2CO. 

(3)  When   oxalic  acid   or  an  oxalate  is  heated  with  concen- 
trated  sulphuric  acid,  a  mixture  of  equal  volumes  of  carbon 
monoxide  and  carbon  dioxide  is  evolved  ;  thus  :  — 


The  carbon  dioxide  may  readily  be  separated  from  the  carbon 
monoxide  either  by  passing  the  mixed  gases  through  a  solution 
of  caustic  soda  or  by  collecting  the  mixture  over  water  rendered 
alkaline  by  this  substance. 

(4)  Pure  carbonic  oxide   is   formed   when   formic  acid  or   a 
formate  is  heated  with  concentrated  sulphuric  acid  :  — 

CH202  =  CO  +  H,0. 

On  the  other  hand,  carbonic  oxide  can  be  converted  into  formic 
acid  by  heating  it  with  caustic  potash,  potassium  formate  being 
produced  (Berthelot)  :  — 


(5)  Carbon  monoxide  can  be  readily  prepared  in  quantity  by 
heating  finely  powdered  potassium  ferrocyanide  with  from  8  to 
10  times  its  weight  of  strong  sulphuric  acid  ;  l   in  this  reaction, 
potassium  sulphate,  ammonium  sulphate,  and  iron  sulphate  are 
formed,    the   following    equation  representing  the   decomposi- 
tion :  — 

K4FeC6N6  +  6H.,S04  +  6H2O  =  6CO  +  2K2SO4  +  3(NH4)2S04 

+  FeSO4. 

The  water  required  for  the  reaction  is  contained  in  the 
ferrocyanide  itself,  which  crystallizes  with  three  molecules  of 
water,  and  also  in  the  commercial  acid,  which  invariably  con- 
tains some.  According  to  Grimm  and  Ramdohr  2  it  appears 
that  sulphur  dioxide  and  carbon  dioxide  are  evolved  in  the 
beginning  of  the  reaction,  but  afterwards  pure  carbon  monoxide 
is  given  off. 

(6)  Carbonic  oxide  may  be  readily  prepared  by  passing  carbon 
1  Fownes,  Phil.  Mag.  24,  21.  2  Annalen,  98,  127. 


708  THE  NON-METALLIC  ELEMENTS 

dioxide  over  zinc  dust  heated  in  a  glass  tube  to  rather  less  than 
a  red  heat,  the  resulting  gas  being  washed  through  caustic  soda.1 

This  reaction  is  sometimes  made  use  of  for  the  preparation  of 
the  gas  on  the  large  scale. 

Properties. — Carbon  monoxide  is  a  colourless,  tasteless  gas, 
which  possesses  a  peculiar  though  slight  smell,  and  has  a 
specific  gravity  of  0'9678  (Craikshank),  0'96702  (Leduc).2  The 
relative  density  remains  constant  up  to  1200°,  but  at  1690°  the 
gas  partially  decomposes  with  formation  of  carbon  dioxide,  free 
carbon  being  deposited.3  It  has  been  liquefied  by  Cailletet, 
Wroblewski,  and  Olszewski ;  its  critical  temperature  is  — 139°  5, 
the  corresponding  pressure  being  35*5  atmospheres,  it  boils  at 
-190°  and  solidifies  at  -211°  (Cailletet).4  In  water  it  is  only 
very  slightly  soluble,  its  absorption  coefficient,  according  to 
Bunsen  and  Pauli,  being  obtained  from  the  following  equation — 

c  =  0-032874  -  0-00081632*  +  0-000016421*2. 

It  is,  however,  easily  soluble  in  an  acid  or  ammoniacal  solu- 
tion of  cuprous  chloride,  Cu2Cl9. 

410  Carbon  monoxide  easily  burns  in  the  air  with  a  bright 
blue  flame,  carbon  dioxide  being  produced.  The  lambent  flame 
observed  on  the  top  of  a  large  coal  fire  is  due  to  the  combustion 
of  this  gas.  When  it  is  mixed  with  oxygen  and  ignited  by  the 
application  of  a  flame  or  by  an  electric  spark,  a  violent  explosion 
occurs.  If  however  the  mixture  of  carbonic  oxide  and  oxygen 
be  thoroughly  freed  from  moisture  by  long-continued  preserv- 
ation over  phosphorus  pentoxide,  it  is  found  to  be  impossible  to 
produce  explosion  by  passing  an  electric  spark  through  the  gap. 
The  addition  of  a  trace  of  water-vapour  or  of  gases,  which, 
by  reaction  with  oxygen,  are  capable  of  producing  water,  such 
as  sulphuretted  hydrogen,  hydrogen,  hydrocarbons,  etc., 
immediately  renders  the  mixture  explosive.  The  combination 
of  carbon  monoxide  with  oxygen  therefore  does  not  appear  to 
take  place  directly,  and  it  is  probable  that  the  oxidation  of 
the  gas  is  effected  by  means  of  the  water-vapour  present,  which 
acts  as  a  carrier  of  oxygen,  the  following  reactions  taking  place : 

=  C02+H2. 
=  2H20. 

1  Noack,  Bcr.  16,  75.  2  Compt.  Rend.  115,  1072. 

3  Meyer  and  Langer,  Ber.  18,  134  c. 

4  Compt.  Rend.  99,  706  ;  100,  350  ;  Monatsh.  Ohem.  6,  204. 


CARBON   MONOXIDE  709 

This  view  of  the  chemical  changes  which  occur  when  carbon 
monoxide  burns  is  supported  by  the  fact  that  the  rapidity  of 
explosion  of  a  mixture  of  oxygen  with  carbon  monoxide  in  a 
tube  one  meter  long  is  greater  with  a  large  quantity  of  aqueous 
vapour  than  when  only  a  trace  is  present.1  This  property  of 
carbon  monoxide  may  be  readily  demonstrated  by  plunging  a 
flame  of  this  gas  burning  in  air  into  a  jar  of  oxygen  in  which 
some  strong  sulphuric  acid  has  been  shaken  up  ;  the  flame  is 
at  once  extinguished. 

When  carbon  is  burned  in  a  free  supply  of  ordinary  air  or 
oxygen,  carbon  dioxide  is  formed.  If,  however,  every  trace  of 
moisture  is  as  far  as  possible  removed  both  from  the  carbon  and 
from  the  oxygen,  it  is  found  that  the  carbon  may  be  heated  nearly 
to  redness  without  taking  fire,  the  oxidation  taking  place  without 
the  evolution  of  light,  and  that  carbon  monoxide  is  almost  the 
sole  product.  In  order  to  obtain  oxygen  sufficiently  dry  for 
experiments  of  this  kind,  it  is  necessary  to  pass  the  gas  through 
sulphuric  acid  and  then  allow  it  to  remain  for  some  weeks 
in  carefully  closed  flasks,  containing  a  considerable  amount  of 
phosphorus  pentoxide.2 

Carbon  monoxide  is  also  largely  formed  by  the  oxidation  of 
carbon  when  heated  in  air  a,t  a  comparatively  low  temperature. 
These  facts  render  it  probable  that  in  the  combustion  of  carbon 
carbon  monoxide  is  the  first  product,  and  is  then  oxidised  to 
carbon  dioxide  by  the  oxygen  of  the  water-vapour  present. 

Carbon  monoxide  combines  directly  with  metallic  nickel  and 
iron  to  form  very  remarkable  substances  of  the  composition 
Ni(CO)4  and  Fe(CO)5.3  These  substances  readily  decompose 
when  heated  into  the  metal  and  carbon  monoxide.  They  will 
be  described  under  the  compounds  of  the  metals. 

Carbonic  oxide  is  a  very  poisonous  gas,  inasmuch  as  it  com- 
bines with  the  haemoglobin  of  the  blood  :  small  animals  die 
almost  instantly  when  placed  in  the  gas,  and  when  even  small 
quantities  are  inhaled  severe  headache,  giddiness,  and  insensi- 
bility readily  occur.  The  accidents,  as  well  as  suicides,  which 
occur  from  burning  charcoal  in  a  chauffer  in  a  small  room,  are 
due  to  the  inhalation  of  this  gas  formed  by  incomplete  com- 
bustion, and  deaths  occurring  from  sleeping  upon  lime-kilns  and 

1  Dixon,  Phil.  Trans.  1884,  ii.  617. 

a  Baker,  Journ.  Chem.  Soc.  1885,  i.  349  ;  Phil.  Trans.  1888,  A.  571. 
3  Journ.  Chem.  Soc.  1890,  i.  749  ;  1891,  1090  ;  Compt.  Rend.  H3,  679  ;  Proc. 
Chem.  Soc.  1891,  126. 


710  THE  NON-METALLIC  ELEMENTS 


brick-kilns  are  probably  also  produced  by  this  gas.  The 
poisonous  nature  of  the  gas  from  red-hot  charcoal  was  known 
and  expatiated  upon  as  early  as  the  year  1716,  when  F. 
Hoffman  published  his  work,  Considerations  on  the  Fatal  Effects 
of  the  Vapour  from  Burning  Charcoal. 

The  remarkable  poisonous  action  of  carbonic  oxide  appears  to 
depend  upon  the  fact  that  the  whole  dissolved  oxygen  is  thereby 
expelled,  the  blood  acquiring  a  light,  purple-red  colour.  The 
absorption  spectrum  of  the  carbonic-oxide-haBmoglobin  is  dis- 
tinguished by  the  fact  that  the  bands  between  D  and  b  are 
situated  nearer  to  b  than  is  the  case  with  oxyhsemoglobin  (Hoppe- 
Seyler),  and  that,  unlike  the  latter,  it  remains  unchanged  in 
presence  of  reducing  agents.  This  test  serves  as  a  means  of 
detecting  cases  of  poisoning  with  the  gas. 

Carbonic  oxide  may  also  be  detected  in  small  quantities  in 
the  air  by  passing  the  air  to  be  examined  through  blood  and 
then  making  a  spectroscopic  examination  of  the  latter.  The 
gas  is  either  estimated  by  explosion  with  oxygen  or  more 
generally  by  absorption  with  acid  cuprous  chloride. 

The  composition  of  carbon  monoxide  is  determined  by  mixing 
the  gas  with  oxygen,  exploding  the  mixture,  and  measuring  the 
carbon  dioxide  formed.  It  is  thus  ascertained  that  one  volume 
of  the  gas  unites  with  half  a  volume  of  oxygen  to  form  one 
volume  of  carbon  dioxide.  Now  the  latter  is  known  to  contain 
its  own  volume  of  oxygen  (p.  714),  and  it  hence  follows  that 
carbon  monoxide  contains  half  its  own  volume  of  oxygen.  The 
molecular  weight  of  the  gas  is  shown  by  its  density  to  be  about 
27'79,  and  this  amount  of  it  therefore  contains  15'88  of  oxygen 
and  11*91  of  carbon,  the  formula  of  the  gas  being  CO. 

* 

CARBON  DIOXIDE  on  CARBONIC  ANHYDRIDE,  C02  =  43'67. 

411  This  gas  belongs  to  the  class  of  acid-forming  oxides  for- 
merly termed  acids,  and  is  still  best  known  under  its  old  name 
•of  carbonic  acid.  It  has  already  been  stated  in  the  historical 
introduction  that  this  gas  was  first  distinguished  from  common 
air  by  Van  Helmont,  who  termed  it  gas  sylvestre.  He  obtained 
it  by  the  action  of  acids  on  alkaline  or  calcareous  substances,  and 
showed  that  it  is  also  formed  by  the  combustion  of  charcoal, 
and  in  the  fermentation  and  decay  of  carbonaceous  matter,  and 
that  it  likewise  occurs  in  the  mineral  water  at  Spa,  in  the 
•Grotto  del  Cane  near  Naples,  and  in  other  localities.  Van 
Helmout  describes  the  suffocating  action  it  exerts  on  animal 


CARBON    DIOXIDE  711 


life  as  well  as  its  effects  in  extinguishing  flame.  Fr.  Hoffmann 
made  further  observations  on  the  gas  contained  in  effervescing 
mineral  waters,  and  he  states  that  this  is  frequently  given 
•off  in  such  quantity  that  when  the  water  is  enclosed  in 
bottles  these  are  burst  by  the  force  of  the  gas.  He  also  shows 
that  this  substance,  to  which  he  gave  the  name  of  spiritus 
mineralis,  has  the  power  of  reddening  certain  blue  vegetable 
colouring  matters,  and  hence  he  considers  it  to  be  a  weak  acid. 
Although  many  other  chemists  investigated  the  properties  of 
this  gas,  it  was  not  until  the  time  of  Black  that  it  was  dis- 
tinctly shown  to  differ  essentially  from  common  air.  Black 
(1755)  proved  that  this  substance  is  a  peculiar  constituent  of 
the  carbonated  or  mild  alkalis,  being,  in  them,  combined  or 
fixed  in  the  solid  state,  whence  it  was  termed  by  him  fixed  air. 
In  the  year  1774  Bergman  published  a  complete  history  of  this 
peculiar  air,  to  which  he  gave  the  name  acid  of  air,  because 
of  its  occurrence  in  the  atmosphere.  Its  chemical  nature  was 
first  properly  explained  by  Lavoisier,  who  showed  that,  whilst 
mercuric  oxide  heated  alone  gives  off  pure  oxygen  gas,  fixed  air 
is  evolved  when  it  is  heated  with  carbon,  proving  that  this 
latter  gas  is  an  oxide  of  carbon. 

Carbon  dioxide  is  a  body  which  is  widely  distributed  in  nature ; 
as  we  have  already  seen,  it  forms  a  small  but  constant  and 
essential  constituent  of  the  atmosphere ;  it  is  likewise  invariably 
contained  in  soil,  being  one  of  the  chief  products  of  the  decay 
of  all  organic  substances.  From  the  soil  it  is  taken  up  by  rain 
and  spring  water,  and  it  is  to  this  substance  that  the  latter,  to  a 
great  extent,  owes  its  fresh  and  pleasant  taste.  It  occurs  in 
chalybeate  and  acidulous  waters  in  large  quantities,  whilst  in 
both  ancient  and  modern  volcanic  districts  it  is  emitted  in  very 
large  volumes  from  the  fumeroles  and  rents  in  the  ground,  for 
instance  in  the  old  craters  in  the  Eifel,  at  Brohl  on  the  Rhine, 
as  well  as  in  the  Auvergne,  particularly  in  the  neighbourhood  of 
Vichy  and  Hauterive,  where  the  gas  is  actually  employed  for 
the  manufacture  of  white  lead.  Especially  remarkable  for  the 
evolution  of  this  gas  in  very  large  quantities  is  the  Poison 
Valley  in  Java,  which  also  is  an  old  crater,  and  the  Grotto  del 
Cane  near  Naples,  which  is  of  such  a  construction  that  the 
heavy  carbonic  acid  gas,  entering  from  the  fissures  in  the  floor 
of  the  cave,  at  a  depth  of  from  two  to  three  feet  below  the 
mouth  of  the  cave,  collects  up  to  this  depth,  and  small  animals 
such  as  dogs,  when  thrown  into  the  cave,  respiring  the  impure 


712  THE  NON-METALLIC  ELEMENTS 

air,  fall  down,  whilst  a  man  breathing  the  pure  air  above  this 
level  is  unaffected  by  the  gas. 

The  carbonic  acid  contained  in  the  air  is  derived  from  a 
variety  of  sources ;  it  is  formed  by  the  respiration  of  man  and 
animals,  as  well  as  in  the  act  of  combustion  of  organised 
material,  and  in  its  decay  and  decomposition.  The  amount  of 
atmospheric  carbonic  acid  varies  between  certain  narrow  limits, 
but  on  an  average  reaches  3  volumes  in  10,000  volumes  of  air. 
In  the  presence  of  the  sunlight,  plants  have  the  power,  through 
their  leaves,  of  decomposing  this  carbonic  acid,  taking  up  the 
carbon  to  form  their  own  tissue,  and  eliminating  the  oxygen 
gas ;  hence  the  amount  of  carbonic  acid  in  the  air  does  not 
increase  beyond  the  limits  named  (Saussure). 

Carbon  dioxide  is  an  acid-forming  oxide  giving  rise  to  a 
series  of  salts  termed  the  carbonates,  many  of  which  occur  in 
nature  as  minerals.  Amongst  these  is  especially  to  be  men- 
tioned calcium  carbonate,  CaCO3,  which  occurs  in  two  distinct 
crystalline  forms  as  calc-spar  and  arragonite,  whilst  it  is  found 
in  a  massive  crystalline  form  in  marble  and  limestone ;  calcium 
carbonate  also  forms  the  chief  constituent  of  the  shells  of 
mollusca  and  foraminifera,  the  remains  of  which  constitute  the 
chalk  formation  as  well  as  the  greater  part  of  all  the  lime- 
stones. The  double  carbonate  of  magnesium  and  calcium 
(MgCa)CO3  also  occurs  in  large  masses  as  dolomite.  Amongst 
other  naturally  occurring  carbonates  may  be  mentioned  mag- 
nesite,  MgCO3 ;  iron  spar  or  spathic  iron  ore  (FeMnCaMg) 
CO3 ;  witherite,  BaC03 ;  strontianite,  SrCO3 ;  and  calamine, 
ZnCO3. 

412  Preparation. — (1)  In  order  to  prepare  carbon  dioxide  a 
carbonate,  such  as  marble  or  chalk,  is  brought  into  a  gas- 
evolution  flask,  and  dilute  hydrochloric  acid  poured  upon  it, 
when  the  gas  is  rapidly  evolved  with  effervescence : — 

CaC03  +  2H01  =  C02  +  CaCl2  +  H20. 

The  gas  thus  obtained  invariably  carries  over  small  quantities 
of  hydrochloric  acid  vapour  with  it,  from  which  it  may  be  freed 
by  passing  through  a  solution  of  bicarbonate  of  soda.  In 
order  to  obtain  a  constant  stream  of  carbon  dioxide  the  same 
apparatus  may  be  employed  which  was  made  use  of  for  the 
preparation  of  sulphuretted  hydrogen  gas  (Fig.  100). 

(2)  Another  method  of   obtaining  a  constant  current  is  to 


PROPERTIES   OF   CARBON  DIOXIDE  713 

pour  concentrated  sulphuric  acid  over  chalk,  and   add  a  very 
small  quantity  of  water: — 

CaC03  +  H2S04  =  C02  +  CaS04  +  H2O. 

The  dilute  acid  cannot  be  employed  for  this  purpose,  since 
the  calcium  sulphate  formed,  which  is  soluble  in  the  con- 
centrated acid,  does  not  dissolve  in  the  dilute  acid,  and  thus 
prevents  its  further  action  on  the  chalk. 

(3)  The  gas  thus  obtained   from   chalk  possesses  a  peculiar 
smell,  which    is   due   to   the   presence   of  small    quantities  of 
volatile  organic  matter  always  contained    in    the    chalk.      In 
order  to  prepare  a   very  pure  gas,   sodium  carbonate  may  be 
decomposed  with  pure  dilute  sulphuric  acid  ;  thus : — 

Na2CO3  +  H2SO4  =  CO2  +  Na,SO4  +  H2O. 

(4)  Carbon  dioxide  may  be  obtained  on  a  large  scale  for  the 
preparation  of  bicarbonate  of  soda,  white  lead,  and  other  com- 
mercial   products    by    "  burning "   limestone,   or    even   by   the 
combustion  of  charcoal  or  coke. 

413  Properties. — Carbon  dioxide  is  a  colourless  gas  possessing 
a  slightly  pungent  smell  and  acid  taste.  It  does  not  support 
either  combustion  or  respiration;  hence  a  flame  is  extinguished 
when  plunged  into  the  gas,  and  animals  thrown  in  become 
insensible  and  suffer  death  from  suffocation.  At  the  same  time, 
however,  it  does  not  exert  a  poisonous  action  on  the  animal 
economy,  as  indeed  may  be  gathered  from  the  fact  that  it  is 
constantly  taken  into  the  lungs  and  emitted  from  them ;  it 
destroys  life,  however,  because  it  does  not  contain  any  free 
oxygen.  According  to  Berzelius,  common  air  containing  «Vth 
of  its  volume  of  carbon  dioxide  can  be  breathed  without  pro- 
ducing any  serious  effects ;  but  from  Angus  Smith's  later 
experiments1  it  appears  that  when  air  contains  only  0*20  per 
cent,  by  volume  of  this  gas,  its  effect  in  lowering  the  action  of 
the  pulse  is  rendered  evident  after  the  respiration  has  continued 
for  about  an  hour.  It  seems,  therefore,  premature  to  say  that 
the  smallest  increase  of  atmospheric  carbonic  acid  may  not  be 
productive  of  hurtful  results.  In  close  spaces  inhabited  by  man 
the  quantity  of  carbon  dioxide  is  naturally  much  larger  than  in 
the  open  air.  The  oppressive  feeling  which  respiration  in  such 
air  is  apt  to  produce  is  not  however  in  general  due  to  the 
presence  of  this  gas,  but  rather  to  the  volatile  organic  emana- 
1  Air  and  Rain,  p.  209,  "On  Some  Physiological  Effects  of  Carbonic  Acid." 


714  THE  NON-METALLIC  ELEMENTS 


tions,  the  presence  of  which  is  indicated  by  the  unpleasant 
smell  observed  on  entering  such  rooms  from  the  fresh  air. 

Carbon  dioxide  possesses,  according  to  Regnault,  a  specific 
gravity  of  T529,  and  hence,  being  much  heavier  than  air,  it 
may  be  poured  like  water  from  one  vessel  to  another;  this 
fact  can  be  strikingly  exhibited  by  bringing  a  lighted  taper  into 
the  vessel  into  which  the  carbon  dioxide  has  been  poured,  when 
it  will  be  instantly  extinguished.  This  property  is  made  use  of 
to  test  the  presence  of  the  gas  in  old  wells,  cellars,  and  coal-pits, 
where  it  frequently  accumulates,  and  is  termed  choke  damp. 
Before  the  workman  descends  it  is  usual  to  lower  down  a 
burning  candle,  and  thus  ascertain  whether  the  air  is  pure 
enough  for  respiration.  Air  containing  4  per  cent,  of  carbon 
dioxide  extinguishes  a  candle-flame,  but  it  will  support  respira- 
tion for  a  short  time. 

Composition  of  Carbon  Dioxide. — When  carbon  burns-  to  the 
dioxide,  the  volume  of  the  gas  which  is  formed  is  the  same 
as  that  of  the  oxygen  which  is  needed  to  produce  it.  This  fact 
may  readily  be  shown  by  help  of  the  apparatus  which  is  shown 
in  Fig.  108,  under  sulphur  dioxide,  the  experiment  being  con- 
ducted as  there  described,  except  that  a  small  piece  of  freshly 
heated  charcoal  is  placed  in  the  small  combustion  pan  instead 
of  sulphur.  The  molecular  weight  of  the  gas,  calculated  from 
its  density,  is  43*67,  and  this  amount  of  it  therefore  contains 
31'76  parts  of  oxygen  and  11*91  parts  of  carbon,  the  molecular 
formula  being  CO2. 

Dumas  and  Stas l  have  accurately  ascertained  the  atomic 
weight  of  carbon  by  a  series  of  careful  experiments.  For  this  pur- 
pose they  made  use  of  the  apparatus  which  is  shown  in  Fig.  190, 
in  which  they  burnt  diamond  and  purified  graphite  in  a  stream 
of  oxygen  gas.  The  carbon  dioxide  formed  was  completely 
absorbed  by  means  of  caustic  potash,  and  the  composition  of 
the  carbon  dioxide  calculated  from  the  weight  of  it  formed, 
together  with  the  weight  of  carbon  burnt.  In  order  to  carry 
out  this  investigation  it  was  necessary,  in  the  first  place,  to  in- 
sure the  absence  of  every  trace  of  carbon  dioxide  from  the  oxygen 
gas  employed.  The  oxygen  used  was,  therefore,  collected  in  a 
large  Woulff's  bottle — (a) — and  preserved  over  water  rendered 
alkaline  by  caustic  potash.  The  gas  was  driven  out  from  this 
gas-holder  in  the  course  of  the  experiment  by  means  of  a  dilute 
caustic  potash  solution  (&).  The  oxygen  then  passed  through  a 
1  Ann.  Chim.  Phys.,  [1]  40,  991- 


ATOMIC  WEIGHT  OF  CARBON 


long  wide  tube  filled  to  the 
point  (c)  with  pumice-stone 
moistened  with  strong 
caustic  potash  solution, 
whilst  at  (d)  it  came  in 
contact  with  pieces  of  dry 
solid  caustic  potash,  and 
at  (e)  with  glass  moisten- 
ed with  boiled  sulphuric 
acid.  In  order  to  insure 
the  absolute  dryness  of  the 
gas  it  passed  through  a  U 
tube,  marked  (/),  filled 
with  pumice-stone  moist- 
ened with  sulphuric  acid. 
This  tube  was  weighed 
before  and  after  the  ex- 
periment. The  pure  dry 
oxygen  then  passed  into  a 
porcelain  tube,  which  was 
heated  to  redness  in  a 
tube-furnace,  the  substance 
undergoing  combustion  be- 
ing placed  at  (y)  in  a 
platinum  boat.  The  boat 
containing  the  substance 
was  accurately  weighed 
both  before  and  after  the 
experiment,  inasmuch  as 
even  the  purest  diamond 
and  graphite  always  leave 
on  combustion  traces  of 
inorganic  ash,  and  this 
must,  of  course,  be  sub- 
tracted from  the  total 
amount  of  substance  taken. 
The  other  end  of  the  tube 
contained,  at  (A),  a  quan- 
tity of  perfectly  oxidised 
copper  scales,  CuO,  which 
served  for  the  purpose  of 
oxidizing  to  carbon  dioxide 


716  THE  NON-METALLIC  ELEMENTS 

any  monoxide  which  might  be  formed.  The  carbon  dioxide  then 
passed  through  the  tube  (i)  containing  pumice-stone  moistened 
with  sulphuric  acid,  and  then  into  two  Liebig's  potash-bulbs 
(k  and  /).  In  order  to  be  certain  that  the  whole  of  the 
carbon  dioxide  was  absorbed,  and  that  no  moist  air  passed 
away  from  the  potash -bulbs,  the  excess  of  oxygen  was  passed 
through  two  tubes  (m  and  n),  the  first  of  which  contained 
pumice-stone  moistened  with  potash,  and  the  last  solid  potash. 
In  this  way  five  combustions  of  natural  graphite  were  made, 
four  of  artificial  graphite,  and  five  of  diamond.  As  a  mean  of 
the  closely  agreeing  results,  it  was  found  that  800  parts  of 
oxygen  combined  with — 

Natural  Graphite.  Artificial  Graphite.  Diamond. 

299-94  299-95  300'02 

The  mean  of  these  numbers  is  299*97.  Hence  2  atoms  of 
oxygen,  or  31*76  parts  by  weight,  combine  with  1T9  parts  by 
weight  of  carbon.  Similar  numbers  have  been  obtained  by  other 
chemists,  and  the  most  accurate  value  is  probably  11*91.  This 
number  is  taken  as  the  atomic  weight  of  carbon,  because  it 
is  the  least  amount  ever  found  in  a  molecule  of  one  of  its 
compounds. 

414  Liquefaction  of  Carbon  Dioxide. — Carbon  dioxide  can  be 
liquefied  both  by  cold  and  by  pressure.  Faraday  was  the  first  to 
obtain  this  result  by  pouring  some  sulphuric  acid  into  the  closed 
limb  of  a  bent  tube  made  of  strong  glass,  whilst  over  it  he 
pushed  a  piece  of  platinum  foil  on  which  a  lump  of  carbonate 
of  ammonia  was  placed.  After  closing  the  open  end  of  the  tube 
the  sulphuric  acid  was  cautiously  allowed  to  flow  over  the  car- 
bonate, and  in  this  way  the  carbon  dioxide  which  was  evolved 
was  condensed  by  its  own  pressure.  Liquid  carbon  dioxide  pre- 
pared in  this  way  cannot  be  employed  for  further  experiments, 
as  on  endeavouring  to  open  the  tube  it  usually  bursts  with  a 
loud  explosion.  Gore,1  however,  made  an  ingenious  arrange- 
ment in  which  the  carbon  dioxide  is  generated  in  a  small  bent 
tube  closed  with  a  stopper  of  gutta-percha,  to  which  is  fixed  a 
small  glass  cup.  In  this  way  the  solvent  action  of  the  liquid  on 
other  bodies  was  first  examined. 

In  order  to  condense  carbon  dioxide  on  a  larger  scale  Thilorier 2 
constructed  an  apparatus  which  consisted  of  a  strong  cast-iron 
cylinder,  termed  the  generator,  capable  of  being  closed  hermeti- 

1  Phil.  Trans.  1861,  p.  83.  2  Thilorier,  Annalen,  30,  122. 


LIQUEFACTION  OF  CARBON   DIOXIDE 


7r 


cally,  the  original  form  of  which  is  shown  in  Fig.  191.  In 
this  cylinder  he  placed  bicarbonate  of  soda,  whilst  sulphuric 
acid  was  placed  in  a  separate  vessel  lowered  into  the  cylinder. 
The  cylinder  was  closed  by  means  of  the  screws  at  the  top, 
and  then  inclined  so  that  the  sulphuric  acid  came  in  contact 
with  the  bicarbonate  of  soda,  evolving  carbon  dioxide  in  such 
quantity  that  it  was  liquefied  by  its  own  pressure.  The  liquid 
carbon  dioxide  was  then  separated  from  the  sulphate  of  soda 
formed  at  the  same  time,  by  distilling  it  into  a  similar  cast- 
iron  vessel,  termed  the  receiver.  Owing  to  the  brittle  nature 
and  non-homogeneity  of  the  cast-iron  of  which  the  cylinders 
were  made,  and  in  consequence  of  the  enormous  pressure 
which  they  had  to  withstand,  it  was  feared  that  serious  acci- 


FIG.  191. 

dents  might  occur  in  the  production  of  the  liquid  according 
to  Thilorier's  plan,  and  Hare  suggested  that  the  apparatus 
should  be  made  of  wrought  iron.  Not  long  after  this  suggestion 
had  been  made  an  accident  occurred  :  one  of  Thilorier's  cylinders 
exploded  with  fearful  force  whilst  it  was  being  charged,  and  a 
young  chemist  of  the  name  of  Hervey  lost  his  life.  The  wrought- 
iron  apparatus,  which  was  employed  after  this  time,  is  shown  in 
Fig.  192,  the  construction  having  been  much  improved  byMareska 
and  Donny.1  Instead  of  wrought  iron  they  employed  a  leaden 
cylinder,  surrounded  by  another  one  made  of  copper,  this,  in 
its  turn,  being  bound  round  with  strong  hoops  of  wrought  iron. 
In  order  to  prepare  liquid  carbon  dioxide  with  this  apparatus, 

1  Mem.  cour.  et  mem.  Sava?its  itrang.  Acad.  Roy.  Eruxelles,  18. 


718 


THE  NON-METALLIC  ELEMENTS 


the  requisite  quantity  l  of  bicarbonate  of  soda  is  placed  in  the 
generator,  and  a  quantity  of  dilute  sulphuric  acid,  sufficient  for 
its  decomposition,  is  poured  into  the  narrow  copper  cylinder. 
The  apparatus  is  closed  with  a  strong  screw-tap,  and  then 
made  to  revolve  on  its  axis  so  that  the  acid  comes  slowly  in 
contact  with  the  carbonate.  For  the  purpose  of  distilling  the 
carbon  dioxide,  which  is  formed  by  this  decomposition,  into  the 
horizontal  receiver,  a  copper  tube  is  screwed  on,  connecting  the 
one  vessel  with  the  other.  When  the  screw-taps  are  opened  the 


FIG.  192. 

liquid  dioxide  distils  over,  the  receiver  being  kept  cool  and  the 
generator  being  placed,  at  the  end  of  the  operation,  in  hot  water. 
By  repeating  this  operation  a  large  quantity  of  liquid  carbon 
dioxide  can  easily  be  obtained. 

The  apparatus,  however,  which  is  now  most  commonly  em- 
ployed for  the  preparation  of  liquid  carbon  dioxide  on  the  small 
scale  or  for  lecture  purposes  is  that  made  by  Natterer 2  of  Vienna 
and  Bianchi  of  Paris.  In  this  apparatus,  the  general  arrange- 
ment of  which  is  seen  in  Fig.  193,  carbon  dioxide,  generated  by 

1  2|  Ibs.  of  bicarbonate  of  soda,  6£  Ibs.  of  water,  and  ]^  Ib.  of  oil  of  vitriol. 

2  J.  Pr.  Chem.  35,  169. 


LIQUEFACTION    OF  CARBON    DIOXIDE  719 


FIG.  194. 


FIG.  193. 


720  THE  NON-METALLIC  ELEMENTS 

the  action  of  dilute  sulphuric  acid  on  bicarbonate  of  soda,  is  con- 
densed by  means  of  a  force-pump  into  a  wrought-iron  pear-shaped 
vessel,  seen  in  section  in  Fig.  194,  furnished  with  suitable  cocks, 
t,  Fig.  194,  the  air  which  was  contained  in  the  flask  being  first 
of  all  displaced  by  the  gas.  During  the  operation  the  reservoir 
(V)  is  surrounded  by  a  freezing  mixture  contained  in  a  vessel, 
screwed  on  to  the  plate  a  b,  Fig.  193. 

415  Liquid  carbon  dioxide  is  now  a  commercial  article  and 
sold  in  iron  bottles  containing  8  kg.,  being  mostly  prepared  in 
certain  large  breweries  from  the  carbon  dioxide  evolved  in  the 
process  of  fermentation.  It  is  also  made  from  the  natural  gas 
evolved  from  certain  springs,  especially  at  Burgbrohl  on  the 
Rhine,  and  from  the  gas  prepared  from  marble  or  sodium 
bicarbonate.  The  liquefaction  is  accomplished  by  compressing 
the  gas  by  means  of  powerful  pumps  into  recipients  kept  cool 
by  a  stream  of  water.  It  is  used  for  producing  cold,  as  well  as 
high  pressures,  and  therefore  employed  in  the  manufacture  of 
cast  steel.  It  is  also  employed  for  beer-engines,  in  the  manu- 
facture of  salicylic  acid,  and  for  the  production  of  aerated 
waters. 

Liquid  carbon  dioxide  is  a  colourless  very  mobile  liquid 
slightly  soluble  in  water,  upon  the  surface  of  which  it  swims. 
Its  specific  gravity  is  0'9951  at  -10°,  O9470  at  0°,  and  O8266 
at  -f  200.1  These  numbers  show  that  liquid  carbon  dioxide 
expands  more  upon  heating  than  a  gas,  and  its  coefficient  of 
expansion  is,  therefore,  larger  than  that  of  any  known  body. 
The  boiling-point  of  liquid  carbon  dioxide  is  —  78*2°  under  a 
pressure  of  760  mm.,  and  its  tension  at  different  temperatures 
is  given  in  the  following  table  :  — 


Temperature. 

Pressure  in  mm.  of 
mercury. 

Temperature. 

Pressure  in  mm.  of 
mercury. 

-  25° 

13007*02 

+  15° 

39646-86 

-  15 

17582-48 

+  25 

50207-32 

-     5 

23441  "34 

+  35 

62447-30 

+     5 

30753-80 

+  45 

76314-60 

Liquid  carbon  dioxide  conducts  electricity  badly ;  it  does  not 

act  as  a  solvent  on  most  substances,  does  not  redden  dry  litmus 

paper,  and  possesses  no  very  striking  chemical  properties  (Gore). 

When   the    stopcock    of   a  vessel   containing   liquid   carbon 

dioxide  is  opened,   a  portion   of  the   liquid  which   rushes  out 

solidifies  in    the    form    of   white   snow-like  flakes    (Thilorier). 

This  is  caused  by  the  absorption  of  heat,  produced  by  the  rapid 

1  Andreef,  Annalen,  HI. 


SOLID  CARBON  DIOXIDE 


721 


evaporation  of  another  portion  of  the  liquid.  For  the  purpose 
of  obtaining  larger  quantities  of  this  carbon  dioxide  snow,  the 
apparatus  shown  in  Figs.  195  and  196,  suggested  by  Natterer, 
is  employed.  It  consists  of  two  brass  cylinders,  AB  and  CD,  one 
of  which  can  be  fixed  inside  the  other,  each  cylinder  being 
fastened  to  a  hollow  handle. 
The  liquid  carbon  dioxide  is 
allowed  to  pass  quickly  through 
the  tube  d  (the  position  of 
which  inside  the  box  is  shown 
in  Fig.  196),  from  the  nozzle  n, 
Fig.  194,  of  the  reservoir.  A 
portion  of  the  liquid  undergoes 
evaporation  and  passes  out  as 
gas  through  the  fine  holes  in 
the  handles,  whilst  the  larger 
portion  remains  behind  as  a 
solid  finely  divided  crystalline 
mass,  which  is  a  very  light  sub- 
stance, capable  of  being  pressed 
together  like  snow.  It  evapor- 
ates much  less  quickly  than  the 
liquid  dioxide,  because  it  is 
much  colder,  and  in  addition  it 
has  to  take  up  the  amount  of 
heat  necessary  for  liquefaction. 
Notwithstanding  its  excessive- 
ly low  temperature  it  may  be 
touched  without  danger,  the 
gas  which  it  constantly  emits 
forming  a  non-conducting  at- 
mosphere round  it.  If,  however, 
it  be  pressed  hard  upon  the 
skin,  solid  carbon  dioxide  pro- 
duces a  blister  exactly  like  that 

produced    by   contact    with   a  FlG8<  195  and  196 

red-hot  body.     This  snow-like 

mass  may  readily  be  pressed  into  cylindrical  wooden  moulds.  The 
cylinders  thus  obtained  have  a  density  of  about  1'2,  and  require 
a  considerable  time  for  evaporation  in  the  air.1  When  solid 
carbon  dioxide  is  mixed  with  ether,  and  the  mixture  brought 

1  Landolt,  Ber.  17,  309. 
47 


722  THE  NON-METALLIC  ELEMENTS 

into  the  vacuum  of  an  air-pump,  the  temperature  sinks  to 
—  110°.  A  tube  containing  liquid  carbon  dioxide  brought  into 
this  solidifies  to  a  transparent  ice-like  solid  mass  (Mitchell  and 
Faraday).  When  liquid  ammonia  is  placed  over  sulphuric  acid 
in  a  vacuum  and  allowed  to  evaporate,  such  a  diminution  of  tem- 
perature is  obtained  that  gaseous  carbonic  acid  may  be  liquefied. 
Exposed  to  a  pressure  of  from  3  to  4  atmospheres,  this  solidifies, 
and  the  transparent  mass  thus  obtained,  when  pressed  with  a 
glass  rod,  separates  into  cubical  masses.1  It  is  very  remarkable 
that  solid  carbon  dioxide  melts  at  —65°,  that  is  at  a  temperature 
lying  much  above  its  boiling  point  at  the  atmospheric  pressure. 

A  process  has  been  patented  for  carrying  out  the  manufacture 
of  solid  carbonic  acid  by  a  similar  method  on  the  large  scale. 
The  solid  material  may  be  transported  in  bags  or  casks,  or  in  the 
iron  chamber  in  which  it  is  produced.2 

In  the  paragraph  (24)  concerning  the  continuity  of  the 
gaseous  and  liquid  states  of  matter,  it  has  been  pointed  out 
that  a  certain  temperature  may  be  reached  for  any  given  gas, 
above  which  this  gas  cannot  be  liquefied,  however  much  the 
pressure  may  be  increased.  This  temperature  is  termed  by 
Andrews 3  the  critical  point,  that  of  carbon  dioxide  being 
found  by  him  to  be  30*92°,  whilst  according  to  more  recent 
observations  it  is  31*35°,  the  pressure  at  this  temperature,  or 
the  critical  pressure,  being  72*9  atmospheres.4  In  order  to 
determine  this  point,  Andrews  employed  the  apparatus  shown 
in  Figs.  197  and  198.  The  dry  and  pure  gas  is  contained  in 
the  glass  tube  a  b,  which  is  closed  at  a  and  open  at  b,  the  gas 
being  shut  in  by  the  thread  of  mercury  c.  This  tube  is  firmly 
bedded  in  the  brass  end-piece  d,  and  this,  in  its  turn,  is  bolted 
on  to  the  strong  copper  tube  which  carries  a  second  end-piece 
at  its  lower  extremity.  Through  this  the  steel  screw  e  passes 
packed  with  leather  washers  to  render  the  cylinder  perfectly 
air-tight.  The  cylinder  is  completely  filled  with  water,  and 
the  pressure  on  the  gas  is  increased  up  to  400  atmospheres  by 
turning  the  steel  screw  e  into  the  water.  The  closed  and 
capillary  end  of  the  tube  containing  the  compressed  gas  is 
sometimes  surrounded  with  a  cylinder  into  which  water  of  a 
given  temperature  is  brought.  A  second  modification  of  the 
apparatus  is  shown  in  Fig.  198.  The  capillary  pressure-tube 
is  bent  round  so  as  to  render  it  possible  to  place  it  in  a  freezing 

1  Loir  and  Drion,  Compt.  Rend.  52,  748.         2  English  Patent  13,684,  1891. 
3  Phil.  Trans.  1869,  partii,  575.  4  Amagat,  Compt.  Rend.  114,  1093. 


LIQUID  CARBON  DIOXIDE 


mixture  under  the  receiver  of  an  air-pump.  The  action  both 
of  great  pressure  and  great  cold  upon  the  condensed  gas  can 
thus  be  examined. 

If  the  screw  be  turned  round  when  the  gas  possesses  a 
temperature  below  the  critical  point,  liquid  carbon  dioxide  is 
formed  when  a  certain  pressure  is  reached,  and  a  layer  of  this 
liquid  can  be  distinctly  seen  lying  with  the  gas  above  it.  If 
the  same  experiment  be  repeated  at  a  temperature  above  the 
critical  one,  no  liquid  is 
seen  to  form  even  when  the 
pressure  is  increased  to  150 
atmospheres.  The  volume 
gradually  diminishes,  no  line 
of  demarcation  or  other 
alteration  in  appearance  can 
be  observed,  and  the  tube 
appears  as  empty  as  it  did 
before  the  gas  was  submitted 
to  pressure.  If  the  tem- 
perature be  now  lowered 
below  31-35°,  the  whole 
mass  becomes  liquid,  and 
when  the  pressure  is  dimin- 
ished begins  to  boil,  two 
distinct  layers  of  liquid  and 
gaseous  matter  being  again 
observed.  Thus,  whilst  at  all 
temperatures  below  31 '35° 
carbon  dioxide  gas  cannot 
be  converted  into  a  liquid 
without  a  sudden  condensa- 
tion, at  temperatures  above 
this  point  gaseous  carbon 
dioxide  may,  by  the  applica- 
tion of  great  pressure  and  subsequent  cooling  to  below  the  critical 
point,  be  made  to  pass  into  a  distinctly  liquid  condition  without 
undergoing  any  sudden  change  such  as  is  observed  in  the  case 
of  ordinary  liquefaction. 

416  Carbon  dioxide  is  a  very  stable  body,  requiring  for  its 
decomposition  an  extremely  high  temperature.  When  passed 
over  pieces  of  porcelain  in  a  porcelain  tube,  heated  to  a  tempera- 
ture of.  from  1200°  to  1300°,  it  decomposes  to  a  small  extent 


FIGS.  197  and  198. 


724  THE  NON-METALLIC  ELEMENTS 

into  carbon  monoxide  and  oxygen  (Deville)  ;  and  the  same 
decomposition  is  brought  about  by  the  electric  spark  (Dalton 
and  Henry).  In  this  way  only  a  small  portion  of  the  gas  is 
decomposed,  as  when  a  certain  quantity  of  oxygen  has  been 
formed  this  again  combines  with  the  carbon  monoxide.  Under 
favourable  circumstances  the  recombination  of  the  gases,  accom- 
panied by  the  passage  of  a  flame,  can  be  actually  observed  to 
take  place  periodically.1  If,  however,  hydrogen,  or  mercury,  or 
any  oxidizable  body  be  present,  the  whole  of  the  carbon  dioxide  is 
converted  into  monoxide  (Saussure).  This  decomposition  is  best 
shown  by  allowing  the  electric  spark  to  pass  by  means  of  iron 
poles  through  a  measured  volume  of  carbon  dioxide  ;  after  the 
action  is  completed,  the  volume  of  carbon  monoxide  formed  is 
found  to  be  exactly  equal  to  that  of  the  dioxide  taken,  thus  :  — 

2C02  =  2CO  +  O2, 

the  oxygen  being  completely  absorbed  by  the  metallic  iron 
(Buff  and  Hofmann). 

A  piece  of  burning  magnesium  wire  or  ribbon  continues  to 
burn  when  plunged  into  carbon  dioxide,  the  oxide  of  the  metal 
being  formed  and  carbon  liberated  :  — 


When  carbon  dioxide  is  passed  over  heated  potassium  or 
sodium,  the  carbonates  of  these  metals  are  formed,  and  carbon 
is  set  free  ;  thus  :  — 


Liquid  carbon  dioxide  is  also  attacked  by  the  alkali  metals 
(Gore). 

Carbon  dioxide  is  soluble  in  water,  its  solubility  beng  re- 
presented by  the  following  equation  :  — 

c  =  1-7967  -  0-07761*  +  0-0016424*2, 

or  one  vol.  of  water  at  0°  dissolves  1*7967  vols.  of  carbon  dioxide. 

5         „         1-4497     „ 
10         „         1-1847     „ 

15         „         1-0020     „ 
20         „         0-9014     „ 

1  Hofmann,  Ber.  23,  3303. 


CAEBONIC  ACID  725 


It  is  more  easily  soluble  in  alcohol  of  specific  gravity  0*792, 
the  coefficient  of  solubility  being  given  by  the  following  equa- 
tion : — 

c  =  4-32955  -  0-09395t  +  0'00124t2. 

When  the  pressure  is  much  smaller  than  that  of  the  atmo- 
sphere, carbon  dioxide  follows  Dalton  and  Henry's  law  of  the 
absorption  of  gases  in  water,  but  deviation  is  observed  from  this 
law  at  higher  pressures,  increasing  gradually  as  the  pressure  is 
increased. 

The  following  table  shows  the  volume  of  carbon  dioxide, 
measured  at  0°  and  760  mm.  dissolved  by  one  volume  of  water 
at  0°  (Column  S),  the  pressure  being  given  in  the  column  (P).1 

P  S  S 

P 

in  atmospheres  Vols.  at  0° 

1  1-797  1-797 

5  8-65  1-730 

10  16-03  1-603 

15  21-95  1-463 

20  26-65  1-332 

25  30-55  1-222 

30  33-74  1-124 

Q 

If  the  law  of  Henry  and  Dalton  were  correct,  the   value  of 

would  be  constant.  It  appears  from  the  table,  however,  that  this 
fraction  gradually  decreases  in  value,  the  solubility  increasing 
at  a  slower  rate  than  the  pressure. 


CARBONIC  ACID  AND  THE  CARBONATES. 

417  The  aqueous  solution  of  carbon  dioxide  contains  in  solu- 
tion a  dibasic  acid,  termed  carbonic  acid,  CO(OH)2.  This  acid 
does  not  exist  iri  the  pure  concentrated  state,  and  in  this  respect 
resembles  sulphurous  acid  and  other  acids  whose  corresponding 
oxides  are  gaseous.  Its  existence  is,  however,  ascertained  by 
the  fact  that  the  aqueous  solution  of  carbon  dioxide  reddens 
litmus  paper,  whereas  dry  carbon  dioxide,  whether  in  the 
gaseous  or  liquid  state,  produces  no  such  action.  It  has  also 
been  observed  that  if  the  water  be  saturated  with  carbon  dioxide 
under  pressure,  and  if  the  pressure  be  then  at  once  removed, 
1  Wroblewski,  Compt.  Rend.  94,  1355. 


726  THE  NON-METALLIC  ELEMENTS 

the  gas  quickly  makes  its  escape  by  effervescence,  the  bubbles 
being  so  minute  that  the  whole  liquid,  during  the  disengage- 
ment of  the  gas,  appears  milky.  If,  however,  the  liquid  be 
allowed  to  remain  in  contact  with  the  gas  for  some  consider- 
able time  after  saturation,  before  the  pressure  is  removed,  the 
gas  escapes,  on  removal  of  the  pressure,  in  large  bubbles  which 
adhere  chiefly  to  the  sides  of  the  glass  vessel  in  which  the 
liquid  is  contained.  The  difference  may  be  explained  by  the 
fact  that,  to  begin  with,  the  carbon  dioxide  is  mechanically 
dissolved  in  the  water,  but  that  after  remaining  in  contact  with 
the  water  for  some  time  an  actual  combination  takes  place 
between  these  two  substances,  which  may  be  represented  by  the 
following  equation  :  — 

/OH. 
O  =  C  =  O     +     H-O-H      =     O=C 


If  a  piece  of  porous  substance  like  sugar  or  bread  be  brought 
into  a  liquid,  such  as  soda-water  or  champagne,  which  has  been 
saturated  under  pressure  with  carbon  dioxide,  and  which  has 
been  exposed  to  the  air  for  some  little  time,  so  that  the  effer- 
vescence due  to  the  diminution  of  pressure  has  ceased,  a  strong 
renewal  of  the  effervescence  is  observed.  Water  which  contains 
a  small  quantity  of  common  salt  in  solution  dissolves  carbon 
dioxide  more  easily  than  common  water.  This  depends  on  the 
fact  that  a  chemical  decomposition  takes  place,  a  part  of  the 
common  salt,  NaCl,  being  converted  by  the  carbonic  acid  present 
into  acid  sodium  carbonate,  NaHCO3,  free  hydrochloric  acid, 
HC1,  being  liberated.  The  presence  of  the  latter  acid  may 
easily  be  shown  by  the  addition  to  the  liquid  of  a  small  quantity 
of  ultramarine,  this  substance  losing  its  blue  colour  in  presence 
of  hydrochloric  acid,  whilst  a  solution  either  of  common  salt  or 
of  carbonic  acid  fails  to  act  upon  it. 

If  a  current  of  carbon  dioxide  be  passed  through  a  solution 
of  chloride  of  lead,  an  insoluble  chlorocarbonate  of  lead  is 
formed  1  :  — 

/Cl 

Pb< 

/Cl  \0v 

2Pb<      +C02+H2O=  >qO+2HCL 

Na  <y 


Hugo  Miiller,  Jmtrn.  Cliem.  Soc.  1870,  37. 


THE  CARBONATES  727 


In  a  similar  way,  when  carbon  dioxide  is  passed  through  a 
solution  of  common  sodium  phosphate,  acid  sodium  phosphate 
and  acid  sodium  carbonate  are  formed  ;  thus : — 

HNa2PO4  +  H2CO3  =  H2NaPO4  +  HNaCO3. 

Carbonic  acid  readily  decomposes  into  water  and  carbon  dioxide. 
In  consequence  of  this,  litmus  paper,  which  has  been  turned  red 
in  the  aqueous  acid,  becomes  blue  on  drying.  When  carbon 
dioxide  gas  is  passed  into  a  solution  of  blue  litmus,  this  solution 
becomes  first  violet  and  then  of  a  wine-red  colour ;  if  this  red 
solution  be  then  heated,  carbon  dioxide  is  evolved  in  large 
bubbles,  and  after  boiling  for  a  few  seconds  the  liquid  again 
becomes  blue. 

Although  carbonic  acid  itself  is  such  an  extremely  unstable 
compound,  the  carbonates  are  very  stable.  Being  a  dibasic  acid, 
it  forms  two  series  of  salts — namely,  the  normal  and  the  acid 
carbonates.  When  carbon  dioxide  is  passed  through  a  solution 
of  the  hydroxide  of  an  alkali -metal,  a  normal  carbonate  is  first 
formed  ;  thus  : — 

2KOH  +  CO2  =  K2CO3  +  H20. 

These  normal  carbonates  of  the  alkali-metals  are  soluble  in 
water,  and  have  an  alkaline  reaction.  When  they  are  treated 
with  an  excess  of  carbon  dioxide,  the  solution  becomes  neutral, 
and  then  contains  an  acid  carbonate  : — 

K,CO3  +  C02  +  H20  =  2HKC03. 

If  a  solution  of  the  hydroxides  of  the  metals  of  the  alkaline 
earths,  such  as  lime-water  or  baryta- water,  be  treated  in  a  similar 
way,  a  white  precipitate  of  the  normal  carbonate  of  these  metals 
is  obtained  in  the  first  instance,  these  salts  being  almost  in- 
soluble in  water ;  thus  : — 

Ca(OH)2  +  C02  =  CaCO,  +  H2O. 

If  more  carbon  dioxide  gas  be  passed  through  the  milky  solution 
the  precipitate  dissolves,  and  the  solution  contains  an  acid 
carbonate.  These  acid  carbonates  of  the  metals  of  the  alkaline 
earths  are  only  known  in  solution.  When  their  solutions  are 
boiled,  carbon  dioxide  is  given  off,  and  the  normal  carbonate  is 
again  precipitated.  All  the  other  normal  carbonates  are  in- 
soluble in  water.  A  few,  such  as  magnesium  carbonate  and 


728 


THE  NON-METALLIC  ELEMENTS 


ferrous  carbonate,  dissolve  like  the  carbonates  of  the  alkaline 
earth  metals  in  an  excess  of  carbonic  acid.  Carbonic  acid  being 
a  very  weak  acid,  its  salts  are  readily  decomposed  by  the  greater 
number  of  the  acids,  when  the  solution  is  not  too  dilute,  carbon 
dioxide  being  evolved  as  gas.  It  may  be  readily  recognised,  as 
it  produces  a  white  precipitate  when  brought  in  contact  with 
clear  lime-  or  baryta-water. 

418  In   order  to  determine  the  quantity  of  carbon  dioxide 
contained  in  a  carbonate,   several  methods   may  be  employed. 

It  is  easy  to  determine  this  quantity  by 
the  loss  of  weight  which  the  salt  under- 
goes when  an  acid   is  added,  and  for 
this  purpose  the  apparatus  constructed 
by   Geissler,  which  is  thus  described 
by    Fresenius,    is    one    of    the    best. 
The    apparatus,    the    construction    of 
which  is  shown   in  Fig.   199,  consists 
of  three   parts,    A,  B,  and  C.      C  is 
ground  into  the  neck  of  A,  so  that  it 
may  close  air-tight,  and  yet  admit  of 
its  being  readily  removed  for  the  pur- 
pose of  filling  and  emptying  A.  b  c  is 
a  glass  tube,  open  at  both  ends,  and 
ground  water-tight  into  0  at  the  lower 
end,  c  ;  it  is  kept  in  the  proper  position 
by  means  of  the  movable  cork,  i.     The 
cork,  e,  must   close   air-tight,  and    so 
must  the  tube,  d,  in   the  cork.     The 
weighed  substance  to  be  decomposed 
is  put  into  A,  water  is  added  to  the 
extent  indicated  in  the  engraving,  and 
the  substance  shaken  towards  the  side 
of  the  flask.     C  is  now  filled  nearly  to 
the   top  with  dilute  nitric  or  hydro- 
chloric acid,  with  the  aid  of  a  pipette,  after   having  previously 
moved  the   cork,  i,  upwards  without  raising  b.     The  cork   is 
then  again  turned  down,  C  again  inserted  into  A,  B  somewhat 
more  than   half  filled  with  concentrated  sulphuric  acid,  and  b 
closed  at  the  top,  by  placing  over  it  a  small  piece  of  caoutchouc 
tubing  with  a  glass  rod  fitted  into  the  other  end.     After  weighing 
the  apparatus,  the  decomposition  is  effected  by  opening  b  a  little 
and  thus  causing  acid  to  pass  from  C  into  A.     The  carbonic  acid 


FIG.  199. 


GKAPHITIC  ACID  729 


passes  through  the  bent  tube  k  into  the  sulphuric  acid,  where  it 
is  dried.  It  then  leaves  the  apparatus  through  d.  When  the 
decomposition  is  effected,  A  is  gently  heated,  the  stopper  from  b 
removed,  and  the  carbon  dioxide  still  present  sucked  out  at  d. 
The  apparatus,  when  cold,  is  again  weighed,  and  the  difference 
between  the  two  weighings  will  be  that  of  the  carbon  dioxide 
expelled. 

A  more  accurate  method  consists  in  passing  the  carbon 
dioxide  evolved  by  the  decomposition  of  the  substance  by  dilute 
acids  through  a  series  of  drying  tubes  and  then  absorbing  it  in 
a  weighed  solution  of  potash,  and  thus  ascertaining  its  weight 
directly. 

The  method  for  the  determination  of  the  carbon  dioxide  con- 
tained in  the  air  has  already  been  explained  (p.  539). 

In  order  to  determine  the  quantity  of  this  gas  dissolved  in 
water,  ammonia  is  added  to  a  solution  of  barium  chloride  ;  this 
is  heated  to  the  boiling  point  and  filtered  ;  50cc.  of  this  solution 
are  then  brought  into  a  flask  of  300cc.  capacity,  and  the  water 
under  investigation  added  in  measured  quantity,  the  flask  being 
again  stoppered  up  and  allowed  to  stand  for  a  considerable 
length  of  time  until  the  precipitated  carbonate  of  barium  has 
separated  out.  The  solution  is  then  warmed,  and  the  flask 
again  stoppered.  The  solution  is  allowed  to  become  clear,  the 
clear  liquid  is  poured  off  as  completely  as  possible  from  the 
precipitate,  the  flask  is  filled  up  with  water  which  has  been  well 
boiled,  is  again  allowed  to  deposit,  and  the  operation  is  repeated 
several  times.  The  precipitate  is  then  brought  upon  a  tared 
filter  and  weighed,  after  being  carefully  dried  at  110°.  The 
quantity  of  carbon  dioxide  can  be  readily  calculated  from  the 
weight  of  barium  carbonate  obtained. 

GRAPHITIC  ACID,  CnH405. 

419  It  has  already  been  stated  under  the  head  of  graphite 
that  in  certain  properties  this  substance  differs  remarkably  from 
the  other  modifications  of  carbon.  Sir  Benjamin  Brodie  1  has 
shown  that,  when  acted  upon  by  certain  oxidizing  agents, 
graphite  is  converted  into  a  compact  substance  which  contains 
oxygen  and  hydrogen.  In  order  to  prepare  graphitic  acid,  an 
intimate  mixture  of  one  part  of  purified  graphite  and  three  parts 
of  potassium  chlorate  is  treated  with  so  much  concentrated 
1  Phil.  Trans.  1859,  p.  249. 


730  THE  NON-METALLIC  ELEMENTS 


nitric  acid  that  the  mass  becomes  liquid.  It  is  then  heated  for 
three  or  four  days  on  a  water-bath.  The  solid  residue,  after 
having  been  washed  with  water  and  dried  at  100°,  is  subjected 
four  or  five  times  to  a  similar  treatment,  until  no  further  change 
is  observed.  Graphitic  acid  is  a  stable  yellow  substance  existing 
in  thin  microscopic  crystals,  which  have  the  property  of  redden- 
ing moistened  blue  litmus  paper,  and  are  slightly  soluble  in  pure 
and  insoluble  in  acidified  water.  It  is  doubtful  whether  the 
substance  is  in  reality  possessed  of  acid  properties. 

According  to  Berthelot 1  and  Luzi 2  the  products  of  oxidation 
obtained  from  different  specimens  of  graphite  differ  in  several 
important  respects,  and  yield  different  series  of  derivatives. 

When  graphitic  acid  is  heated,  it  swells  up  and  yields  a  fine 
powder  consisting  of  pyrographitic  oxide,  C.,2H2O4,  a  substance 
which  is  almost  entirely  dissolved  by  the  oxidizing  mixture  of 
nitric  acid  and  potassium  chlorate,  a  little  of  the  original 
graphitic  acid  being  regenerated.  When  heated  with  hydriodic 
acid,  graphitic .  acid  is  converted  %into  hydrographitic  acid, 
which  contains  more  hydrogen  than  graphitic  acid  itself,  and 
does  not  swell  up  and  yield  pyrographitic  acid  when  heated. 
Oxidizing  agents  reconvert  it  into  graphitic  acid.  There  is 
still  considerable  doubt  as  to  the  composition  of  these  sub- 
stances, and  their  chemical  nature  as  well  as  the  relation  of 
the  various  derivatives  to  each  other  are  but  very  imperfectly 
understood. 

Neither  charcoal  nor  diamond  yields  similar  compounds,  and 
Brodie  believes  that  graphite  may  be  considered  to  be  a  peculiar 
radical,  to  which  he  gives  the  name  of  graphon.  As  already 
mentioned,  diamond  is  not  attacked  by  the  oxidizing  mixture  of 
nitric  acid  and  potassium  chlorate,  whilst  ordinary  charcoal  is 
converted  into  a  brown  mass,  soluble  in  water.  Berthelot  has 
made  use  of  this  property  for  the  purpose  of  estimating  the 
quantity  of  charcoal,  graphite,  and  diamond  present  in  a 
mixture.3  The  finely  powdered  substance  is  treated  by  the 
method  described  for  the  preparation  of  graphitic  acid  ;  care, 
however,  must  be  taken  that  not  more  than  five  grams  of  the 
mixture  are  used  at  once,  as  otherwise  explosions  may  take 
place.  In  order  to  separate  the  diamond  from  the  graphitic 
acid,  the  residue  is  gently  ignited,  and  again  treated  with  the 
oxidizing  mixture.  The  process  is  repeated  until  the  whole  of 

1  Ann.  Chem.  Phys.  [4]  19,  405.  2  Ber.  24,  4085  ;  25,  1378. 

3  Ann.  Chim.  Phys.  [4],  19,  399. 


CARBONYL  CHLORIDE  731 

the   graphitic  acid   has  disappeared,  but  any  diamond  which 
may  be  present  remains  unaltered. 


CARBON  OXYCHLORIDE  OR  CARBONYL  CHLORIDE,  COC12  =98-17. 

420  When  equal  volumes  of  dry  chlorine  and  carbon  mon- 
oxide gas  are  brought  together  in  the  dark,  no  action  takes 
place,  but  when  the  mixture  is  exposed  to  light  combination 
•ensues,  especially  in  the  sunlight.  J.  Davy,  who  discovered 
this  compound  in  the  year  1811,  termed  it  phosgene  gas  (from 
<£w?,  light,  and  «yeiW&>,  I  give  rise  to).  Carbonyl  chloride  is, 
at  the  ordinary  temperature,  a  gas  possessing  a  peculiar,  very 
unpleasant,  pungent  smell,  and  having  a  specific  gravity  of 
3'4604  (Thomson).  At  a  low  temperature  it  can  be  condensed, 
forming  a  colourless  liquid,  boiling  at  +801  and  having  a 
specific  gravity  at  0°  of  1*432.  The  compound  is  decomposed 
by  water  into  hydrochloric  acid  and  carbon  dioxide  ;  thus  :  — 


For  the  purpose  of  preparing  pure  carbonyl  chloride  the 
method  proposed  by  Wilm  and  Wischin2  is  the  best.  The 
mixed  gases  issuing  at  about  the  same  rate  are  brought  into  a 
large  glass  balloon  having  a  capacity  of  about  ten  litres  ;  from 
this  balloon  the  mixed  gases  pass  into  a  second  one,  which,  like 
the  first,  is  exposed  to  sunlight.  It  is  best  to  employ  a  slight 
excess  of  chlorine,  this  being  afterwards  got  rid  of  by  passing 
the  gas  through  a  tube  filled  with  lumps  of  metallic  antimony. 
The  gas  thus  purified  can  be  liquefied  by  passing  into  a  tube 
surrounded  by  ice,  or,  better,  by  a  freezing  mixture. 

Carbonyl  chloride  is  also  formed  when  a  mixture  of  twenty 
parts  of  trichloromethane  (chloroform),  four  hundred  of  sulphuric 
acid,  and  fifty  of  potassium  dichromate  are  heated  together  on  a 
water-bath  (Emmerling  and  Lengyel),  thus  :  — 

2CHC1,  +  K2Cr207  +  5H2S04  =  2COC12  + 
2KHSO4  +  Cr2(SO4)3  +  C12  +  5H2O. 

Carbonyl  chloride  is  also  obtained  when  carbon  tetrachloride 
is  acted  upon  by  fuming  sulphuric  acid.3 

1  Emmerling  and  Lengyel,  Annalen,  suppl.  Band  7,  101. 

2  Annalen,  147,  147.  3  Ber.  26,  1993. 


732  THE  NON-METALLIC  ELEMENTS 

It  is  the  chloride  of  carbonic  acid,  just  as  sulphuryl  chloride, 
SO2C12,  is  the  chloride  of  sulphuric  acid,  and  when  treated 
with  ammonia  is  converted  into  the  amide  of  carbonic  acid, 
known  as  carbamide  or  urea,  CO(NH2)2  (p.  739). 

Carbonyl  chloride  is  largely  used  in  the  preparation  of  colour- 
ing matters,  for  which  purpose  it  is  sometimes  made  by  passing 
equal  volumes  of  carbon  monoxide  and  chlorine  over  heated 
animal  charcoal ;  under  these  circumstances  combination  takes 
place  in  the  dark.  Carbonyl  chloride  is  also  formed  by  the 
spontaneous  oxidation  of  chloroform  when  it  is  exposed  to  the 
light  in  the  presence  of  air.  Chloroform  which  is  to  be  used  as 
an  anaesthetic  should  therefore  be  kept  in  the  dark,  and  the 
bottles  containing  it  should  be  kept  filled,  as  the  presence  of 
carbonyl  chloride  in  it  renders  it  extremely  dangerous  if  used 
for  inhalation. 

CARBON  OXYBROMIDE  OR  CARBONYL  BROMIDE, 
COBr2=  186-51. 

421  When  a  mixture  of  carbon  monoxide  and  bromine  vapour 
is  exposed  to  the  light,  combination  takes  place  slowly,  but  the 
gas  thus  prepared  retains  a  yellow  colour.     Potash  decomposes 
it  with   the  formation  of  potassium  carbonate  and  potassium 
bromide.1 

Carbonyl  bromide  has  also  been  obtained  in  the  impure  state 
by  the  oxidation  of  bromoform.2 

CARBON  AND  SULPHUR. 

CARBON  BISULPHIDE,  CS2  =  75'55. 

422  This  compound  was  accidentally  discovered  by  Lampadius 
in  1796,  by  heating  pyrites  with  charcoal.     In  their  investiga- 
tion of  carbonic  oxide  in  the  yea.r  1802,  Clement  and  Desormes 
wished    to    ascertain    whether   charcoal    invariably    contained 
combined  hydrogen  ;  they  examined  the  action  of  sulphur  on  red- 
hot  charcoal,  and  obtained  the  same  liquid  which  had  been  pre- 
viously discovered  by  Lampadius.    This  liquid  they  first  believed 
to  be  a  compound  of  hydrogen  and  sulphur,  but  they  soon  con- 
vinced themselves  that  it  only  contained   carbon  and  sulphur. 
Notwithstanding  these  experiments,  the  nature  of  the  compound 
remained  doubtful,  until  Vauquelin  ascertained  that  its  vapour, 

1  Schiel,  Annalen,  suppl.  Band  2,  311.  2  Emmerling,  Ber.  13,  874. 


CARBON  BISULPHIDE 


733 


passed  over  red-hot  metallic  copper,  is  converted  into  carbon 
and  copper  sulphide. 

Carbon  bisulphide  is  prepared  on  the  large  scale  by  passing 
the  vapour  of  sulphur  over  red-hot  charcoal.  For  this  purpose 
a  large  upright  cast-iron  cylinder  (Fig.  200),  ten  or  twelve  feet 
long  and  one  to  two  feet  in  diameter,  is  employed.  This 
cylinder  is  placed  above  a  furnace  and  surrounded  by  brickwork, 
and  at  the  same  time  it  is  provided  with  a  lid  to  admit  of  the 
whole  being  filled  with  charcoal.  A  second  opening  (a),  fur- 
nished with  a  hopper,  exists  at  the  bottom  of  the  cylinder,  and 
this  serves  to  bring  the  sulphur  into  the  apparatus.  The 
sulphur  evaporates,  and  in  the  state  of  vapour  combines  with 
the  red-hot  carbon,  impure  carbon  bisulphide  distilling  over  by 


FIG.  200. 

the  tube  (c),  and  collecting  in  the  vessel  (d)  under  water.  The 
tubes  (e)  serve  as  condensers,  to  separate  the  vapour  of  carbon 
bisulphide  from  sulphuretted  hydrogen  formed  during  the  re- 
action, owing  to  the  presence  of  hydrogen  in  the  charcoal,  the 
sulphuretted  hydrogen  being  absorbed  by  passing  over  the 
layers  of  slaked  lime  contained  in  the  purifier  (/).  In  actual 
work  the  yield  of  bisulphide  is  about  20  per  cent,  below  the 
theoretical  amount.  The  crude  substance  invariably  contains 
sulphur  in  solution,  from  which,  however,  it  may  be  separated 
by  distillation ;  but  other  sulphur  compounds,  which  impart  to 
the  crude  material  a  most  offensive  odour,  are  also  contained  in 
the  distillate.  In  order  to  remove  these  impurities,  different 
processes  are  in  use.  The  substance  was  formerly  purified 
by  frequent  re-distillation,  over  oil  or  fat,  by  which  means 


734  THE  NON-METALLIC  ELEMENTS 

the  disagreeably  smelling  compounds  were  held  back.  Another 
means  of  purification  employed  is  that  of  shaking  the  liquid 
with  mercury,  and  allowing  it  to  remain  for  a  long  time  in 
contact  with  corrosive  sublimate  in  the  cold,  and  then  distilling 
it  off  white  wax. 

Pure  carbon  bisulphide  is  a  colourless,  mobile,  strongly  re- 
fracting liquid,  possessing  a  sweetish  srnell  not  unlike  that  of 
ether  or  chloroform,  having  a  specific  gravity  at  0°  of  1*29232 
(Thorpe)  and  boiling  at  46°.  It  solidifies  at  —  116°,remelting  at 
—  110°  (Wroblewski  and  Olszewski).  When  carbon  combines 
with  sulphur  to  form  carbon  bisulphide,  heat  is  absorbed  which 
is  given  out  again  when  it  is  decomposed.  Like  many  sub- 
stances of  this  class,  it  can  be  caused  to  undergo  explosive 
decomposition,  the  heat  thus  evolved  being  sufficient  to  propa- 
gate the  explosion  throughout  the  mass  of  the  gas.  When  a 
small  quantity  of  fulminating  mercury  is  exploded  in  a  glass 
tube  containing  vapour  of  carbon  bisulphide,  the  vapour  is 
decomposed  into  carbon  and  sulphur,  which  are  deposited  on 
the  sides  of  the  tube.1  The  same  decomposition  can  be 
brought  about  by  the  explosion  of  the  yellowish-brown  powder 
which  is  formed  by  the  action  of  sodium  on  carbon  bisulphide, 
and  has  the  formula  C5S2. 

Pure  carbon  bisulphide  is  exceedingly  inflammable,  taking 
fire  in  the  air  at  149°,  according  to  Frankland,  and  burning  with 
a  bright  blue  flame.  A  mixture  of  one  volume  of  carbon 
bisulphide  vapour  and  three  volumes  of  oxygen  detonates  with 
great  violence  on  inflammation,  so  that  great  care  is  needed  in 
the  manufacture,  and  frequent  and  severe  explosions  are  caused 
by  its  inflammability.  A  mixture  of  the  vapour  of  carbon  bisul- 
phide and  nitric  oxide  burns  with  a  very  bright  blue  flame,  par- 
ticularly rich  in  chemically  active  rays.  Carbon  bisulphide  is 
powerfully  poisonous,  and  its  vapour  soon  produces  fatal  effects 
on  small  animals  exposed  to  its  action.  It  not  only  acts  as  a 
poison  when  inhaled  in  large  quantity,  but  it  produces  very 
serious  effects  upon  the  nervous  system  when  inhaled  for  a 
considerable  time  even  in  very  small  amount. 

The  vapour  of  carbon  bisulphide  acts  as  a  powerful  anti- 
putrescent,  and  Zoller  2  has  shown  that  meat  and  other  putrescible 
bodies  may  be  preserved  fresh  for  a  long  time  if  kept  in  an 
atmosphere  containing  the  vapour  of  this  compound. 

Carbon  bisulphide  is  largely  used  in  the  arts,  especially  in  the 

1  Thorpe,  Journ.  Chem.  Soc.  1889,  i.  220.  2  Ber.  10,  707. 


THIOCARBONIC  ACID  735 


indiarubber  and  woollen  manufactures,  in  the  first  case  as  a 
solvent  for  the  caoutchouc,  and  in  the  second  as  a  means  of 
regaining  the  oil  and  fats  with  which  the  wool  has  to  be  treated. 
As  it  dissolves  iodine  in  large  quantity,  but  does  not  dissolve 
appreciably  in  water,  it  is  employed  for  the  purpose  of  determin- 
ing the  amount  of  moisture  contained  in  commercial  iodine.  A 
remarkably  characteristic  reaction  of  carbon  bisulphide  is  that 
it  possesses  the  power  of  combining  with  triethylphosphine, 
P(C2H5)3,  a  derivative  of  common  alcohol,  to  form  a  solid 
compound,  crystallizing  in  magnificent  red  crystals,  and  having 
the  composition  P(C2H5)3CS2. 

Other  compounds  of  carbon  and  sulphur  have  been  described, 
but  these  consist  for  the  most  part  of  amorphous  brown  sub- 
stances, and  are  probably  mixtures.  A  compound  tricarbon 
disulpkide,  C3S2,  has  recently  been  described  by  Lengyel, 1  who 
prepared  it  by  passing  the  vapour  of  carbon  bisulphide  over  an 
electric  arc  passing  between  carbon  poles.  It  is  a  deep-red 
liquid,  which  is  slowly  volatile  at  the  ordinary  temperature,  the 
vapour  causing  a  copious  flow  of  tears.  It  distils  partly 
unchanged  under  diminished  pressure,  but  is  partly  converted 
at  the  same  time  into  a  black  solid  amorphous  modification 
of  the  same  composition.  It  combines  with  bromine  to  form  a 
yellow  compound  of  the  composition  C3S2Br6,  which  has  a  not 
unpleasant  aromatic  smell. 

THIOCARBONIC  ACID,  H2CS3. 

423  The  salts  of  this  acid,  which  is  sometimes  termed  sul- 
phocarbonic  acid,  are  formed  by  processes  analogous  to  those 
by  which  the  carbonates  are  produced.  This  is  shown  by  the 
reaction,  discovered  by  Berzelius,  in  which  carbon  bisulphide 
is  brought  in  contact  with  a  solution  of  the  sulphide  of  an 
alkali-metal ;  thus : — 

CS2  +  Na2S  =  Na2CS3, 
corresponding  to 

CO2  +  Na2O  =  Na.2C03. 

When  alcohol  is  added  to  the  solution  thus  obtained,  the  thio- 
carbonate  separates  out  in  the  form  of  a  heavy  liquid  of  a  slightly 
brown  colour,  which  on  the  addition  of  cold  dilute  hydrochloric 
acid  decomposes  into  free  thiocarbonic  acid.  This  forms  a 

1  Her.  26.  2960. 


736  THE  NON-METALLIC  ELEMENTS 

yellow  oil,  possessing  a  most  disagreeable  penetrating  odour ; 
when  slightly  heated  it  is  resolved  into  carbon  bisulphide  and 
sulphuretted  hydrogen. 

The  thiocarbonates  of  the  alkali-metals  and  those  of  the 
alkaline  earths  are  soluble  in  water. 

The  soluble  thiocarbonates  give  a  brown  precipitate  with  copper 
salts,  a  yellow  precipitate  with  dilute  silver  nitrate  solution, 
and  a  red  precipitate  on  the  addition  of  a  lead  salt.  These 
precipitates  rapidly  become  black,  owing  to  the  formation  of 
the  corresponding  sulphides. 

THIOCARBONYL  CHLORIDE,  CSC12  =  114-11. 

424  This  compound  was  obtained  by  Kolbe1  by  acting  for 
some   weeks   with   dry   chlorine    gas  upon  carbon    bisulphide. 
According  to  Carius,2  it   can   be  easily  obtained  by  heating 
phosphorus   pentachloride    and   carbon  bisulphide  together  in 
sealed  tubes  at  100°,  when  the  following  decomposition  takes 
place  : — 

PCL  +  CS2  =  PSC13  +  CSCly 

It  is  most  readily  obtained  by  acting  on  the  compound  CSC14 
described  below,  with  tin  and  hydrochloric  acid,  and  forms  a 
strongly  smelling  liquid  which  boils  at  73'5°,  has  a  specific 
gravity  of  1 '5  085  at  15°,  and  is  only  slowly  attacked  by  water.  3 

The  compound  CSC14  is  obtained  by  the  action  of  chlorine  on 
carbon  bisulphide  in  presence  of  a  little  iodine,  and  is  a  most 
unpleasant-smelling  liquid,  boiling  with  slight  decomposition  at 
149°. 

CARBONYL  SULPHIDE  OR  CARBON  OXYSULPHIDE,  COS  =  59*55. 

425  This  gas,  discovered  by  Than,4  is  formed  when  a  mixture 
of  sulphur  vapour  and  carbon  monoxide  is  passed  through  a 
moderately  heated  tube.     It  is  also  obtained  by  numerous  other 
reactions,  such  for  example  as  the  action  of  sulphur  trioxide 5 
or  chlorosulphonic  acid 6  on  carbon  bisulphide.     It  is  usually 
prepared  by  the  action  of  dilute  acids  on  potassium  thiocyanate, 

1  Annalen,  45,  41.         2  Annalen,  112,  193.         3  Klason,  Ber.  20,  2378. 
v  4  Annalen  Suppl.  5,  236.  5  Armstrong,  Ber.  2,  712. 

6  Dewar  and  Cranston,  Zeitsch,  Chem.  1869,  734. 


CARBONYL  SULPHIDE  737 

the  thiocyanic  acid  first  formed  splitting  up  into  carbonyl 
sulphide  and  ammonia — 

HCNS  +  H20  =  COS  +  NH3, 

the  latter  combining  with  the  excess  of  acid  present.  To 
obtain  the  pure  gas  the  following  method  is  employed : — 

To  a  cold  mixture  of  five  volumes  of  sulphuric  acid  and 
four  volumes  of  water  such  a  quantity  of  potassium  thiocyanate 
(sulphocyanate  of  potassium)  is  added  that  the  mass  just  re- 
mains liquid.  The  evolution  of  gas,  which  commences  without 
heating,  is  regulated  either  by  cooling  or  gently  warming  the 
mixture,  and  by  care  a  constant  current  of  gas  can  thus 
be  obtained.  The  decomposition  which  takes  place  is  as 
follows  : — 

NCSK  +  H20  +  2H2S04  =  COS  +  KHSO4  +  (NH,)HSO4. 

The  gas  invariably  contains  the  vapours  of  hydrocyanic  acid, 
HCN,  and  bisulphide  of  carbon.  The  first  is  removed  by 
passing  the  gas  through  a  tube  filled  with  cotton-wool  which 
has  been  rubbed  in  oxide  of  mercury,  and  the  second  by 
passing  it  over  pieces  of  cut  caoutchouc  contained  in  a  tube 
placed  in  a  freezing  mixture.  According  to  Hofmann,  the 
vapour  of  carbon  bisulphide  cannot  be  thus  completely  re- 
moved, but  it  may  be  got  rid  of  by  passing  the  gas  over 
cotton-wool  saturated  with  an  ethereal  solution  of  triethyl- 
phosphine,  (C2H5)8P. 

Carbonyl  sulphide  is  a  colourless  gas,  having  a  specific 
gravity  of  2*1046,  and  a  peculiarly  resinous  smell,  at  the  same  time 
resembling  that  of  sulphuretted  hydrogen.  It  becomes  liquid 
at  0°  under  a  pressure  of  12'5  atmospheres,  and  the  liquid  when 
poured  out  solidifies.  It  is  very  inflammable,  taking  fire  even 
when  brought  in  contact  with  a  red-hot  splinter  of  wood,  and 
burning  with  a  blue  slightly  luminous  flame.  A  mixture  of  one 
volume  of  the  gas  with  one  and  a  half  volumes  of  oxygen  in- 
flames with  slight  explosion,  burning  with  a  bright  blue  flame ; 
if,  however,  it  be  mixed  with  seven  volumes  of  air,  the  mixture 
burns  without  explosion.  A  platinum  wire  heated  to  whiteness 
by  the  electric  current  decomposes  the  gas  completely,  without 
alteration  of  volume,  into  sulphur  and  carbon  monoxide. 
Carbonyl  sulphide  is  soluble  in  water ;  one  volume  of  the  gas 
dissolves  in  an  equal  volume  of  water,  and  imparts  to  the 
solution  its  own  peculiar  smell  and  taste.  The  sulphur- 

48 


738  THE  NON-METALLIC  ELEMENTS 

waters  of  Harkany  and  Panic!  in  Hungary  possess  similar 
properties,  and  probably  contain,  as  other  sulphur-waters  may 
do,  carbonyl  sulphide.  The  aqueous  solution  gradually  decom- 
poses with  formation  of  sulphuretted  hydrogen  and  carbon 
dioxide  thus  :  — 

COS  +  H0  =  SH  +  C02. 


The  gas  is    slowly  absorbed    by  aqueous   solutions  of  caustic 
alkalis  with  formation  of  a  carbonate  and  a  sulphide  :  — 

COS  +  4KOH  =  K2CO3  +  K2S  +  2H20. 

Alcoholic  potash  also  absorbs  it  rapidly. 

CARBAMIC  ACID.     CO 


426  This  acid,  the  monamide  of  carbonic  acid,  is  not  known 
in  the  free  state,  although  we  are  acquainted  with  its  salts  and 
its  ethereal  salts.  Ammonium  car  bam  ate  is  formed  when  dry 
carbon  dioxide  is  brought  into  contact  with  dry  ammonia 
(J.  Davy,  H.  Rose)  :— 


It  is  best  prepared  by  passing  a  rapid  stream  of  the  two 
gases  into  well-cooled  absolute  alcohol.  A  deliquescent  crystal- 
line powder  is  formed,  which  smells  of  ammonia  and  is  readily 
soluble  in  water.1  Ammonium  carbamate  is  also  contained  in 
the  commercial  carbonate  of  ammonia,  and  can  be  obtained 
from  this  source  by  allowing  the  commercial  substance  to  stand 
in  contact  with  saturated  aqueous  ammonia  for  forty  hours  at 
a  temperature  of  from  20°  to  25°;  on  cooling,  the  carbamate 
separates  out.  The  specific  gravity  of  the  vapour  of  ammonium 
carbamate  between  37°  and  100°  is  0'892  (Naumann) ;  hence  it 
appears  that  this  body  is  completely  dissociated  on  evaporation 
into  carbon  dioxide  and  ammonia.  If  the  gaseous  mixture  thus 
obtained  be  allowed  to  cool,  ammonium  carbamate  is  again 
formed,  but  only  slowly,  owing  to  the  fact  that  the  combination 
is  not  a  direct  one,  an  intramolecular  change  taking  place.2 
When  brought  in  contact  with  acids,  ammonium  carbamate 
evolves  carbon  dioxide,  whilst  with  alkalis  it  evolves  ammonia. 

1  Basarow,  J.  Pr.  Chem.  [2],  1,  283 ;  Mente,  Annalen,  248,  234. 

2  Naumann,  Annalen,  150,  1- 


CARBAMIDE  OR  UREA  739 

The  sodium  and  potassium   salts  have   also  been  prepared, 
and  also  a  calcium    salt  which  has  probably  the  constitution 


\  (  ).Ca(OH). 

Carbaminic  chloride  or  chloroformamide,  NH2.COC1,  is  pre- 
pared by  passing  carbonyl  chloride  over  ammonium  chloride. 
It  usually  exists  as  a  very  pungent-smelling  liquid,  but  it  may 
be  obtained  in  long  prisms  melting  at  50°;  the  liquid  boils  at 
61°—  62°. 


CARBAMIDE  OR  UREA,  CO(NH2) 


22. 


427  Urea,  which  is  largely  contained  in  urine  and  in  other 
liquids  of  the  animal  body,  was  first  described  in  the  year  1773 
by  H.  M.  Rouelle  as  Extractum  saponaceum  urince.  It  was,  how- 
ever, more  accurately  investigated  in  the  year  1790  by  Fourcroy 
and  Vauquelin.  The  discovery  by  Wohler,  in  the  year  1828,1 
that  when  an  aqueous  solution  of  ammonium  cyanate  is  heated, 
this  compound  undergoes  a  molecular  change  and  is  converted 
into  urea,  is  one  of  the  most  important  discoveries  of  modern 
chemistry,  inasmuch  as  it  was  the  first  case  in  which  a  compound 
formed  in  the  animal  body  was  prepared  from  its  inorganic 
constituents : — 

NC(ONH4)  =  CO(NH2)2. 

Urea  is  also  obtained  by  the  action  of  ammonia  on  carbonyl 
chloride 2 : — 

COC12  +  2NH3  =  CO(NH2)2  +  2HC1. 

It  was  obtained  first  in  this  manner  by  John  Davy,  mixed 
with  ammonium  chloride,  in  1812,  sixteen  years  before  Wohler's 
synthesis,  but  he  did  not  recognize  its  identity,  although  he 
determined  the  relative  volumes  of  the  gases  entering  into 
combination  3 

It  is  likewise  produced  by  heating  ammonium  carbamate  or 
common  carbonate  of  ammonia  to  a  temperature  of  from  130° 
to  140°  (Basarow) :— 


ro         2    _  po         2  _L  H  o 

J  i  ONH4  -        >  i  NH2  4 

1  Wohler,  Pogg.  Ann   [1828],  12,  253. 

2  Natanson,  Annalen,  98,  287  ;  Neubaner,  Annaleit,  101,  342. 

3  Phil.  Trans.  1812  ;  see  also  Matthews,  Chem.  Newx,  1890. 


740  THE  NON-METALLIC  ELEMENTS 

Urea  is  found  in  the  urine  of  all  mammalia,  especially  that  of 
the  carnivora,  and  also  in  small  quantities  in  that  of  birds  and 
reptiles,  as  well  as  in  the  excreta  of  the  lower  animals.  It 
forms  an  important  constituent  of  the  vitreous  humour  of  the 
eye,  and  invariably  occurs  in  small  quantity  in  blood,  and  some- 
times in  perspiration  and  pus.  In  order  to  prepare  urea  from  the 
urine  the  liquid  is  evaporated  down  on  a  water-bath  to  dryness, 
the  residue  treated  with  alcohol,  the  alcoholic  solution  evapo- 
rated, and  the  residue  again  treated  with  absolute  alcohol.  On 
evaporating  this  last  solution,  urea,  slightly  coloured  with 
extractive  matter,  remains  behind. 

A  second  method  is  to  evaporate  down  urine  to  a  syrupy 
consistency,  and  then  add  pure  nitric  acid,  when  nitrate  of  urea 
separates  out  as  a  crystalline  powder,  which  is  decomposed  on 
the  addition  of  potassium  carbonate,  with  formation  of  potassium 
nitrate  and  free  urea.  These  bodies  can  then  be  easily  separated 
by  treatment  with  alcohol,  in  which  the  urea  dissolves.  A  third 
method  is  that  recommended  by  Berzelius :  a  concentrated 
solution  of  oxalic  acid  is  added  to  the  evaporated  urine,  when  a 
precipitate  of  the  insoluble  oxalate  of  urea  is  thrown  down ;  this 
is  boiled  with  chalk,  when  the  pure  urea  is  left  in  solution. 

The  preparation  of  urea  from  ammonium  cyanate  is,  however, 
by  far  the  best.  For  this  purpose  potassium  cyanate  is  first 
prepared  by  heating  dried  potassium  ferrocyanide  with  potassium 
bichromate  in  the  manner  described  on  p.  761.  This  may  be 
then  dissolved  in  water,  and  an  equivalent  quantity  of  ammonium 
sulphate  added,  or  the  requisite  quantity  of  the  latter  may  be 
added  to  the  mother  liquors  obtained  in  preparing  the  cyanate. 
Potassium  sulphate  is  formed,  the  greater  part  of  which  crys- 
tallizes out,  the  small  portion  still  remaining  in  solution  being 
got  rid  of  by  evaporation  and  subsequent  crystallization.  The 
liquid  is  then  evaporated  to  dryness,  and  the  urea  contained  in 
the  dry  residue  is  extracted  with  boiling  alcohol,  and  may  be 
further  purified  by  recrystallization  from  amyl  alcohol.1 

Urea  dissolves  in  its  own  weight  of  cold  water  and  in  every 
proportion  in  boiling  water.  It  also  dissolves  in  five  parts  of 
cold  and  one  part  of  boiling  alcohol,  but  is  almost  insoluble  in 
ether.  It  crystallizes  in  long  striated  quadratic  prisms,  and 
possesses  a  cooling  taste  resembling  that  of  nitre.  On  heating, 
it  melts  at  132°,  and  decomposes  when  heated  to  a  higher 
point.  Although  urea  does  not  possess  an  alkaline  reaction,  it 

1  Erdmann,  Ber.  26,  2438. 


SALTS  OF  UREA  741 


combines  easily  with  acids,  and  forms  a  series  of  salts  which 
crystallize  well,  and  of  which  the  following  are  the  most 
important : — 

Hydrochloride  of  urea,  CO(NH2)2HC1.  This  salt  is  formed 
when  dry  hydrochloric  acid  acts  upon  urea.  In  consequence  of 
the  heat  which  is  evolved  in  the  act  of  combination,  the  com- 
pound appears  in  the  form  of  a  yellowish  oil,  but  this  crystal- 
lizes on  cooling.  On  addition  of  water  it  is  resolved  into 
its  constituents. 

Nitrate  of  urea,  CO(NH2)2~NO3H.  This  is  a  very  charac- 
teristic compound,  being  soluble  in  water  but  almost  insoluble 
in  nitric  acid.  Hence  if  pure  nitric  acid  be  added  to  a  toler- 
ably concentrated  solution  of  urea,  a  crystalline  precipitate  falls, 
which  on  recrystallization  from  solution  in  hot  water  is  deposited 
in  prismatic  crystals. 

Oxalate  of  urea,  2CO(NH2)2,C204H2.  This  salt  separates  out 
in  the  form  of  monosymmetric  tablets  when  a  solution  of  urea 
is  mixed  with  a  concentrated  solution  of  oxalic  acid. 

Urea  not  only  unites  with  acids  but  also  with  a  variety  of 
salts  and  oxides.  Thus,  for  instance,  it  forms  with  common 
salt  the  compound  CO(NH2)2  -f  NaCl  4-  H20.  This  substance 
crystallizes  from  aqueous  solution  in  large  glittering  rhombic 
prisms.  Urea  also  forms  similar  compounds  with  certain  other 
chlorides  and  nitrates. 

When  treated  with  a  solution  of  caustic  potash  in  the  cold, 
it  does  not  yield  any  ammonia,  but  on  warming  it  is  gradually 
resolved  into  ammonia  and  carbon  dioxide.  When  heated  with 
water  above  100°  it  is  also  decomposed,  and  it  is  at  once  acted 
upon  by  nitrous  acid  or  anhydride,  with  liberation  of  the 
nitrogen  of  both  compounds  in  the  free  state  and  formation  of 
water  if  the  solution  be  warm : — 

CO(NH2)2  +  N203  =  C02  +  2N2  +  2H20. 
In  the  cold  ammonium  carbonate  is  also  formed 1 : — 
2CO(NHS)2  +  N20S  =  (NH4)2C03  +  2N2  +  C02. 

Neutral  potassium  permanganate  solution  has  no  action  on 
urea  in  the  cold,  and  very  little  at  100°,  but  the  acidified  solution 
causes  the  rapid  evolution  of  1  volume  of  nitrogen  and  2  volumes 
of  carbon  dioxide  at  100°.  Solutions  of  urea  evaporated  with 
silver  nitrate  yield  ammonium  nitrate  and  silver  cyanate. 
1  Glaus.  Ber.  4,  1440 


742  THE  NON-METALLIC  ELEMENTS 

428  In  order  to  detect  the  presence  of  urea  in  a  liquid  it  is 
evaporated  down  on  a  water-bath,  the  residue  extracted 
with  alcohol,  the  liquid  evaporated,  and  a  few  drops  of  pure 
nitric  acid  added  to  the  residue.  If  urea  is  present  the  nitrate 
of  urea  is  precipitated,  and  this  is  recognised,  inasmuch  as  it 
consists  of  microscopic  crystalline  scales  forming  rhombic  or 
six-sided  tables,  having  an  angle  of  82°,  as  measured  by  means 
of  the  microgoniometer. 

It  has  already  been  stated  that  urine  when  not  sterilized  de- 
composes with  separation  of  ammonium  carbonate.  This  change 
occurs  in  in  presence  of  certain  micro  organisms,  notably  micro- 
coccus  urine  and  bacillus  coli  communes  which  can  be  separated 
from  the  liquid  by  nitration.  If  the  filter  containing  these 
micro-organisms  be  now  washed  with  water  and  dried  at  a 
temperature  not  exceeding  40°,  they  retain  their  activity, 
and  if  this  paper  be  coloured  with  turmeric  it  may  be  used  for 
detecting  the  presence  of  urea  in  neutral  liquids.  In  about  ten 
to  fifteen  minutes  after  the  paper  is  moistened  with  a  neutral 
solution  of  urea  a  portion  of  the  urea  is  converted  into  ammonium 
carbonate,  and  the  paper  becomes  of  a  brown  tint.  In  this  way 
one  part  of  urea  can  be  detected  in  10,000  of  liquid.1 

Urea  is  the  last  product  of  the  action  of  the  atmospheric 
oxygen  on  the  nitrogenous  components  of  the  human  body,  and 
the  quantity  which  is  excreted  within  a  given  time  gives  us  a 
measure  of  the  changes  which  are  going  on  ;  for  although  there 
are  other  nitrogenous  products  contained  in  the  urine,  yet  their 
quantity  is  so  small  in  comparison  to  that  of  the  urea  that  in 
most  cases  they  may  be  disregarded.  For  the  medical  man 
and  the  physiologist  it  is  therefore  of  the  greatest  conse- 
quence that  a  quick  and  accurate  method  should  exist  for 
determining  the  amount  of  urea  in  the  urine.  One  of  the  best 
processes  for  this  purpose  is  that  proposed  by  Liebig.  Jt  de- 
pends upon  the  fact  that  when  mercuric  nitrate  is  added  to  a 
solution  of  urea,  a  white  insoluble  precipitate  falls  down, 
possessing  the  composition  2CO(NH2)2+  3HgO  +  Hg(N03)2.  It 
is  convenient  that  each  cc.  of  the  mercuric  solution  should  pre- 
cipitate O'Ol  gram,  of  urea,  and  a  solution  of  this  strength  is 
obtained  by  dissolving  71  '48  grams,  of  metallic  mercury  in 
nitric  acid,  evaporating  to  drive  off  as  much  of  the  acid  as 
possible,  and  diluting  the  solution  to  one  liter.  Before  this 
solution  is  added  to  the  urine  it  is  necessary  to  precipitate  the 
phosphatesand  sulphates  which  the  latter  con  tains  in  solution.  For 
1  Musculus,  Compt.  Rend.  78,  132. 


DETERMINATION  OF  UREA  743 

this  purpose  two  volumes  of  urine  are  mixed  with  one  volume 
of  a  mixture  of  equal  volumes  of  baryta-solution  and  cold 
saturated  solution  of  barium  nitrate.  The  standard  mercury 
solution,  is  then  added  by  a  burette  to  15  cc.  of  the  filtrate, 
corresponding  to  10  cc.  of  urine,  until  no  further  precipitate 
occurs.  The  end  of  the  reaction  is  easily  ascertained  by  adding 
a  solution  of  sodium  carbonate  to  a  drop  of  the  liquid,  which 
assumes  a  yellow  colour  as  soon  as  a  slight  excess  of  mercury  is 
present.  One  cc.  of  the  mercury  solution  corresponds  to  0*01 
gram,  of  urea. 

According  to  Bunsen's  method,  the  urine  is  heated  with  a 
solution  of  barium  chloride  in  dilute  ammonia  to  a  temperature 
of  230°,  and  the  barium  carbonate  which  separates  out  is 
weighed  :  — 

CO(NH2)2  +  BaClg  -f-  2H20  =  BaC03  -f-  2NH4C1. 

A  third  method  for  the  determination  of  urine,  known  as  the 
Davy-Knop  method,  depends  on  the  fact  that  urea  when 
brought  into  contact  with  an  alkaline  hypochlorite  or  hypo- 
bromite  evolves  pure  nitrogen,  the  carbon  dioxide  wliich  is 
formed  being  absorbed  by  the  free  alkali  :  — 

CO(NH2)2  +  3NaOBr  =  CO2  +  N2  +  SNaBr  +  2H20. 

For  this  purpose  a  freshly  prepared  alkaline  solution  of  sodium 
hypobromite  is  prepared  by  dissolving  bromine  in  excess  of 
caustic  soda,  and  this  is  mixed  with  a  solution  of  urea,  the 
nitrogen  gas  which  is  evolved  by  this  reaction  being  collected 
in  a  graduated  tube.  For  the  details  of  this  process  reference 
must  be  made  to  the  original  papers.1 

(  NTTOTT 
HYDROXYCARBAMIDE,  CO      T 


429  For  the  preparation  of  this  compound,  a  solution  of  hy- 
dro xylamine  nitrate  in  absolute  alcohol  is  cooled  to  —10°,  and 
to  this  a  concentrated  solution  of  potassium  cyanate  is  added. 
On  standing,  hydroxycarbamide  separates  out  in  white  needles 
which  melt  at  128—130°. 

ISURETINE,  CH4ON2. 

This  isomeride  of  urea  is  formed  when  an  alcoholic  solution 

of  hydroxylamine   is  warmed  with  a  concentrated  solution  of 

1  Hiifner,  J.  Pr.  Chem.  [2],  3  ;  Russell  and  West,  Journ.  Chcm.  Soc.  1874,  249. 


744  THE  NON-METALLIC  ELEMENTS 

hydrocyanic  acid  to  a  temperature  of  from  40°  to  50°.  On 
evaporating  the  solution  isuretine  separates  out  in  the  form 
of  long  colourless  rhombic  crystals  which  have  an  alkaline  re- 
action, and  melt  at  105°.  This  substance  forms  with  acids 
a  series  of  easily  crystallizable  salts,  and  its  constitution  is 
represented  by  the  following  formula : — 

NH2.CH:N.OH. 


BIURET,  C2O2H5N3. 

430  This  compound,  discovered  by  Wiedemann,1  is  formed 
when  urea  is  heated  for  some  time  to  150  —  160°.  The  decom- 
position which  takes  place  is  represented  as  follows  :  — 


co 


NH2 


/ 

co/ 

XNH2  XNH2. 

When  the  residue  is  heated  with  water,  cyanuric  acid  remains 
behind,  and  biuret  separates  out  on  cooling  the  solution.  This 
may  be  purified  by  dissolving  in  hot  water  and  reprecipitating 
with  dilute  ammonia.  Biuret  forms  long  white  needle-shaped 
crystals,  100  parts  requiring  for  their  solution  6,493  parts  of 
water  at  15°,  whilst  at  106°,  the  boiling-point  of  a  saturated 
solution,  222  parts  only  are  needed.2 

When  a  few  drops  of  cupric  sulphate  and  then  an  excess  of 
caustic  soda  are  added  to  a  solution  of  biuret  in  water,  the 
liquid  assumes  a  colour  varying  from  red  to  violet,  according  to 
the  quantity  of  copper  salt  added.  By  means  of  this  reaction 
it  is  easy  to  show  the  passage  of  urea  into  biuret  ;  it  is  only 
necessary  to  heat  a  small  quantity  of  urea  in  a  test-tube  until 
ammonia  begins  to  be  evolved,  the  melted  mass  being  poured 
into  hot  water  and  treated  as  described. 

1  Annalen,  68,  323.  2  Hofmanii,  Ber.  4,  262. 


CARBONYLDIUREA  745 


CARBONYLDIUREA,  C3H6N4O3. 

431  When  urea  is  heated  to  100°  with  carbonyl  chloride  in  a 
sealed  tube,  this  substance  is  formed  ;  thus  :  — 


CO 


It  is  a  white  powder,  which  separates  from  boiling  water  as  a 
crystalline  powder. 

On  heating  biuret  with  carbonyl  chloride  a  substance  very 
similar  to  the  one  now  described  is  obtained,  to  which  the  name 
of  carbonyldi-biuret  has  been  given,1  and  which  has  the  follow- 
ing constitution  :  — 

CO(NH.CO.NH.CO.NH2)2. 


OXYTHIOCARBAMIC  ACID,  CO   I 

432  Berthelot  first  obtained  the  ammonium  salt  of  the  above 
acid.  This  salt  separates  out  in  crystals,  when  dry  carbonyl 
sulphide  is  brought  in  contact  with  ammonia.  The  acid  itself 
is  not  known. 

.  When  heated  in  a  closed  tube  the  ammonium  salt  is  con- 
verted into  urea  2  : — 


THIOCARBAMIC  ACID,  CS  | 


433  The  constitution  of  this  compound,  originally  described 
under  the  name  of  hydro  thiosulphoprussic  acid,3  was  first  pointed 
out  by  Debus.4  The  ammonium  salt  of  this  acid  is  formed  by  the 

1  E.  Schmidt,  J.  Pr.  Chem.  [2],  5,  35. 

2  Kretzschmar,  J.  Pr.  Chem.  [2],  7,  474. 

3  Zeise,  AnndUn,  48,  95.  4  Annalen,  73,  26. 


746  THE  NON-METALLIC  ELEMENTS 

union  of  carbon  bisulphide  with  dry  ammonia  in  presence  of 
absolute  alcohol ;  the  salt  separates  out,  after  some  time,  in 
prismatic  crystals.  When  hydrochloric  acid  is  added  to  its 
aqueous  solution,  free  thiocarbamic  acid  separates  out.  This 
substance  is  an  oil  at  the  ordinary  temperature,  but  below  10°  it 
forms  a  crystalline  mass.1  It  has  a  smell  resembling  that  of 
sulphuretted  hydrogen,  possesses  an  acid  reaction,  and  is  easily 
decomposed  into  sulphuretted  hydrogen  and  thiocyanic  acid, 
NCSH. 


THIO-UKEA,  OR  THIOCARBAMIDE,  CS(NH2)2. 

434  Reynolds  2  was  the  first  to  prepare  this  analogue  of  urea. 
It  is  formed  by  a  reaction  analogous  to  that  by  which  common 
urea  is  obtained.  A  molecular  change  takes  place  in  ammonium 
thiocyanate  NCS(NH4)  when  heated  to  140°,  corresponding  to  that 
observed  in  the  case  of  ammonium  cyanate.  At  the  same  time 
a  quantity  of  guanidine  thiocyanate  is  formed,  while  a  portion 
of  the  thiocyanate  remains  unchanged.3  In  order  to  separate 
these  three  bodies,  the  melted  mass  is  treated  with  two- 
thirds  its  weight  of  cold  water,  when  the  greater  portion 
of  the  thio-urer,  remains  behind.  This  is  then  dried  on 
a  porous  plate,  and  purified  by  recrystallization  from  hot 
water.  According  to  Glaus,  it  is  not  necessary  to  prepare  am- 
monium thiocyanate  for  this  purpose,  but  a  solution  of  the 
crude  salt  obtained  by  dissolving  carbon  bisulphide  in  alcoholic 
ammonia  may  be  employed ;  this  is  quickly  evaporated,  until 
ammonia,  ammonium  sulphide,  and  carbon  bisulphide  are  abun- 
dantly evolved,  and  then  the  residue  treated  as  above  described. 
As  long  as  thio-urea  is  not  perfectly  pure  it  crystallizes  in 
silky  needles,  but  in  the  pure  condition  it  forms  magnificent 
large  rhombic  prisms  or  thick  tables.  It  dissolves  in  eleven  parts 
of  cold  water,  and  on  heating  with  water  to  140°  is  reconverted 
into  ammonium  thiocyanate.  Heated  by  itself  to  170 — 180°  it 
is  converted  into  guanidine  thiocyanate  and  ammonium  thio- 
carbonate.  Like  common  urea,  thio-urea  forms  compounds  with 
acids,  salts,  and  oxides.  A  characteristic  compound  is  the  nitrate 
CS(NH2)2,  NO3H,  which  crystallizes  exceedingly  well. 

Seleno-urea,  CSe(NH2)2,  has  also  been  prepared.4 

1  Mulder,  J.  Pr.  Chem.  101,  401,  and  103,  178.       2  Journ.  Chem.  Soc.  1855, 1. 

3  J.  Volhard,  Ber.  7,  92,  and  J.  Pr.  Chem.  [2],  9,  6. 

4  Verneuil,  Compt.  Rend.  99,  1154. 


CYANOGEN  COMPOUNDS  747 

CARBON  AND  NITROGEN. 

CYANOGEN  COMPOUNDS. 

435  The  history  of  these  compounds  commences  with  the  dis- 
covery of  Prussian  blue,  made  accidentally  early  in  the  eighteenth 
century  by  a  colour-maker  of  the  name  of  Diesbach.  The  fact 
was  shortly  afterwards  communicated  to  the  alchemist  Dippel, 
who  ascertained  the  conditions  under  which  the  formation  of  the 
colouring  matter  takes  place.  The  method  for  preparing  the 
colour  was,  however,  first  published  by  Woodward,  who  states 
that  it  was  obtained  by  calcining  the  alkali  obtained  by  heating 
equal  parts  of  cream  of  tartar  arid  saltpetre  with  ox-blood.  The 
residue  of  the  calcination  was  then  lixiviated,  and  green  vitriol 
and  alum  added  to  the  solution,  whereby  a  greenish  precipitate 
was  thrown  down,  which,  on  treatment  with  hydrochloric  acid, 
yielded  the  blue  colour.1  About  the  same  time  John  Brown 
found  that  animal  flesh  could  be  employed  instead  of  blood,  and 
Geoffroy,  in  1725,  showed  that  other  animal  matters  could  be 
used  for  the  same  purpose. 

Up  to  this  time  very  remarkable  opinions  were  held  concerning 
the  composition  of  this  colouring  matter.  Geoffroy,  for  example, 
assumed  that  the  colour  was  due  to  metallic  iron  which,  owing 
to  the  presence  of  the  alum,  was  in  an  extremely  fine  state  of 
division.  In  1752  Macquer  found  that  Prussian  blue  could 
be  manufactured  without  the  use  of  alum,  and  that  when  the 
colouring  matter  is  boiled  with  an  alkali,  oxide  of  iron  remains 
behind,  and  a  peculiar  body  is  formed  which  is  found  in 
solution.  To  this  substance  (ferrocyanide  of  potassium)  the 
name  of  phlogisticated  potash  was  given,  and  it  was  believed 
that  the  iron  contained  in  the  Prussian  blue  was  coloured 
blue  by  a  peculiar  combustible  substance.  During  the  years 
1782-5  Scheele  occupied  himself  with  the  investigation  of 
this  compound,  and  he  showed  that  when  phlogisticated 
potash  is  distilled  with  sulphuric  acid,  a  very  volatile  and 
inflammable  body  is  formed,  which  is  soluble  in  water,  and 
possesses  the  property,  when  treated  with  alkalis  and  green 
vitriol,  of  forming  a  very  beautiful  blue  colour.  To  this  body 
the  name  of  prussic  acid  was  given.  In  1787,  the  investigation 
of  this  compound  was  taken  up  by  Berthollet,  who  proved  that 
iron  was  contained  in  the  prussiate  of  potash  as  an  essential  con- 
stituent, together  with  alkali  and  prussic  acid  ;  and  that  prussic 

1  Phil.  Trans.  1724. 


748  THE  NON-METALLIC  ELEMENTS 

acid,  the  salts  of  which  yield  ammonia  and  carbonic  acid  as  pro- 
ducts of  decomposition,  consists  of  carbon,  nitrogen,  and  hydrogen. 
These  results  were  afterwards  confirmed  by  the  investigations  of 
other  chemists,  but  it  is  especially  to  the  labours  of  Gay-Lussac 
that  we  owe  a  clear  statement  of  the  true  composition  of 
this  acid  and  its  salts.  His  researches  are  of  great  theoretical 
interest,  as  it  is  in  them  that  we  find  the  fact,  for  the  first  time  , 
clearly  pointed  out,  that  a  compound  substance  may  exist  which 
is  capable  of  acting  in  many  respects  as  if  it  were  an  element, 
Gay-Lussac1  showed  in  1811  that  prussic  acid  is  a  compound 
consisting  of  hydrogen  combined  with  a  radical  containing 
carbon  and  nitrogen,  to  which  he  gave  the  name  of  cyanoyem 
(from  Kvavos  dark  blue  and  yewdco  I  produce).  He  proved, 
moreover,  that  the  salts  obtained  by  the  action  of  prussic  acid 
upon  a  base  are  compounds  of  this  cyanog&ne  with  rnetals.  In 
corroboration  of  this  view,  in  1815  he  was  able  to  show  that  this 
radical  cyanogen  can  be  prepared  and  is  capable  of  existing  in 
the  free  state. 

By  the  name  of  compound  radical  we  signify  a  group  of  atoms 
which  plays  the  part  of  a  single  atom,  or,  employing  Liebig's 
classical  definition,  we  may  say  that  cyanogen  is  a  radical, 
because 

(1)  "  It  is  a  never-varying  constituent  in  a  series  of  compounds  : 

(2)  "It  can   be  replaced  in  these  compounds  by  other  simple 
bodies  : 

(3)  "  In  its  compounds  with  a  simple  body,  this  latter  may  be 
easily  separated  or  replaced  by  equivalent  quantities  of  other 
simple  bodies." 

"  Of  these  three  chief  and  characteristic  conditions  of  a  com- 
pound radical,  at  least  two  must  be  fulfilled  if  the  substance  is 
to  be  regarded  as  a  true  compound  radical."  2 

In  addition  to  cyanogen,  a  number  of  other  compound  radicals 
are  known,  some  of  which  have  already  been  mentioned.  These 
radicals  are  distinguished  one  from  another,  as  the  elements 
are,  by  their  quantivalence.  Thus,  nitrogen  peroxide  NO2  is 
a  monad  radical  occurring  in  nitric  acid  and  in  many  of  its 
derivatives,  and  to  it  the  name  of  nitroxyl  has  been  given.  Its 
constitution  is  represented  as  follows  :  — 


Annales  de  Chimie,  77,  128  ;  95,  136.  2  Annalen,  25,  3. 


CYANOGEN  GAS  749 


indicating  that  the  two  atoms  of  dyad  oxygen  are  connected 
with  the  nitrogen  each  by  two  of  the  combining  units  of  the 
pentad  atom  of  the  latter.  Hence,  one  combining  unit  of  the 
nitrogen  remains  free,  and  the  group  acts  consequently  as  a 
monad  radical. 

Sulphur  dioxide  or  sulphuryl,  SO2,  serves  as  an  instance  of 
a  dyad  radical.  This  radical  is  contained  in  sulphuric  acid,  and 
a  large  number  of  compounds  derived  from  it.  It  is  termed  a 
dyad  because,  of  the  six  combining  units  of  the  sulphur  atom, 
four  only  are  saturated  by  combination  within  the  molecule,  and 
its  constitution  may,  therefore,  be  represented  by  the  following 
formula  : — 

,O 


Many  phosphorus  compounds  contain  a  triad  radical,  phos- 
phoryl  PO,  which,  however,  is  not  known  in  the  free  state.  It 
contains  the  pentad  phosphorus  atom  combined  with  the  dyad 
oxygen  atom,  and  its  constitution  can  be  represented  as 
follows  :  — 


Cyanogen,  ON,  is  a  monad  radical,  as  it  contains  the  triad 
atom  of  nitrogen  and  the  tetrad  atom  of  carbon  ;  its  constitution 
is  represented  thus  :  — 

N  =  C— 

The  radical  cyanogen,  to  which  the  symbol  Cy.  instead  of 
the  formula  ON  is  sometimes  given,  combines  like  chlorine 
with  hydrogen  and  the  metals,  and  its  compounds  may,  there- 
fore, be  compared  with  those  of  chlorine,  to  which  they 
have  the  strongest  resemblance  in  their  physical  and  chemical 
properties  :  — 

Hydrocyanic  acid,  CyH.  Hydrochloric  acid,  C1H. 

Potassium  cyanide,  CyK.  Potassium  chloride,  C1K. 

Potassium  cyanate,  CyOK.  Potassium  hypochlorite.  C1OK. 

Free  cyanogen,  Cy2.  Free  chlorine,  C12. 

CYANOGEN  GAS,  OR  DICYANOGEN,  C2N2  or  Cy2. 

436  This  gas,  which  was  discovered  by  Gay-Lussac,  is  formed 
when  the  cyanides  of  mercury,  silver,  or  gold  are  heated.  For 


750  THE  NON-METALLIC  ELEMENTS 

this   purpose  it   is  usual  to   employ   mercuric   cyanide,  which 
decomposes  thus : — 

Hg(CN)2  =  Hg  +  C2N2. 

The  mercury  salt  placed  in  a  tube  of  hard  glass  fitted  with 
a  cork  and  gas-delivery  tube,  or  in  a  small  hard  glass  retort,  is 
heated  to  dull  redness,  and  the  gas  collected  over  mercury. 

The  heat  of  combination  of  mercuric  cyanide  is  large,  and  a 
very  high  temperature  therefore  is  required  to  bring  about 
this  decomposition.  If  mercuric  chloride  be  mixed  with  the 
cyanide,  the  cyanogen  comes  off  at  a  lower  temperature,  as  the 
mercury  then  combines  with  the  mercuric  chloride,  forming 
mercurous  chloride,  and  the  whole  reaction,  represented  by  the 
equation 

Hg  (ON),  +  HgCl2  =  (CN)2  +  2HgCl, 

takes  place  with  slight  evolution  of  heat. 

Cyanogen  gas  can  also  be  obtained  by  other  reactions. 
For  instance,  it  is  formed  when  oxalate  of  ammonium  is  heated 
with  phosphorus  pentachloride  ;  thus  : — 

C204(NH4)2  =  c2N2  +  4H20. 

It  is  also  formed  by  gently  igniting  an  intimate  mixture 
of  two  parts  of  well-dried  ferrocyanide  of  potassium,  Fe(CN)f;K4, 
with  three  parts  of  mercuric  chloride.1  According  to  Ber- 
zelius,  however,  the  gas  thus  prepared  contains  free  nitrogen, 
and  he  recommends  the  employment  of  potassium  cyanide  in 
place  of  the  ferrocyanide.2  There  is  no  doubt  that  in  this 
reaction  mercuric  cyanide  is  first  obtained,  and  this  is  then 
decomposed  as  already  described. 

Cyanogen  is  most  readily  obtained  by  gradually  adding  a 
concentrated  solution  of  potassium  cyanide  to  a  solution  of 
two  parts  of  crystallized  copper  sulphate  in  four  parts  of  water, 
and  then  warming;  cyanogen  is  evolved  and  cuprous  cyanide 
simultaneously  separates  out  from  the  solution.  If  the  cuprous 
cyanide  be  washed  by  decantation,  and  then  treated  with  ferric 
chloride,  the  remainder  of  the  cyanogen  is  evolved.3 

Cyanogen  gas  is  found  in  small  quantities  in  the  gases  pro- 
ceeding from  the  blast  furnaces,4  and  it  is  likewise  formed  when 
a  mixture  of  ammonia  and  coal-gas  is  burnt  in  a  Bunsen 

1  Kemp,  Phil.  Mag.  23,  179.  2  Berzelius,  Jahresb.  24,  84. 

3  Jacquemin,  Ann.  Chim.  Phys.  [6],  6,  1-40.      4  J.  Pr.  Chem.  [1],  42,  145. 


CYANOGEN  GAS  751 


burner.1     The  following  equation  probably  explains  the  reaction 
which  takes  place  in  the  latter  case  : — 

4CO  +  4NH3  +  .02  =  2C2N2  +  6H2O. 

Cyanogen  is  a  colourless  gas  possessing  a  peculiar  pungent 
odour  resembling  that  of  peach  kernels.  It  is  poisonous,  and 
burns,  when  brought  in  contact  with  flame,  with  a  charac- 
teristic purple-mantled  flame,  with  formation  of  carbon  dioxide 
and  nitrogen.  When  cyanogen  gas  is  mixed  with  an  excess 
of  oxygen  and  an  electric  spark  passed  through  the  mixture, 
an  explosion  occurs,  and,  on  cooling,  the  gas  possesses  exactly 
the  same  volume  as  before  the  explosion  ;  thus : — 

C2N2  +  202  =  N2+ 2C02. 

Cyanogen  is  decomposed  when  subjected  to  the  action  of 
electric  sparks,  carbon  being  deposited,  and  small  quantities  of 
paracyanogen  formed. 

The  specific  gravity  of  the  gas  is  1*806  (Gay-Lussac) ;  when 
exposed  to  pressure  or  to  cold,  cyanogen  condenses  to  a  colour- 
less liquid  which  boils  at  —  20'7°,  and  possesses  at  17°  a  specific 
gravity  of  0'866.  When  cooled  to  a  still  lower  temperature  the 
liquid  freezes,  forming  a  crystalline  mass  which  melts  at  —  34'4°.2 
According  to  Bunsen  3  the  tension  of  cyanogen  gas  at  different 
temperatures  is  as  follows : — 

Temperature.  Tension  in  atmospheres. 

-207°  1 

-10  1-85 

0  2-7 

+  10  3-8 

15  4-4 

20  5-0 

At  the  ordinary  atmospheric  temperature,  one  volume  of 
cyanogen  dissolves  in  four  volumes  of  water,  and  in  twenty - 
three  volumes  of  alcohol.  The  aqueous  solution  soon  becomes 
turbid,  with  separation  of  a  brown  powder  known  as  azulmic 
acid,  and  the  solution  contains  ammonium  oxalate,  together 
with  smaller  quantities  of  ammonium  carbonate,  hydrocyanic 
acid,  and  urea  (Wohler).  The  formation  of  these  products  of 
decomposition  will  be  explained  later  (Vol.  III.,  Part  ii.,  2nd 

1  Levoir,  Jahresb.  76,  445.       2  Davy  and  Faraday,  Phil  Trans.  1823,  196. 
3  Pogg.  Ann.  46,  101. 


752  THE  NON-METALLIC  ELEMENTS 


edition,  pp.  139,  343).  Cyanogen  gas  when  passed  over  heated 
potassium  unites  directly  with  the  metal,  forming  potassium 
cyanide. 

Paracyanogen,  (CN)n. — In  the  preparation  of  cyanogen  from 
mercuric  cyanide  a  brown  powder  remains  behind.  This  sub- 
stance possesses  the  same  percentage  composition  as  cyanogen, 
but  its  molecular  weight  is  unknown ;  when  it  is  heated  in  a 
current  of  carbon  dioxide  or  nitrogen  it  gradually  volatilizes, 
being  converted  into  cyanogen  gas. 

HYDROCYANIC  ACID,  OR  PRUSSIC  AciD,1  HCN. 

437  We  owe  the  discovery  of  hydrocyanic  acid  to  Scheele  in 
1782.  It  was  subsequently  examined  by  Ittner,2  and  by  Gay- 
Lussac ;  the  former  of  these  chemists  preparing  it  for  the  first 
time  in  the  anhydrous  condition.3  For  this  purpose  he  heated 
mercuric  cyanide  with  hydrochloric  acid  in  a  flask,  leading  the 
gas,  which  was  evolved,  through  a  long  tube,  half  filled  with 
pieces  of  marble  in  order  to  retain  any  vapours  of  hydrochloric 
acid  which  might  be  carried  over,  the  second  half  of  the  tube 
being  filled  with  chloride  of  calcium  for  the  purpose  of  absorbing 
any  aqueous  vapour.  The  gas,  thus  dried,  was  passed  into  a 
receiver  placed  in  a  freezing  mixture,  where  it  was  condensed. 
According  to  this  method  only  two-thirds  of  the  theoretical 
amount  of  acid  are  obtained,  as  the  mercuric  chloride  unites 
with  a  portion  of  the  prussic  acid  to  form  a  compound  which 
is  decomposed  only  at  a  high  temperature.  If,  however,  sal- 
ammoniac  be  added,  this  salt  combines  with  the  mercuric 
chloride,  and  the  whole  of  the  hydrocyanic  acid  is  set  free.4 

Pure  hydrocyanic  acid  is  also  obtained  by  leading  sulphu- 
retted hydrogen  gas  over  dry  mercuric  cyanide  heated  to  about 
30°.  The  salt  is  placed  in  a  long  horizontal  tube  of  which  the 
front  portion  is  filled  with  carbonate  of  lead  for  the  purpose  of 
retaining  the  excess  of  sulphuretted  hydrogen ;  as  soon  as  this 
begins  to  turn  black  the  evolution  of  sulphuretted  hydrogen  is 
stopped  (Vauquelin).5 

Hydrocyanic  acid  is  generally  prepared  from  ferrocyanide 
of  potassium,  which,  as  Scheele  showed,  is  decomposed  by  dilute 
sulphuric  acid.  A  process  described  by  Wohler  enables 

1  The  name  acide  prussique  was  first  used  by  Guyton  de  Morveau. 

2  Beitrage  z.  Geschichte  d.  Blausaure,  1809. 

3  Ann.  Uhim.  78    128,  and  95,  136. 

4  Bussy  and  Buignet,  Ann.  Ohim.  Phys.  [4],  3,  232.       5  Annalen,  73,  218. 


HYDROCYANIC  ACID 


753 


us  to  prepare  any  desired  quantity,  either  of  the  aqueous 
acid  of  different  strengths,  or  of  the  anhydrous  compound. 
A  cold  mixture  of  14  parts  of  water  and  7  parts  of  concen- 
trated sulphuric  acid  is  poured  upon  10  parts  of  coarsely 
powdered  ferrocyanide  of  potassium  contained  in  a  large  retort 
the  neck  of  which  is  placed  upwards  in  a  slanting  direction. 
If  the  anhydrous  acid  be  required,  the  vapour  is  allowed  to  pass 
through  cylinders  or  U-tubes  filled  with  calcium  chloride  and 
surrounded  by  water  heated  to  30°,  after  which  the  gas,  thus 
dried,  is  condensed  by  passing  into  a  receiver  surrounded  by  ice 
or  a  freezing  mixture.  To  prepare  the  aqueous  acid  the  drying 
apparatus  may  be  omitted  and  the  gas  evolved  from  a  flask  as 
shown  in  Fig.  201,  passed  through  a  Liebig's  condenser,  and  then 


FIG.  201. 

led  into  the  requisite  quantity  of  distilled  water.  The  action 
of  the  dilute  sulphuric  acid  upon  the  prussiate  of  potash 
is  represented  by  the  following  equation  : — 

2Fe(CN)6K4-!-3H2S04  =  3K2S04  +  Fe(CN)6K2Fe  +  6HCN. 

As  is  seen  from  this  equation,  only  half  of  the  cyanogen  con- 
tained in  the  ferrocyanide  is  obtained  as  hydrocyanic  acid,  but 
notwithstanding  this  loss  the  method  is  simpler  and  cheaper 
than  those  previously  described. 

Amongst  the  various  other  methods  which  have  been  described 
for  the  preparation  of  this  acid,  one  proposed  by  Clarke  may  be 
mentioned,  inasmuch  as  by  this  method  a  dilute  acid  may  readily 
be  prepared  having  an  approximately  known  strength.  For  this 

49 


754  THE  NON-METALLIC  ELEMENTS 

purpose  nine  parts  of  tartaric  acid  are  dissolved  in  sixty  parts  of 
water  and  the  solution  poured  into  a  iiask  so  that  it  is  nearly 
rilled.  Four  parts  of  potassium  cyanide  are  then  added,  and  the 
flask  closed  by  means  of  a  cork.  The  mixture  is  then  well 
shaken,  and  allowed  to  stand  for  some  time  until  the  whole  of 
the  cream  of  tartar  (acid  tartrate  of  potassium)  has  separated 
out,  after  which  the  dilute  hydrocyanic  acid  is  poured  off.  This 
should  contain  3*6  per  cent,  of  hydrocyanic  acid,  and  only  a 
small  amount  of  cream  of  tartar  in  solution  ;  the  reaction  being 
represented  by  the  following  equation  :— 


KCN  +  C4H606  =  HCN  +  C4Hr)KOc. 

Hydrocyanic  acid  is  also  formed  when  a  series  of  electric 
sparks  is  passed  through  a  mixture  of  acetylene  and  nitrogen  ;  L 
thus  :  — 

C2H2  +  N2  =  2HCN. 

It  is  also   obtained    by   the   passage   of  the   silent   discharge 
through  a  mixture  of  cyanogen  and  hydrogen.2 

438  Properties.  —  Pure  anhydrous  hydrocyanic  acid  is  a  colour- 
less very  mobile  liquid  possessing  a  characteristic  smell  resembling 
that  of  bitter  almonds,  and  producing  when  inhaled  in  small 
quantities  a  peculiar  irritation  of  the  throat.  Its  specific  gravity 
is  07058  at  7°  and  0'6969  at  18°  (Gay-Lussac)  ;  it  boils  at  261°, 
and  forms  a  colourless  vapour  which  possesses  the  specific 
gravity  0'947.  At  —  15°  the  liquid  solidifies  to  a  mass  of  colour- 
less feathery  crystals.  If  a  drop  of  the  liquid  acid  be  brought 
on  a  glass  rod  into  the  air,  evaporation  takes  place  so  quickly, 
and  so  much  heat  is  thereby  absorbed,  that  a  portion  of  the 
liquid  freezes.  Hydrocyanic  acid  is  miscible  in  all  proportions 
with  water,  alcohol,  and  ether.  A  very  singular  phenomenon  is 
observed  when  hydrocyanic  acid  is  mixed  with  water,  inasmuch 
as  a  diminution  of  temperature  occurs  without  any  increase  of 
the  volume  of  the  liquid  ;  on  the  contrary,  a  diminution  of 
volume  takes  place,  and  this  is  the  greatest  when  equal  volumes 
of  water  and  acid  are  mixed.  In  this  mixture  the  proportions 
correspond  to  the  relations,  3H2O  +  2HCN,  and,  in  this  case, 
the  diminution  in  bulk  which  occurs  is  from  100  to  94  (Bussy 
and  Buignet).  The  anhydrous  acid  as  well  as  its  concentrated 
aqueous  solution  is  easily  inflammable,  and  burns  with  a  beauti- 
ful violet  flame.  When  not  absolutely  pure,  it  decomposes  very 

1  Berthelot,  Com.pt.  Rend.  77,  1141.  2  Boillot,  Conipt.  Rend.  76,  1132. 


HYDKOCYANIC  ACID  755 

readily  on  exposure  to  light,  with  separation  of  a  brown  body, 
and  formation  of  ammonia,  whilst,  in  the  case  of  the  pure  dilute 
acid,  formic  acid,  ammonia,  and  other  bodies  are  produced.  The 
tendency  to  decomposition  is  much  diminished  by  the  presence 
of  traces  of  mineral  acid  or  of  formic  acid.  Concentrated  mineral 
acids,  as  well  as  boiling  alkalis,  decompose  hydrocyanic  acid  into 
ammonia  and  formic  acid,  water  being  taken  up  ;  thus  : — 

HCN  +  2H20  =  NH3  +  HO.COH. 

On  the  other  hand,  hydrocyanic  acid  and  water  are  formed  when 
the  ammonium  salt  of  formic  acid  is  quickly  heated. 

Pure  hydrocyanic  is  one  of  the  most  powerful  and  rapid  of 
known  poisons.  When  a  small  quantity  of  the  vapour  of  the 
pure  substance  is  drawn  into  the  lungs  instant  death  ensues  ; 
small  quantities  produce  headache,  giddiness,  nausea,  dyspnoea, 
and  palpitation. 

A  few  drops  brought  into  the  eye  of  a  dog  kill  it  in  thirty 
seconds,  whilst  an  internal  dose  of  0'05  grain  is  usually  sufficient 
to  produce  fatal  effects  upon  the  human  subject,  but  cases  have 
been  known  in  which  O'l  grain  has  been  taken  without  death 
ensuing.  As  antidotes,  ammonia  and  chlorine  water  have  been 
proposed,  and  these  appear  to  be  efficacious,  although  we  are 
unable  to  explain  their  mode  of  action,  for  ammonia  under 
ordinary  conditions  only  forms  ammonium  cyanide,  and  chlorine 
cyanogen  chloride,  both  of  which  are  bodies  as  poisonous  as 
prussic  acid  itself.  Prussic  acid  is  used  as  a  medicine,  and  is  a 
constituent  of  several  officinal  preparations,  such  as  laurel  water, 
bitter-almond  water,  &c.,  \\Iiich  are  obtained  by  distilling  the 
leaves  of  the  common  laurel,  or  bitter  almonds  with  water. 
These  plants  do  not  contain  the  prussic  acid  ready  formed ;  but, 
in  common  with  most  of  the  plants  of  the  same  family,  contain 
amygdalin,  a  complicated  compound  which,  under  certain  cir- 
cumstances, splits  up  into  sugar,  oil  of  bitter  almonds,  and 
prussic  acid. 

439  To  estimate  the  quantity  of  hydrocyanic  acid  contained 
in  these  preparations,  an  exce&s  of  potash  solution  is  added  to  a 
measured  or  weighed  quantity  of  the  liquid,  and  then  by  means 
of  a  burette  a  solution  of  nitrate  of  silver  containing  6 '3  gram, 
in  one  liter  is  dropped  in  until  a  permanent  precipitate  appears. 
Each  cc.  corresponds  to  two  milligrams  of  anhydrous  prussic  acid. 
In  this  reaction  the  double  cyanide,  AgCN  f  KCN,  is  formed, 
which  is  not  decomposed  by  alkalis,  and  is  soluble  in  water. 


756  THE  NON-METALLIC  ELEMENTS 

Hence  as  soon  as  exactly  half  the  quantity  of  prussic  acid 
present  is  converted  into  silver  cyanide,  one  drop  more  of 
the  silver  solution  will  produce  a  permanent  precipitate  of  silver 
cyanide.1 

To  detect  hydrocyanic  acid,  as  in  cases  of  poisoning,  the 
suspected  matter  is  acidulated  with  tartaric  acid,  and  the  prussic 
acid  distilled  otf  by  means  of  a  water-bath.  The  distillate  is 
made  alkaline  with  caustic  soda,  and  a  mixture  of  a  ferrous  and 
ferric  salt  (a  solution  of  ferrous  sulphate  oxidized  by  exposure 
to  the  air)  is  added,  and  then  an  excess  of  hydrochloric  acid. 
Prussian  blue  remains  undissolved  if  prussic  acid  be  present. 
If  the  quantity  contained  be  very  small,  the  solution  appears 
first  of  a  green  colour,  and,  on  standing,  deposits  dark  blue 
flakes.  When  dilute  prussic  acid  is  mixed  with  yellow  ammo- 
nium sulphide,  and  the  liquid  evaporated  to  dryness  over  a 
water-bath,  ammonium  thiocyanate  is  formed.  The  presence  of 
this  body  is  made  known  by  means  of  ferric  chloride,  which  pro- 
duces in  a  solution  of  thiocyanate  a  deep  blood-red  coloration.2 

THE  CYANIDES. 

440  Hydrocyanic  acid  turns  blue  litmus  paper  red,  but  it  is 
so  weak  an  acid  that  its  soluble  salts  are  decomposed  by  the 
carbonic  acid  of  the  air,  and  they  therefore  smell  of  hydrocyanic 
acid,  and  have  an  alkaline  reaction  even  when  their  solution 
contains  excess  of  hydrocyanic  acid. 

Cyanides  can  be  prepared  in  a  variety  of  ways.  Carbon  and 
nitrogen  do  not  unite  together  even  under  the  action  of  the 
electric  spark,  but  if  these  elements  are  heated  in  presence  of 
an  alkali,  a  cyanide  is  formed  Thus,  for  instance,  potassium 
cyanide  is  obtained  when  nitrogen  is  led  over  a  heated  mixture 
of  carbon  and  potash  : — 

N2  +  4C  +  K.2C03  =  2KCN  +  SCO. 

If,  instead  of  potash  caustic  baryta  is  taken,  barium  cyanide  is 
obtained : — 

N2  +  3C  +  BaO  =  Ba  (CN)2  +  CO. 

According  to  Kuhlman,  ammonium  cyanide  is  formed  when 
ammonia  is  passed  over  red-hot  charcoal : — 

2NH3  +  C  =  NH4CN  +  H9. 

1  Liebig,  Annalen.  71,  102.  2  Liebig,  Annalen,  61,  127. 


THE  CYANIDES  757 


Cyanides  are  also  easily  formed  when  nitrogenous  organic 
bodies  such  as  hoofs,  clippings  of  hides,  wool  and  blood,  are 
heated  with  an  alkali  such  as  potash.  The  potassium  cyanide 
thus  obtained  is  prepared  on  the  large  scale,  and  forms  the 
starting-point  of  the  cyanogen  compounds.  As  this  substance 
is  difficult  to  purify  on  account  of  its  great  solubility,  it  is  con- 
verted into  ferrocyanide  of  potassium  or  yellow  prussiate  of 
potash,  which  is  used  for  the  preparation  of  the  other  cyanogen 
compounds.  p 

The  cyanides  of  the  alkali  metals,  and  those  of  the  metals  of 
the  alkaline  earths,  are  soluble  in  water,  and  smell,  as  has 
been  already  stated,  of  hydrocyanic  acid,  as  they  are  decom- 
posed by  the  atmospheric  carbonic  acid.  They  are,  therefore^ 
just  as  poisonous  as  the  free  acid  itself.  The  cyanides  of  the 
other  metals,  with  the  exception  of  mercuric  cyanide,  are 
insoluble  in  water.  They  dissolve,  however,  in  the  cyanides 
of  the  alkali  metals,  with  the  formation  of  soluble  double 
cyanides.  These  double  cyanides  may  be  divided  into  two 
distinct  classes.  The  compounds  of  the  first  class  are  easily 
decomposed  by  dilute  acids  with  the  formation  of  an  insoluble 
metallic  cyanide,  and  free  hydrocyanic  acid ;  and  possess  all  the 
usual  properties  of  the  cyanides ;  of  these,  silver-potassium 
cyanide  and  nickel-potassium  cyanide  may  serve  as  examples. 

AgCN.KCN  +  HNO3  =  AgCN  +  HCN  +  KNO3. 
Ni(CN)2.  2KCN  +  2HC1  =  Ni(CN)2  +  2HCN  +  2KC1. 

.  The  double  cyanides  of  the  second  class,  although  containing, 
like  the  others,  two  different  metals,  possess  wholly  distinct 
properties.  The  best  known  of  these  are  the  yellow  prussiate 
of  potash  or  ferrocyanide  of  potassium,  Fe(CN)2  4KCN,  and 
the  red  prussiate  of  potash  or  ferricyanide  of  potassium,  Fe(CN)3 
3KCN.  If  to  a  solution  of  the  first  of  these  salts  dilute  hydro- 
chloric acid  be  added,  no  hydrocyanic  acid  is  evolved,  but  a 
wrhite  crystalline  precipitate  is  obtained  having  the  composition 
H4Fe(CN)6.  This  compound  is  a  powerful  acid,  to  which  the 
name  of  fcrrocyanic  acid  has  been  given,  and  in  which  the  four 
atoms  of  hydrogen  can  be  either  partially  or  wholly  replaced  by 
metals.  In  the  same  way  the  potassium  can  also  be  replaced 
in  the  red  prussiate  of  potash  by  hydrogen,  and  thus  fcrricyanic 
acid,  H3Fe(CN)6,  can  be  obtained. 

Similar  double  compounds  are  known,  especially  those  of 
cobalt,  manganese,  platinum,  and  of  the  metals  allied  to  it. 


758  THE  NON-METALLIC  ELEMENTS 

In  these  compounds  the  second  metal  does  not  act  as  a  base,  but 
has  combined  with  the  cyanogen  to  form  an  acid  radical  (for 
example,  Fe(CN)6),  which  then  unites  with  hydrogen  and 
metals  in  the  same  manner  as  other  acid  radicals  such  as 
chlorine.  (See  also  Vol.  II.,  under  Iron.) 

441  Detection  and  determination  of  cyanogen  and  the  cyanides. — 
The  detection  of  cyanogen  in  the  cyanides,  such  as  those  of  the 
alkalis  and  alkaline  earths,  which  are  easily  decomposed  by 
dilute  acid,  is  simple  enough.  If  the  solution  is  not  already 
alkaline,  a  solution  of  caustic  soda  is  added,  and  next,  a  few  drops 
of  a  solution  of  ferrous  sulphate  (green  vitriol)  which  has  been 
partially  oxidized  by  exposure  to  the  air ;  an  excess  of  hydro- 
chloric acid  is  then  added,  when  a  precipitate  of  Prussian  blue 
is  thrown  down.  In  this  reaction  the  ferrous  salt  in  the 
presence  of  caustic  soda  and  a  cyanide  gives  rise  to  the  form- 
ation of  sodium  ferrocyanide,  which  then  undergoes  the  usual 
reaction  with  the  ferric  salt  which  is  present.  Cyanides,  which 
do  not  evolve  hydrocyanic  acid  on  the  addition  of  a  dilute  acid, 
may  be  decomposed  by  fusing  them  with  dry  carbonate  of  soda. 
The  fused  mass  is  then  dissolved  in  water,  and  the  filtered 
liquid  treated  according  to  the  above-mentioned  process. 

In  the  soluble  cyanides,  as  well  as  in  those  which  are  de- 
composed by  dilute  acids,  the  quantity  of  cyanogen  can  readily 
be  determined  by  precipitation  with  nitrate  of  silver.  A  precipi- 
tate of  cyanide  of  silver  is  obtained,  which,  after  washing  and 
drying  at  110°,  is  weighed.  This  method,  however,  cannot  be 
employed  in  the  cases  of  mercuric  cyanide,  potassium  ferro- 
cyanide, and  analogous  substances,  in  which  cases  it  is  necessary 
to  obtain  hydrocyanic  acid  from  the  compound  by  some  ap- 
propriate method,  such  as  treatment  with  sulphuretted  hydrogen, 
distillation  with  dilute  sulphuric  acid,  &c.  Concerning  the 
further  details  of  the  qualitative  and  quantitative  analysis  of 
such  compounds  we  must  refer  to  treatises  on  analytical 
chemistry. 


COMPOUNDS  OF  HYDROCYANIC  ACID  WITH  THE  HYDRACIDS  OF 
THE  CHLORINE  GROUP  OF  ELEMENTS. 

442  Although  hydrocyanic  acid  possesses  weak  acid  properties, 
yet  it  behaves  in  many  respects  as  a  weak  base  which,  like 
ammonia,  is  capable  of  direct  union  with  certain  acids.  This 
property  depends  upon  the  fact  that  its  chemjpal  constitution  is 


CYANOGEN  CHLORIDE  759 


analogous  to  that  of  ammonia,  for  hydrocyanic  acid  is  formed 
when  ammonia  is  heated  with  chloroform  under  pressure; 
thus  : — 

CHC13  +  4NH3  =  NCH  +  3NH4C1. 

According  to  this  mode  of  formation,  hydrocyanic  acid  may 
be  considered  to  be  ammonia  in  which  the  three  atoms  of 
hydrogen  have  been  replaced  by  the  triad  radical  methenyl  OH, 
whence  it  might  be  termed  methenylamine. 

Hydrocyanic  acid  forms  two  compounds  with  hydrochloric 
acid.  The  monohydrochloridc  HCN,HC1  is  prepared  by  passing 
hydrogen  chloride  into  anhydrous  hydrocyanic  acid  at  15°  and 
subsequent  warming  in  a  sealed  tube  to  35-40°.  It  is  a 
crystalline  very  hygroscopic  substance,  which  is  decomposed 
by  water  into  ammonium  chloride  and  formic  acid.1  The 
sesquihydrochloride  2HCN,3HC1  is  obtained  by  passing  hydrogen 
chloride  into  a  mixture  of  anhydrous  hydrocyanic  acid  and 
ethyl  acetate,  and  forms  a  very  hygroscopic  crystalline  mass, 
which  yields  with  water  the  same  products  as  the  previous 
compound.2  Similar  derivatives  have  been  obtained  with 
hydrobromic  acid,  having  the  composition  HCN,HBr  and 
2HCN,3HBr  respectively.  The  hydriodide  HCN,HI  has  also 
been  prepared. 

Hydrocyanic  acid  also  combines  with  metallic  chlorides  to 
form  crystalline  compounds,  as 

SbCl5  +  3HCN,  SnCl4  +  2HCN  and  TiCl4  -I-  2HCN. 


COMPOUNDS  OF   CYANOGEN   WITH   THE    ELEMENTS    OF   THE 
CHLORINE  GROUP. 

CYANOGEN  CHLORIDE,  C1CN  =  61-04. 

443  Berthollet  obtained  this  compound  in  1787  by  the  action 
of  chlorine  on  hydrocyanic  acid,  and  believed  it  to  be  oxy- 
genated hydrocyanic  acid.  Its  true  composition  was  recognised 
by  Gay-Lussac  in  the  year  1815.  According  to  Wohler3this 
compound  is  easily  obtained  by  passing  chlorine  gas  into  a 
saturated  aqueous  solution  of  mercuric  cyanide  containing  some 
of  the  solid  salt,  until  the  liquid  is  saturated  with  the  gas,  and 
the  air  which  remains  above  the  liquid  is  replaced  by  chlorine. 

1  Gautier,  Ann.  Chim.  Phys.  [4],  17,  129. 

2  Claisen  and  Matthews,  Ber.  16,  309.  3  Ann.  Chim.  Phys.  95?  200. 


760  THE  NON-METALLIC  ELEMENTS 

The  vessel  is  then  well  closed  and  placed  in  a  dark  room  till  all 
the  mercuric  chloride  has  dissolved,  or  all  the  chlorine  has 
combined.  In  order  to  remove  any  free  chlorine  which  may 
remain  the  solution  is  shaken  up  with  mercury,  and  the  liquid 
heated  in  order  to  expel  the  cyanogen  chloride,  the  volatile 
product  being  condensed  by  passing  through  a  bent  tube  plunged 
into  a  freezing  mixture. 

According  to  Gautier l  the  liquid  chloride  is  best  obtained  by 
leading  chlorine  into  a  mixture  of  one  part  of  hydrocyanic  acid 
and  four  parts  of  water  contained  in  a  retort  or  flask  which  is 
connected  with  an  inverted  Liebig's  condenser.  As  soon  as 
the  liquid  has  attained  a  green  colour,  the  current  of  chlorine 
is  stopped,  and  an  excess  of  mercuric  oxide  and  calcium  chloride 
is  added  to  the  liquid,  which  is  well  cooled  down  in  a  freezing 
mixture.  The  chloride  of  cyanogen  is  next  distilled  off  and 
condensed  in  a  well-cooled  receiver. 

At  the  ordinary  temperature  cyanogen  chloride  is  a  gas  which 
has  a  very  penetrating  odour  and  causes  a  copious  flow  of  tears ; 
it  is  very  readily  condensed  to  a  liquid,  and  freezes  at  —  18°, 
forming  colourless  prisms,  which  according  to  Regnault 2  melt 
at  —  7°,  the  liquid  formed  boiling  at  127°,  whilst  according  to 
Wurtz3  it  melts  at  — 12  to  — 15°  and  boils  at  15*5°.  Its  vapour 
density  is  2*13.4  When  allowed  to  stand  it  is  partially 
converted  into  the  polymeric  cyanuric  chloride  C3N3C13  (p.  766). 


CYANOGEN  BROMIDE,  CNJBr. 

444  This  compound,  which  was  discovered  in  the  year  1827 
by  Serullas,5  is  formed  by  the  action  of  bromine  on  hydrocyanic 
acid,  or  on  the  metallic  cyanides.  If  bromine  is  added  drop  by  drop 
to  a  well-cooled  aqueous  solution  of  potassium  cyanide,  crystals 
separate  out,  which  consist  of  a  mixture  of  cyanogen  bromide 
and  potassium  bromide.6  When  these  crystals  are  heated  to  a 
temperature  of  from  60°  to  65°,  cyanogen  bromide  sublimes  in 
the  form  of  delicate  transparent  prisms,  which  soon  pass  into 
the  cubical  form.  It  melts  at  52°  and  boils  at  61'3°  under 
750  mm.  pressure,7  has  an  exceedingly  pungent  smell,  and  acts 
very  powerfully  upon  the  eyes.  It  is  also  poisonous ;  one  grain 

1  Bull.  Soe.  Chim.  [2],  403         2  Jahresb.  1863,  70.         3  Annalen,  79,  284. 
*  Bull.  Soc.  Chim.  [2],  4,  105.  5  Ann   Chim.  Phys.  34,  100. 

6  Langlois,  Ann.  Chim.  Phys.  [3],  61,  482. 

7  Mulder,  Bee.  Trav.  Chim.  4,  151  ;  5,  85. 


CYANIC  ACID  761 


dissolved  in  a  small  quantity  of  water  brought  into  the  oesophagus 
of  a  rabbit  produced  instant  death  (Serullas).  According  to 
Bineau  the  specific  gravity  of  its  vapour  is  3'607.  When  not 
absolutely  pure  cyanogen  bromide  is  converted  at  130°  to  140° 
into  cyanuric  bromide,  CgNgBrg.1 


CYANOGEN  IODIDE,  CNI. 

445  This  compound    was    discovered  by  Davy  in  the   year 
1816,2  and  is  easily  formed  by  the  action  of  iodine  on  cyanide 
of  mercury  and  other  metallic  cyanides,   whilst  it   frequently 
occurs  as   an   impurity   in   commercial   iodine.     According   to 
Liebig  it  can  be  best  obtained  by  dissolving  iodine  in  a  warm 
concentrated  solution    of   potassium  cyanide   until  the    liquid 
solidifies  on  cooling  to  a  crystalline  mass.      Cyanogen  iodide 
forms  long  delicate  white  needles,  easily  soluble  in  alcohol  and 
ether,  and  from  these  solutions  the  compound   crystallizes  in 
four-sided  tables.     It  is  sparingly  soluble  in  water.     It  melts 
at  146'5°,  but  begins  to  volatilize  at  a  much  lower  temperature. 
It  has  a  smell  similar  to  that  of  the   bromide,  and  is  equally 
poisonous. 

CYANIC  ACID,  NCOH. 

446  Cyanic  acid  was  first  observed  by  Vauquelin  in  the  year 
1818,  although  it  was  first  distinctly  recognised  as  a  peculiar 
acid  and  its  properties  more  exactly  investigated  by  Wohler  in 
1822.     Its  salts  are  formed  when  cyanogen  gas  is  led  into  an 
alkali;  thus:  — 


When  potassium  cyanide  is  melted  in  the  presence  of  air,  or, 
better,  with  the  addition  of  an  easily  reducible  oxide  or  peroxide, 
potassium  cyanate  is  formed. 

The  most  convenient  method  of  preparing  the  cyanates  is  by 
the  action  of  potassium  bichromate  on  potassium  ferrocyanide. 
For  this  purpose  200  grams  of  completely  dehydrated  potassium 
ferrocyanide  are  mixed  whilst  still  warm  with  150  grams  of 
fused  potassium  bichromate,  and  the  whole  heated  in  an  iron 
dish.  The  black  product  is  then  quickly  extracted  with  a 

1  Eghis,  Zeitsch.  Chem.  [2],  5,  376  ;  Ponamarew,  Ber.  18,  3261. 

2  Gilb.  Ann.  54,  384. 


762  THE  NON-METALLIC  ELEMENTS 

mixture  of  800  cc.  of  80  per  cent,  etrryl  alcohol  and  100  cc.  of 
methyl  alcohol,  potassium  cyanate  separating  from  the  nitrate 
on  cooling.1 

Cyanates  are  readily  decomposed  by  dilute  acids,  but  the 
cyanic  acid  at  the  same  time  takes  up  water,  carbon  dioxide 
and  ammonia  being  formed.  Therefore  on  adding  a  dilute  acid 
to  potassium  cyanate  effervescence  takes  place,  carbon  dioxide 
being  given  off.  This  has  a  pungent  smell,  due  to  the  presence 
of  a  trace  of  cyanic  acid.  By  acting  on  a  cyanate  with  dry 
hydrochloric  acid,  cyanic  acid  is  set  free.  It  at  once  combines, 
however,  with  hydrochloric  acid  to  form  the  compound  HC1 
NCOH,  a  colourless  liquid,  fuming  in  the  air.  It  is  impossible 
to  isolate  cyanic  acid  from  this  compound,  because  it  at  once 
changes  into  the  polymeric  cyanuric  acid  C3N3(OH)3  (p.  766). 

The  only  reaction  by  which  cyanic  acid  can  be  obtained 
is  by  the  decomposition  of  cyanuric  acid  by  heat,  when  it 
is  resolved  into  three  molecules  of  cyanic  acid,  the  vapours  of 
which  must  be  condensed  by  means  of  a  freezing  mixture. 
Cyanic  acid  is  a  colourless  mobile  liquid  having  a  most  pungent 
smell.  On  taking  the  vessel  containing  it  out  of  the  freezing 
mixture  the  liquid  soon  becomes  turbid  and  hot,  and  with  a 
crackling  noise,  or  if  in  large  quantity  with  explosive  ebulli- 
tion, is  soon  converted  into  a  white  porcelain-like  mass  which  is 
a  polymeric  modification,  called  cyamelidc,  the  molecular  weight 
of  which  is  unknown.  On  heating  cyamelide  it  is  reconverted 
into  cyanic  acid. 

Of  the  cyanates  the  most  interesting  salt  is  the  ammonium 
cyanate,  NCO(NH4),  obtained  as  a  white  crystalline  mass  by 
mixing  the  vapour  of  dry  cyanic  acid  with  dry  ammonia.  The 
freshly  prepared  aqueous  solution  gives  the  reactions  of  a  cyanate 
and  of  ammonia,  but  on  standing  for  some  time,  or  on  heat- 
ing, the  ammonium  cyanate  is  transformed  into  the  isomeric 
carbamide  or  urea,  CO(NH2)2.  The  dry  salt  also  undergoes  the 
same  transformation  on  heating,  and  the  change  may  also  be 
shown  by  heating  a  solution  of  potassium  cyanate  to  which  an 
equivalent  quantity  of  ammonium  sulphate  has  been  added. 

THIOCYANIC  ACID,  NCSH. 

447  This  acid,  to   which  the  name  of  sidphocyanic  acid  has 
also  been  given,  was  mentioned  by  Bucholz  in  1798,  but  first 
1  Bell,  Glum.  News,  32,  49  ;  Erdmann,  Ber.  26,  2438. 


THIOCYAX1C  ACID  763 


carefully  examined  by  Porret  in  1808.  He  obtained  its  potas- 
sium salt  by  boiling  a  solution  of  potassium  sulphide  with 
Prussian  blue.  Its  quantitative  composition  was  ascertained 
by  Berzelius  in  1820.1 

Salts  of  this  acid  are  formed  by  the  direct  union  of  sulphur 
with  a  cyanide.  Thus,  for  instance,  potassium  thiocyanate  is 
easily  obtained  by  melting  together  potassium  cyanide  and 
sulphur  or  dried  potassium  ferrocyanide  with  potassium 
carbonate  and  sulphur.  The  ammonium  salt  is  obtained  by 
warming  hydrocyanic  acid  with  yellow  sulphide  of  ammonium  ; 
thus  :  — 

4    S*  +  HCN  = 


Ammonium  thiocyanate  is  easily  obtained  in  large  quantities 
by  warming  a  mixture  of  alcoholic  concentrated  ammonia  and 
bisulphide  of  carbon  which  has  been  allowed  to  stand  for  a  con- 
siderable time.  Ammonium  thiocarbonate  and  thiocarbamate 
are  formed,  and  these  are  decomposed  on  heating,  with  evolu- 
tion of  sulphuretted  hydrogen  (Millon)  ;  thus  :  — 


4  = 


CS 


For  this  purpose  it  is  best  to  take  350  —  400  grams  of  carbon 
bisulphide,  600  grams  of  95  per.  cent,  alcohol,  and  800  grams 
of  ammonia  (sp.gr.  0'912).  As  soon  as  the  carbon  bisulphide 
has  dissolved,  the  solution  is  concentrated,  and  on  cooling  the 
salt  separates  out  in  crystals.2  If  a  solution  of  mercuric  nitrate 
be  added  to  a  solution  of  ammonium  or  potassium  thiocyanate, 
a  heavy  white  precipitate  of  mercuric  thiocyanate,  (CNS).,Hg, 
is  thrown  down.  This  substance  when  dry  is  easily  inflam- 
mable, and  burns  with  a  pale  sulphur  flame,  leaving  a  very 
voluminous  residue  behind.  This  salt  is  used  for  the  prepara- 
tion of  the  so-called  Pharaoh's  Serpents. 

Aqueous  thiocyanic  acid  may  be  readily  obtained  by  decom- 
posing barium  thiocyanate  with  dilute  sulphuric  acid.  This 
solution  may  be  distilled  under  diminished  pressure,  the  acid 
coming  over  with  the  first  portion,  and  if  dried  over  calcium 
chloride  forms  a  yellowish  oil  which  soon  decomposes.3  On 

1  Schweig.  Journ.  31,  42.  2  Claus,  Annalen,  179,  112. 

3  Klason,  J.  Pr.  Chem.  [2],  36,  57. 


764  THE  NON-METALLIC  ELEMENTS 

boiling  the  aqueous  solution,  the  greater  part  of  the  acid  is 
converted  with  absorption  of  water  into  carbonyl  sulphide 
and  ammonia,  whilst  at  the  same  time  some  hydrocyanic 
acid  is  formed.  Thiocyanic  acid  and  its  soluble  salts  are 
coloured  blood-red  by  ferric  chloride,  ferric  thiocyanate  being 
formed.  The  soluble  thiocyanates  produce  with  silver  nitrate  a 
precipitate  of  silver  thiocyanate  which  is  insoluble  even  in  presence 
of  the  stronger  acids.  A  solution  of  potassium  or  ammonium 
thiocyanate  of  known  strength  is,  therefore,  used  for  the 
purpose  of  determining  the  quantity  of  silver  contained  in 
a  solution,  by  adding  some  nitric  acid  and  ferric  sulphate  and 
then  dropping  in  a  standard  solution  of  ammonium  thiocyanate 
from  a  burette  until  the  formation  of  a  slight  red  colour  shows 
that  all  the  silver  has  been  thrown  down.1 


CYANOGEN  SULPHIDE,  OR  THIOCYANIC  ANHYDRIDE,  (CN)2S. 

448  In  order  to  prepare  this  compound,  silver  thiocyanate  is 
added  to  an  ethereal  solution  of  iodide  of  cyanogen,  the  solution 
evaporated,  and  the  residue  treated  with  hot  carbon  bisulphide.2 
If  the  solution  be  cooled  to  0°,  cyanogen  sulphide  separates 
in  rhombic  tables,  which  possess  a  smell  like  that  of  cyanogen 
iodide.  They  melt  at  60°,  but  when  heated  above  30°  they 
begin  to  volatilize.  They  are  very  soluble  in  water,  alcohol, 
and  ether,  and  are  decomposed  by  caustic  alkalis  as  follows : — 

K)0     CN)S  ,CN\0     .... 
H/(         Kj"k         KjC      H2°' 

Cyanogen  Selenide,  (CN)2Se,  is  obtained  by  adding  dry  silver 
cyanide  to  a  solution  of  selenium  bromide  in  bisulphide  of 
carbon.  It  crystallizes  in  tables,  which  are  decomposed  by  water 
into  selenium,  selenious  acid,  and  hydrocyanic  acid.3 


CYANAMIDE,  CN.NH2. 

449  This  body  was  discovered  by  Bineau,4  who  prepared  it 
by  the  action  of  dry  ammonia  on  gaseous  cyanogen  chloride. 
The  solid  mass  thus  obtained  was  supposed  by  him  to  be  the 

1  Volhard,  J.  Pr.  Chem.  [2],  9,  217.          2  Linnemann,  Annalen,  70,  36. 

3  Schneider,  Pogg.   Ann.  129,  364. 

4  Ann.  Chim.  Phys.  [2],  67,  368,  and  70,  251. 


CYANAMIDE  765 


chloride  of  cyanammonium,  C1CN(NH3),  but  Cloez  and  Can- 
nizzaro1  showed  in  1851  that  this  substance  consists  of  a 
mixture  of  ammonium  chloride  and  cyanamide : — 


C  TT 

2  N-^H 

(H 


H 


Cyanamide  is  most  readily  obtained,  however,  by  the  action  of 
mercuric  oxide  upon  a  cold,  not  too  dilute,  solution  of  thio- 
urea 2 : — 

CS(NH2)2  +  HgO  =  CN(NH2)  +  HgS  +  H2O. 

The  mercuric  oxide  must  be  very  finely  levigated,  or  a  dense 
precipitated  oxide  may  be  employed  ;  this  is  suspended  in  water 
and  gradually  added  to  the  solution  of  thio-urea ;  an  excess  of 
the  oxide  must  most  carefully  be  avoided.  The  end  of  the 
reaction  may  be  easily  recognised,  a  drop  of  the  solution  being 
brought  on  to  filter  paper,  and  then  a  drop  of  ammoniacal 
silver  solution  added ;  this  produces  a  black  spot  as  long 
as  thio-urea  is  present.  As  soon  as  all  the  thio-urea  has 
been  decomposed  the  solution  is  filtered  and  evaporated  down 
on  a  water-bath,  and  the  residue  treated  with  ether ;  on  evapora- 
tion of  the  ethereal  solution,  pure  cyanamide  is  left  behind.  It 
forms  small  colourless  crystals,  melting  at  40°,  which  deliquesce 
on  exposure  to  the  air,  and  are  volatile  with  steam.  If  a  few 
drops  of  nitric  acid  be  added  to  its  solution,  cyanamide  is  trans- 
formed into  urea : — 

CN.NH2  +  H20  =CO(NH2)S. 

Cyanamide  acts  as  a  weak  base;  the  hydrochloride  CN.NH2, 
2HC1  is  obtained  as  an  easily  crystallizable  powder  when 
hydrochloric  acid  gas  is  passed  into  a  solution  of  cyanamide 
in  anhydrous  ether  (Drechsel).  The  hydrogen  of  the  cyanamide 
may  also  be  replaced  by  metals ;  thus,  for  instance,  if  an  am- 
moniacal silver  solution  be  added  to  an  aqueous  solution  of 
cyanamide,  an  amorphous  yellow  precipitate,  CN.NAg2,  is 
thrown  down,  which  crystallizes  from  hot  ammonia  in  micro- 
scopic needles.  In  the  same  way,  if  one  part  of  sodium  be 
dissolved  in  15  parts  of  absolute  alcohol,  and  to  the  cold 

1  Compt.  Rend.  32,  62. 

2  Volhard,  J.  Pr.  Chem.  [2],  9,  6  ;  and  Drechsel,  J.  Pr.  Chem.  [2],  9,  284. 


766  THE  NON-METALLIC  ELEMENTS 

liquid  an  alcoholic  solution  of  cyanamide  be  added,  a  light 
crystalline  powder  falls  down,  possessing,  according  to  Drechsel, 
the  composition  CN.NHNa. 

When  cyanamide  is  strongly  heated  it  undergoes  polymerisa- 
tion, first  forming  dicyanamide,  C2N4H4,  and  finally  cyanuramide, 
C3N6IV 

POLYMERISATION  OF  CYANOGEN  COMPOUNDS. 

450  Mention  has  frequently  been  made  of  the  fact  that  many 
of  the  cyanogen  derivatives  readily  undergo  polymerisation — that 
is,  are  converted  into  compounds  having  the  same  percentage 
composition  but  a  higher   molecular  weight.     Thus  cyanogen 
chloride  readily  passes  into  cyanuric  chloride  C3N3C13,  cyanic 
acid   into  cyannric  acid  C3N3(OH)3,  and  cyanamide  into  dicy- 
anamide C2N4H4  and  cyanuramide  C3N6H6.     These  compounds 
have  very  different  properties  from  the  cyanogen   derivatives 
from  which  they  are  produced,   and   contain   in   most  cases  a 
nucleus  of  carbon  and  nitrogen  atoms  united  together  in  such  a 
manner  as  to  form  a  closed   chain.     Thus  the  cyanuric  com- 

/N-°\ 

pounds  all  contain  the  nucleus  C\  ;N,  and  from  them 

\N— C/ 

other  substances  containing  the  same  nucleus  can  be  prepared. 
A  very  large  number  of  compounds  containing  similar  closed 
chains  of  carbon  and  nitrogen  atoms  are  now  known,  and  they 
form  a  very  important  division  of  the  carbon  compounds  usually 
considered  under  the  head  of  Organic  Chemistry;  the  poly- 
merised cyanogen  compounds  will  therefore  be  more  fully 
treated  of  together  with  these  in  a  later  volume. 

GUANIDINE,   CH5N3. 

451  This  powerful  base  was  first  obtained  by  Strecker2  by 
acting  with  potassium  chlorate  and  hydrochloric  acid  on  guanine, 
C5H5N5O,  a  compound  contained  in  guano.     It  can  also  be  ob- 
tained by  various  other  reactions.     For  instance,  it  is  produced 
by  heating  to  100°  chloropicrin  (nitrochloroform),  CC13NO.,,  a 
substitution  product  of  marsh  gas    with  an  alcoholic  solution 
of  ammonia ; 3  thus  : — 

CC13N02  4-  3NH3  -  CH5N3.HC1  +  2HC1  +  HN02. 

1  Annalen,  122,  22.  2  Annalen,  H8,  151. 

8  Hofmann,  Zeitsch.  Chem.  [2],  2,  1073,  and  4,  721. 


GUANIDINE  767 


As  an  excess  of  ammonia  must  be  used,  no  nitrous  acid  is 
liberated,  water  and  free  nitrogen  being  formed  ;  for  this  reason 
it  is  necessary  to  use  strong  glass  tubes  in  order  to  withstand 
the  accumulated  pressure. 

Hydrochloride  of  guanidine  is  also  formed  when  an  alcoholic 
solution  of  cyanamide  is  heated  with  sal-ammoniac1 : — 

CN2H2  +  NH4C1  =  CN8H801. 

It  is,  however,  not  necessary  to  use  pure  cyannmide,  but  cyano- 
gen chloride  may  be  heated  with  alcoholic  ammonia,  or,  still 
better,  cyanogen  iodide  may  be  employed,  as  this  latter  forms 
iodide  of  ammonium,  which  is  easily  soluble  in  alcohol.2  When 
ammonia  acts  upon  carbonyl  chloride  urea  is  formed,  and  at 
the  same  time  small  quantities  of  cyanuric  acid,  ammelide,  and 
guanidine.3 

The  best  method,  however,  of  preparing  guanidine  is  from 
ammonium  thiocyanate,4  which  must  be  heated  for  twenty 
hours  to  a  temperature  of  190°,  when  the  thio-urea  which 
is  formed  decomposes,  with  evolution  of  sulphuretted  hydrogen 
into  cyanamide,  and  this  is  decomposed  by  the  ammonium 
thiocyanate  still  present ;  thus  : — 

CN(NH2)  +  CN  SNH4  =  CNH(NH2)2.CNSH. 

A  residue  of  guanidine  thiocyanate  is  obtained,  whilst  large 
quantities  of  ammonium  thiocarbonate  sublime. 

Other  guanidine  salts  can  be  obtained  from  the  thiocyanate  as 
well  as  the  free  base.  In  order  to  prepare  these,  100  grams  of 
the  salt  are  dissolved  in  lukewarm  water,  and  to  the  concentrated 
solution  58  grams  of  pure  potassium  carbonate  are  added,  the 
solution  evaporated  to  dryness,  and  the  residue  treated  with 
alcohol  in  order  to  dissolve  the  potassium  thiocyanate,  whilst 
carbonate  of  guanidine,  (CH5N3)2.CO3H2,  remains  behind,  and 
can  be  purified  by  recrystallization  from  aqueous  solution.  The 
carbonate  yields,  on  treatment  with  acids,  the  other  guanidine 
salts,  of  which  the  nitrate,  CH5N3.NO3H,  crystallizing  in  tablets, 
is  remarkable  from  its  slight  solubility  in  water.  The  sulphate 
forms  crystals  tolerably  soluble  in  water,  and  yields  the  free 
base  when  treated  with  baryta-water ;  the  solution  may  then 
be  evaporated  in  a  vacuum,  and  the  base  obtained  in  the  form 

1  Erlenmeyer,  Zeitsch   Chcm.  [2],  7,  28.  2  Bannow,  Bcr:  4,  161. 

3  Bouchardat,  Compt.  Rend.  69,  961. 

4  Volhard,  J.  Pr.  Chem.  [2],  9,  6,  and  Delitsch,  ibid.  1. 


768  THE  NON-METALLIC  ELEMENTS 

of  a  crystalline  mass  having  a  caustic  taste  and  strongly 
alkaline  reaction,  which  on  exposure  to  the  air  quickly  absorbs 
moisture  and  carbonic  acid.  When  guanidine  is  heated  with 
dilute  sulphuric  acid,  it  is  resolved  into  urea  and  ammonia ; 
thus : — 

C(NH)(NH2)2  +  H20  =  NH3  +  CO(NH2)2. 

A  cold  solution  of  potash  decomposes  guanidine  thiocyanate  in 
a  similar  way ;  thus  : — 

CNH(NH2)2'NSCH+KOH  =  CO(NH2)24-NCSK  +  NH3. 

These  reactions,  as  well  as  all  the  modes  of  formation  of 
guanidine,  show  that  it  possesses  the  following  constitution: — 

NH2 

I 
C  =  NH 

NH2. 

Nitroguanidine,  NH :  C(NH2)NH.NO2,  is  obtained  by  adding 
guanidine  thiocyanate  to  concentrated  sulphuric  acid,  then  mixing 
with  fuming  sulphuric  acid,  and  after  cooling  adding  fuming  nitric 
acid  to  the  solution.  When  the  resulting  liquid  is  poured  into 
a  large  volume  of  water,  nitroguanidine  separates  out,  and  after 
recrystallization  forms  needles  which  melt  at  230°,  with  evolu- 
tion of  ammonia.1 

Amidoguanidine,  NH :  C(NH2)NH.NH2,  is  prepared  by  the 
careful  reduction  of  the  preceding  compound  with  zinc  dust  and 
the  theoretical  quantity  of  dilute  acetic  acid,  the  whole  being 
carefully  cooled.  It  is  a  crystalline  compound  soluble  in  water, 
and  is  converted  by  boiling  with  acids  or  alkalis  into  hydrazine 
(p.  470),  for  the  preparation  of  which  it  is  used. 


PHOSPHORUS  TRICYANIDE,  P(CN)3. 

452  In  order  to  prepare  this  compound  a  mixture  of  phos- 
phorus trichloride  and  dry  silver  cyanide  is  heated  in  closed 
tubes  for  several  hours  to  a  temperature  of  120°-140°.  When 
cold  the  tubes  are  opened,  the  excess  of  phosphorus  trichloride 
driven  off  by  moderate  heating,  and  the  residue  brought  into 
a  tubulated  retort,  and  heated  in  an  oil-bath  to  130°-140°  in  a 
1  Thiele,  Annalen,  270,  1- 


X 

: 

ARSENIC  TRICYANIl)Ev.v  769 

stream  of  carbon  dioxide.  The  phosphorus  tricyanide  sublimes 
in  long  glistening  white  needles  or  thick  plates.  It  takes  fire 
when  slightly  warmed,  and  burns  in  the  air  with  a  bright  white 
flame ;  it  is  decomposed  by  water  into  hydrocyanic  and  phos- 
phorous acids ; 1  thus : — 

P(CN)3  +  3H2O  =  P(OH)3  +  3HCN. 


ARSENIC   TRICYANIDE,  As(CN)3. 

This  substance  is  obtained  by  the  action  of  finely  divided 
arsenic  on  cyanogen  iodide  in  presence  of  carbon  bisulphide. 
It  forms  yellowish  microscopic  crystals,  and  is  rapidly  attacked 
by  atmospheric  moisture,  and  almost  instantaneously  by  water, 
with  formation  of  arsenious  and  hydrocyanic  acids.2 

BORON  CARBIDE  OR  CARBON  BORIDE,  BC  OR  B2C2. 

This  is  prepared  by  heating  a  mixture  of  boric  anhydride  and 
carbon  in  an  electrical  furnace,  and  forms  a  graphite-like  powder 
which  melts  at  a  very  high  temperature,  burns  with  difficulty  in 
oxygen,  is  insoluble  in  the  usual  solvents,  but  is  decomposed  by 
fusion  with  alkalis.3 

Another  boride  of  carbon,  having  the  composition  CB6,  is 
prepared  by  heating  sixty-six  parts  of  amorphous  boron  and 
twelve  parts  of  carbon,  obtained  from  sugar,  in  an  electrical 
furnace,  or  by  dissolving  the  two  elements  in  iron,  copper,  or 
silver  at  a  very  high  temperature,  and  removing  the  metal  by 
treatment  with  aqua  regia.  It  forms  very  hard  black  lustrous 
crystals,  having  a  density  of  2*41,  which  are  attacked  by  chlorine 
below  1,000°,  and  by  oxygen  very  slowly  at  that  temperature ; 
like  the  previous  compound,  it  is  decomposed  by  fusion  with 
alkalis.  The  powder  is  sufficiently  hard  to  cut  the  diamond, 
but  more  slowly  than  diamond  dust.* 

COAL-GAS. 

453  It  was  noticed  so  long  ago  as  1726  that  when  coal  is 
heated  in  a  closed  vessel  an  inflammable  gas  is  evolved.  In  that 
year  Stephen  Hales  published  his  Vegetable  Staticks,  in  which 

1  Wehrhane  and  Hiibner,  Annalen,  128,  254,  and  132,  277. 

2  Guenez,  Compt.  Rend.  114,  1186. 

3  Miihlhausen,  Zeit.  Anorg.  Chem.  5,  92.     4  Moissan,  Compt.  Rend.  118,  556. 

50 


770  THE  NON-METALLIC  ELEMENTS 

lie  states  that  by  the  distillation  of  128  grains  of  Newcastle 
coal  he  obtained  180  cubic  inches  of  an  inflammable  gas  which 
weighed  51  grains.  Bishop  Watson,  in  his  Chemical  Essays, 
describes  experiments  made  on  coal-gas,  arid  mentions  that  it 
does  not  lose  its  illuminating  power  when  it  is  passed  through 
water.  The  first  to  apply  these  facts  practically  to  the  manu- 
facture of  coal-gas  was  William  Murdoch,  a  Scotchman  living 
at  Redruth  in  Cornwall.  He  distilled  coal  in  an  iron  retort, 
and  lighted  his  house  with  the  gas  which  he  thus  manu- 
factured. Murdoch  was  afterwards  employed  in  the  celebrated 
engine  works  of  Boulton  and  Watt  at  Soho  near  Birmingham. 
Whilst  there  he  improved  his  process  for  the  manufacture  of 
gas,  and  in  1798  the  Soho  factory  was  for  the  first  time  lighted 
with  coal-gas.  In  1802  there  was  a  public  display  of  gas  illumina- 
tion at  Soho  in  honour  of  the  Peace  of  Amiens  ;  and  during  the 
next  three  years  Murdoch's  process  became  so  far  perfected  that 
in  1805  the  large  cotton  mill  of  Messrs.  Phillips  and  Lee  in 
Manchester  was  lighted  with  gas. 

The  success  of  this  undertaking  attracted  the  attention  of 
several  men  of  ability,  especially  Dr.  William  Henry  and  Mr. 
Clegg.  To  the  former  we  owe  the  first  accurate  investigation 
of  the  chemical  composition  of  coal-gas ;  to  the  latter  we  are  in- 
debted for  many  of  the  mechanical  inventions  still  in  use  for  its 
preparation  and  purification,  without  which  the  present  enormous 
extension  of  the  manufacture  would  have  been  impossible. 

The  streets  of  London  were  not  lighted  with  gas  until  1812, 
and  it  was  not  introduced  into  Paris  until  after  the  peace  of 
1815. 

When  coal  is  subjected  to  dry  distillation  at  a  temperature  of 
a  cherry-red  heat,  various  products  are  formed.  These  may 
be  divided  into  three  classes. 

1.  Coal-gas,  a  mixture  of  many  gaseous  compounds. 

2.  Coal-tar,  a  thick,  oily,  strongly  smelling  liquid. 

3.  Gas-liquor  or  Ammoniacal  Liquor,  an    aqueous  distillate 
containing    ammonium    carbonate    and    sulphide,    and    other 
products   in  solution. 

The  apparatus  needed  in  the  manufacture  of  coal-gas  is:  (1) 
the  retorts  in  which  the  distillation  occurs ;  (2)  the  condensers 
in  which  the  liquid  products  of  the  distillation  are  condensed 
and  separated  from  the  gaseous  products ;  (3)  the  washers  and 
scrubbers  in  which  the  last  portion  of  the  liquid  products  and 
the  ammonia  are  removed  ;  (4)  the  purifiers  in  which  the  remain- 


COAL-GAS 


771 


ing  gaseous  impurities  are  removed  ;  (5)  the  gas-holders  in  which 
the  gas  is  stored,  and  whence  it  is  distributed  through  the  gas- 
mains  and  pipes  to  the  place  where  it  is  to  be  burnt. 


FIG.  202. 


FIG.  203. 


FIG.  204. 


FIG.  205. 


FIG.  206. 


FIG.  207. 


454  The  Retorts  originally  introduced  by  Clegg  in  1812 
were  made  of  iron,  but  these  soon  gave  place  to  retorts  made  of 
fire-clay  or  fire-brick.  Several  common  forms  are  shown  in 


772 


THE  NON-METALLIC  ELEMENTS 


Figs.  202,  203,  204.  At  one  time  the  retorts  were  always  closed 
at  one  end,  and  this  is  still  very  often  the  case,  especially  in 
smaller  works,  but  in  the  larger  works  longer  retorts  open  at 
each  end  are  now  usually  employed,  these  being  known  as 
through-retorts.  The  first-named  retorts  vary  from  8 — 10  feet 


in   length,    14 — 18   inches   in  breadth,  and   12 — -16  inches  in 
height,  and  contain  a  charge  of  from  1 J — 3  cwt.  of  coal. 

An  iron  mouthpiece,  shown  in  section  in  Fig.  205  and  in  end 
elevation  in  Fig.  206,  is  attached  to  the  open  end  by  means  of 
bolts,  and  when  the  retort  has  been  charged  the  end  of  the 
mouthpiece  is  closed  by  an  iron  lid  (Fig.  207),  held  in  position  by 
means  of  a  holdfast  and  screw.  An  upright  wide  tube  (d),  cast 


COAL-GAS 


773 


on  to  the  mouthpiece,  carries  away  the  gases,  which  pass  up  the 
vertical  ascension  pipes  H  H  H  H,  Fig.  208,  and  thence  by  the  dip 
pipes  n  n,  Fig.  209,  into  the  horizontal  hydraulic  main  G  G, 
Fig.  208.  The  retorts  are  arranged  so  that  a  number,  varying 
usually  from  five  to  ten,  are  heated  by  means  of  one  furnace,  a 
bed  of  five  retorts  being  shown  in  elevation  in  Fig.  208  and  in 


FIG.  209. 

section  in  Fig.  209.  The  through-retorts  are  similar  in  shape, 
breadth,  and  height  to  those  already  described,  but  are  double 
the  length,  and  contain  a  correspondingly  greater  weight  of 
charge.  Mouthpieces,  ascension  pipes,  &c.,  are  attached  in  this 
case  to  each  end  of  the  retort,  so  that  the  gases  formed  can  pass 
out  at  either  end.  Such  retorts  are  frequently  capable  of  car- 


774  THE  NON-METALLIC  ELEMENTS 

bonising  24  cwt.  of  coal,  and  yielding  12,000  cubic  feet  of  gas, 
per  twenty-four  hours. 

455  The  Condensing  Apparatus. — Large  quantities  of  vola- 
tile products  make  their  way  from  the  retorts  into  the  hydraulic 
main  as  gases,  but  are  there  condensed  into  liquids.  The  end 
of  the  upright  pipe  from  each  retort  (n,  Fig.  209),  termed  the 
dip-pipe,  passes  for  about  one  to  three  inches  under  the  surface 
of  the  liquid  in  the  hydraulic  main  ;  so  that  the  gas  as  it  is 
evolved  can  readily  pass,  but  all  entrance  of  air  into  the  main 
when  the  retort  is  opened  is  prevented.  The  tarry  products  as 
well  as  the  ammoniacal  liquor,  collecting  in  the  hydraulic  main, 
run  off  when  they  reach  a  certain  height  by  a  pipe  which  com- 
municates with  the  tar-well,  where  all  the  liquid  products  of 
the  distillation  are  allowed  to  collect. 

Owing  to  the  several  water-joints  which  are  necessary  in 
various  parts  of  the  apparatus,  the  back  pressure  on  the  gas  in 
the  retorts  is  very  considerable,  and  much  loss  of  gas  ensues 
owing  to  the  necessarily  porous  nature  of  the  fire-clay  retort. 
To  obviate  this  loss,  and  also  to  avoid  the  decomposition  of  the 
dense  hydrocarbons  into  gas-carbon  and  marsh  gas  which  takes 
place  when  the  coal  is  distilled  under  pressure,  an  exhauster 
worked  by  an  engine  is  employed  to  draw  out  the  gas  from 
the  retorts,  and  pass  it  on  through  the  condensers  and  purifiers, 
so  that  in  the  hydraulic  main  the  pressure  is  always  less  than 
that  of  the  atmosphere ;  care  must  however  be  taken  that  the 
vacuum  is  not  so  great  that  air  is  drawn  into  the  hydraulic 
main  through  the  dip-pipe  when  the  retort  lids  are  opened  for 
drawing  the  coke  or  charging  with  fresh  coal. 

The  gas  after  leaving  the  retort  houses  passes  through  the 
atmospheric  condensers,  which  consist  generally  of  a  series  of 
iron  pipes  arranged  either  vertically  or  horizontally.  An  ex- 
ample of  a  vertical  condenser  is  shown  in  Figs.  210,  211.  The 
gas  in  passing  through  the  pipes  is  gradually  reduced  to  the 
atmospheric  temperature,  and  the  more  readily  condensable 
hydro-carbons,  &cv  separate  out  in  the  form  of  tar ;  a  consider- 
able quantity  of  water  is  likewise  condensed  which  dissolves  a 
portion  of  the  ammonia,  sulphuretted  hydrogen,  carbonic  acid, 
and  other  impurities  present  in  the  gas,  yielding  ammoniacal 
liquor.  These  fall  to  the  lowest  portion  of  the  apparatus,  and 
then  pass  away  through  siphons  into  the  tar-well.  After 
leaving  the  condensers  the  gas  passes  to  the  exhausters,  and 
from  this  point  onwards  is  under  pressure  instead  of  vacuum, 


COAL-GAS 


775 


arid  is  forced  through  the  remainder  of  the  apparatus  as  this 
throws  a  considerable  amount  of  back  pressure. 

456  A  considerable  quantity  of  tarry  matter  is  still  retained 
in  suspension  in  the  gas,  and  to  remove  this  and  the  remainder 
of  the  ammonia  it  is  passed  either  through  washers  or  scrubbers, 
or  through  both  these  successively.  Numerous  varieties  of 
washers  are  employed,  but  the  principle  of  almost  all  of  these 


FIG.  210. 


FIG.  211. 


is  the  same,  and  consists  in  dividing  the  gas  up  into  a  number 
of  small  streams  and  forcing  these  through  water  or  the 
weak  liquor  which  separates  out  in  the  hydraulic  main.  By 
this  means  the  tarry  matter  and  most  of  the  ammonia  are 
removed,  and  the  solution  of  ammonia  simultaneously  combines 
with  a  portion  of  the  sulphuretted  hydrogen,  carbonic  acid,  and 
carbon  bisulphide  in  the  gas.  To  completely  remove  the 


776 


THE  NON-METALLIC  ELEMENTS 


ammonia,  the  gas  is  passed  through  the  scrubbers,  which 
consist  of  iron  towers  filled  with  coke  down  which  water  is 
allowed  to  trickle,  thus  exposing  a  very  large  wetted  surface  to 
the  gas.  The  construction  of  these  scrubbers  is  shown  in 
Fig.  212,  the  gas  entering  at  the  bottom  of  the  tower  and 
passing  away  from  the  top,  whilst  the  liquor  formed  falls  to  the 
bottom,  and  thence  by  means  of  siphons  to  the  storage  well. 


FIG.  212. 

457  Purification. — After  leaving  the  scrubbers  the  gas  still 
contains  the  following  impurities :  sulphuretted  hydrogen,  1 — 2 
per  cent,  or  600 — 1,200  grains  per  100  cubic  feet ;  carbonic  acid, 
1—3  per  cent,  or  800—2,400  grains  per  100  cubic  feet ;  carbon  bi- 
sulphide, 15 — 50  grains  per  100  cubic  feet,  and  about  7 — 8  grains 
per  100  cubic  feet  of  other  sulphur  compounds.  Carbonic  acid 


COAL-GAS 


777 


acts  injuriously  on  the  gas,  inasrnuch  as  it  seriously  deteriorates 
its  illuminating  power,  and  the  sulphur  compounds  when  burnt 
give  rise  to  the  formation  of  sulphur  dioxide,  and  eventually  of 
sulphuric  acid,  which  acts  prejudicially  on  metal  and  leather 
work  exposed  to  its  influence.  In  order  to  remove  these 


FIG.  213. 


impurities  as  far  as  possible  the  gas  is  passed  through  layers  of 
various  solid  absorbents  which  are  placed  in  trays  in  large  cast- 
iron  boxes,  generally  termed  the  purifiers,  the  construction  of 
which  is  shown  in  Fi^s.  213,  214. 


FIG.  214. 

Where  no  attempt  is  made  to  remove  the  sulphur  compounds 
other  than  sulphuretted  hydrogen,  the  gas  is  first  passed 
through  a  series  of  boxes  (usually  three  or  four)  containing 
hydrated  oxide  of  iron,  Fe2O3xH2O,  which  absorbs  the  sulphur- 
etted hydrogen,  forming  a  sulphide  of  iron  as  follows : — 

Fe203,H20  +  3H2S  =  Fe2S3  +  4H2O. 


778  THE  NON-METALLIC  ELEMENTS 

As  soon  as  the  gas  passing  the  last  box  is  found  to  contain 
sulphuretted  hydrogen,  the  first  box  of  the  series  is  charged 
with  fresh  oxide  of  iron,  and  this  then  made  the  last  of  the 
series.  The  gas,  after  being  thus  freed  from  sulphuretted 
hydrogen,  is  passed  through  another  series  of  purifiers  contain- 
ing layers  of  moist  slaked  lime  which  absorbs  the  carbon 
dioxide  forming  calcium  carbonate. 

When  the  "  spent "  oxide  of  iron  is  allowed  to  remain  in  a 
moist  condition  exposed  to  the  air  it  gradually  undergoes  oxida- 
tion, and  is  reconverted  into  ferric  oxide  with  separation  of 
sulphur — 

2Fe2S3  +  3O2  =  2Fe2O3  +  3S2 

and  may  then  be  again  placed  in  the  boxes  to  remove  sulphur- 
etted hydrogen,  and  this  process  of  conversion  into  sulphide 
and  revivification  continued  until  the  spent  oxide  contains  50 — 
60  per  cent,  of  free  sulphur,  when  it  is  sold  to  the  sulphuric 
acid  manufacturer. 

If  a  quantity  of  oxygen,  equal  to  about  half  that  of  the 
sulphuretted  hydrogen  in  the  crude  gas,  be  admitted  into  the 
purifiers,  the  above  reaction  goes  on  in  the  boxes  simultane- 
ously with  the  formation  of  sulphide  of  iron,  so  that  a  por- 
tion of  the  spent  oxide  is  revivified  as  fast  as  it  is  formed, 
and  the  boxes  then  last  for  a  much  greater  length  of  time 
without  changing,  thus  effecting  a  considerable  saving  in  the 
cost  of  labour.  It  is  immaterial  for  the  purpose  whether  the 
oxygen  be  added  in  the  pure  state  or  in  the  form  of  air,  but 
when  the  latter  is  used  the  inert  nitrogen  added  to  the  gas 
causes  some  diminution  of  the  illuminating  power,  and  therefore 
necessitates  the  use  of  a  certain  amount  of  richer  coal  to  main- 
tain the  standard  illuminating  power.  On  the  other  hand,  air 
is  obtained  without  cost,  whilst  the  use  of  oxygen  necessitates 
the  employment  of  a  separate  plant  for  preparing  it ;  each  gas 
manager  has  therefore  to  decide  which  plan  will  be  the  less 
costly  under  his  conditions  of  working.  It  must  further  be 
remembered  that  it  is  almost  impossible  to  keep  air  entirely 
out  of  the  gas,  and  there  is  always  a  certain  amount,  varying 
considerably  in  different  works,  already  in  the  gas  at  the  inlet 
to  the  purifiers,  and  the  average  amount  of  this  should  be 
determined  before  deciding  the  amount  of  air  or  oxygen  it  is 
necessary  to  add  for  revivification. 

As  already  stated,  the  above  process  does  not  affect  any  of  the 


COAL-GAS  779 


sulphur  compounds  other  than  sulphuretted  hydrogen,  the 
•carbon  bisulphide,  &c.,  being  allowed  to  remain  in  the  gas.  In 
many  works,  however,  these  impurities  are  also  removed  as  far  as 
possible,  although  the  complete  elimination  has  not  yet  been 
effected  on  the  large  scale.  In  certain  towns  a  limit  is  placed  on 
the  amount  of  sulphur  which  may  be  present  in  the  gas  supplied 
from  the  works;  thus  in  London  the  maximum  allowed  in 
winter  is  22  grains  per  100  cubic  feet,  and  in  summer  17  grains, 
and  the  average  amount  actually  found  in  London  gas  is  generally 
about  10 — 15  grains. 

To  carry  out  the  more  complete  purification  one  of  two 
processes  is  now  usually  employed.  In  the  first,  known  as 
the  Beckton  system,  the  gas  passes  first  through  two  boxes  con- 
taining moist  slaked  lime  which  absorbs  the  carbonic  acid ;  the 
sulphuretted  hydrogen  is  also  absorbed,  but  as  the  carbon  dioxide 
comes  forward  it  decomposes  the  calcium  sulphide  which  had 
been  formed,  and  the  sulphuretted  hydrogen  is  driven  forward 
into  the  next  two  boxes,  which  contain  oxide  of  iron,  where  it  is 
completely  absorbed.  The  gas  next  passes  through  two  lime 
boxes  which  have  previously  been  saturated  with  gas  contain- 
ing sulphuretted  hydrogen  but  free  from  carbon  dioxide,  and 
therefore  contain  chiefly  sulphides  of  calcium.  This  acts  upon 
the  carbon  bisulphide  contained  in  the  gas,  forming  probably 
among  other  compounds  calcium  thiocarbonate,  CaCS3 ;  the 
reaction  is  however  very  complicated,  and  at  present  very  incom- 
pletely understood.  The  gas  then  passes  through  boxes  contain- 
ing Weldon  mud,  which  consists  chiefly  of  a  compound  of  lime 
and  manganese  dioxide,  and  this  absorbs  any  traces  of  sul- 
phuretted hydrogen  which  may  have  passed  the  other  purifiers. 

In  the  second,  or  ail-lime  system,  the  gas  is  passed  through 
a  set  of  boxes,  three  or  four  in  number,  containing  slaked  lime 
throughout;  the  first  two  boxes  absorb  the  greater  portion  of 
the  carbon  dioxide,  whilst  the  sulphuretted  hydrogen  is  ab- 
sorbed mainly  in  the  last  two  boxes,  and  the  sulphides  of 
calcium  formed  act  simultaneously  on  the  carbon  bisulphide  in 
the  gas.  To  prevent  the  passage  of  any  traces  of  sulphuretted 
hydrogen,  the  gas  after  leaving  the  lime  purifiers  passes  through 
"  catch  boxes  "  containing  oxide  of  iron. 

The  great  objection  to  the  last-named  system  is  that  the 
"  spent  lime "  removed  from  the  boxes  has  scarcely  any  com- 
mercial value,  and  possesses  an  extremely  objectionable  odour, 
due  chiefly  to  the  presence  in  it  of  calcium  sulphide,  which 


780  THE  NON-METALLIC  ELEMENTS 

is  decomposed  by  the  carbon  dioxide  and  moisture  in  the  air, 
giving  off  sulphuretted  hydrogen.  It  has  been  found,  however, 
that  the  addition  of  a  small  quantity  of  oxygen  or  air  to  the 
gas  at  the  inlet  to  the  purifiers  renders  the  spent  lime  almost 
odourless,  and  at  the  same  time  assists  the  purification,  the 
charges  lasting  for  a  longer  period  without  changing,  and  the 
process  having  at  least  as  great  an  effect  on  the  carbon  bisul- 
phide as  the  former  plan,  provided  the  amount  of  oxygen  be  not 
excessive.  If,  however,  too  much  oxygen  be  added  to  the  gas, 
the  carbon  bisulphide  is  practically  unacted  upon,  as  the  sul- 
phides are  oxidised  by  the  excess  of  oxygen  before  they  have  an 
opportunity  of  acting  on  the  bisulphide.  Further,  if  an  excess 
of  oxygen  is  accidentally  admitted  to  purifiers  wnich  have  been 
previously  absorbing  the  bisulphide,  this  latter  is  rapidly  given 
off  again,  and  the  gas  may  then  be  found  to  contain  far  more 
sulphur  at  the  outlet  than  at  the  inlet  of  the  purifiers. 

After  leaving  the  purifiers  the  gas  passes  through  the  meter 
to  the  gas-holders,  where  it  is  stored  and  distributed. 

458  The  illuminating  power  and  also  the  yield  of  gas  per  ton 
vary  very  much  with  the  nature  of  the  coal  employed  in  the 
manufacture,  and  also  with  the  conditions  under  which  the 
distillation  takes  place.  Cannel  coal  yields  the  largest 
quantity  of  gas,  possessing  a  high  illuminating  power,  whilst 
ordinary  bituminous  coal  heated  in  the  same  manner  may  yield 
nearly  as  much  gas,  but  its  illuminating  power  is  greatly 
inferior  to  that  obtained  from  cannel.  Bituminous  coals  are, 
as  a  rule,  used  as  the  chief  basis,  but  as  the  illuminating  power 
of  the  gas  they  yield  is  not,  for  the  most  part,  sufficiently  high, 
a  certain  proportion  of  cannel  is  added,  in  order  to  maintain 
the  standard  illuminating  power,  which  varies  very  much 
in  different  localities.  Of  late  years,  however,  the  price  of 
cannel  having  greatly  increased,  many  attempts  have  been 
made  to  find  a  cheaper  enriching  agent,  and  several  processes 
are  now  successfully  at  work.  In  many  cases  the  enriching 
agent  is  oil-gas,  obtained  by  the  dry  distillation  of  the  heavier 
hydrocarbon  oils  (p.  788) ;  in  others  the  vapour  from  the  most 
volatile  portion  of  petroleum,  boiling  below  100°,  is  mixed  with 
the  gas  as  it  passes  from  the  purifiers  to  the*  holders.  Another 
plan  consists  in  preparing  water-gas  by  a  separate  plant  (p.  790), 
and  carburetting  this  by  mixing  it  with  the  vapour  of  oil,  and 
strongly  heating  the  whole,  which  is  then  passed  in  the  requisite 
proportions  into  the  coal-gas. 


BUNSEN'S  PHOTOMETER  781 

459  Determination  of  the  illuminating  power  of  coal-gas. — 
The  commercial  value  of  coal-gas  depends  of  course  chiefly 
upon  its  illuminating  power,  and  it  is  of  great  importance  to 
have  a  ready  method  of  determining  the  latter.  At  the  present 
time  coal-gas  is  largely  used  for  heating  as  well  as  illuminating 
purposes,  and  its  heat  of  combustion  is,  therefore,  also  of  im- 
portance, but  it  has  been  found  that  within  certain  limits  the 
illuminating  power  and  calorific  value  are  very  nearly  propor- 
tional to  one  another,  and  that,  therefore,  from  a  determination 
of  the  one  the  value  of  the  other  may  be  approximately  judged. 

In  determining  the  illuminating  power,  it  is  first  necessary  to 
fix  upon  the  standard  of  light  to  be  employed,  and  in  this 
country  a  sperm  candle  of  definite  construction  and  consuming 
120  grains  of  sperm  (7 '79  grams)  per  hour  is  employed.  This 
standard  is  not  altogether  satisfactory,  but  hitherto  no  other 
standard  has  been  found  to  meet  with  general  approval. 

To  compare  the  value  of  the  coal-gas  flame  and  that  of  the 
candle,  Bunsen's  bar  photometer,  or  some  modification  thereof, 
is  employed,  in  which  advantage  is  taken  of  the  fact  that  the 
intensity  of  the  illumination  from  a  luminous  point  is  inversely 
proportional  to  the  square  of  the  distance  of  the  illuminated 
surface  from  that  point.  All  that  is  necessary  is  to  ascertain 
the  distances  at  which  the  candle  and  standard  gas  flame  both 
produce  the  same  illuminating  effect,  the  illuminating  power 
being  then  calculated  from  these  observa- 
tions. In  Bunsen's  photometer  the  candles 
and  gas  flame  are  fixed  at  the  end  of  a 
graduated  bar,  usually  sixty  inches  in  length, 
and  the  illuminated  surface  consists  of  a 
diaphragm  of  paper,  Fig.  215,  which,  with 
the  exception  of  a  small  circle  in  the 
centre,  has  been  painted  with  a  solution  of 
spermaceti  in  benzene,  thus  rendering  the 
disc,  with  the  exception  of  the  central  portion,  '  *'1G-  21 5- 
transparent.  If,  therefore,  the  disc  is  more 
strongly  illuminated  on  one  side  than  the  other,  a  difference 
is  observed  between  the  two  portions  of  the  disc,  but  when 
both  sides  are  equally  illuminated  this  difference  disappears. 
The  disc  is  fixed  at  right  angles  to  the  bar  in  a  carriage  which 
can  be  moved  in  either  direction  along  the  bar,  and  at  the 
back  of  the  box  containing  the  disc  are  placed  two  inclined 
mirrors  which  enable  the  observer  looking  at  the  diaphragm 


782  THE  NON-METALLIC  ELEMENTS 

in  a  direction  at  right  angles  to  the  bar  to  see  simultaneously 
a  reflection  of  both  sides  of  the  disc.  The  disc  carriage  is  then 
moved  along  the  bar  until  the  -point  is  found  at  which  the 
whole  surface  of  the  disc  on  both  sides  appears  to  be  equally 
illuminated. 

In  making  the  test  two  candles  are  usually  employed,  as 
these  do  not  vary  so  much  as  a  single  candle.  They  are  fixed 
in  a  balance  in  order  that  the  rate  at  which  the  sperm  is  con- 
sumed may  be  accurately  ascertained,  as  this  very  rarely  takes 
place  at  the  specified  rate  of  120  grains  an  hour,  and  a  correction 
must  be  made  accordingly.  The  gas  burnt  is  passed  through 
an  experimental  meter,  which  is  so  regulated  that  the  gas  is 
consumed  at  the  rate  of  five  cubic  feet  an  hour,  this  being  the 
rate  agreed  upon  at  present  as  the  standard  rate  of  consumption. 
The  temperature  and  height  of  the  barometer  are  read  off  at  the 
time  of  testing,  and  a  correction  made,  the  standard  tem- 
perature being  taken  as  60°  F.  and  the  standard  pressure  as 
thirty  inches  of  mercury.  As  a  rule  a  test  is  allowed  to  extend 
over  ten  minutes,  and  the  average  of  the  readings  taken,  these 
being  made  once  each  minute. 

The  illuminating  power  of  the  same  gas  varies  very  much 
with  the  nature  of  the  burner  employed,  and  it  is  therefore 
necessary  to  use  a  standard  burner.  The  same  burner  cannot 
however  be  universally  employed,  as  a  burner  which  gives  good 
results  with  the  quality  of  gas  in  one  town  may  give  less 
satisfactory  results  with  the  higher  or  lower  quality  in  other 
places,  and  the  particular  burner  to  be  used  must  be  decided  in 
each  locality  according  to  the  local  circumstances.  In  London, 
where  the  statutory  quality  of  the  gas  is  sixteen  candles  when 
burnt  at  the  rate  of  five  feet  per  hour,  the  burner  employed  is 
an  Argand,  known  as  the  London  Argand,  in  which  the  number 
of  holes  in  the  steatite  burner,  height  and  width  of  the  glass 
chimney,  &c.,  are  accurately  defined.  This  burner  is  so  con- 
structed that  the  requisite  amount  of  air  for  consuming  sixteen- 
candle  gas  at  the  rate  of  five  feet  per  hour  is  admitted ;  arid 
if,  therefore,  this  burner  is  used  for  determining  the  illuminating 
power  of  gas  of  either  a  decidedly  lower  or  higher  quality,  the 
result  obtained  is  too  low,  for  in  the  first  case  the  inferior  quality 
of  gas,  containing  less  heavy  hydrocarbons,  requires  less  air  for 
its  combustion,  and  the  flame  is,  therefore,  cooled  and  its  illumin- 
ating power  reduced  by  the  excess  of  air  drawn  into  the  burner 
above  the  requirements  of  the  flame.  With  the  higher  quality  of 


COMPOSITION  OF  COAL-GAS  783 

gas  the  air  supply  is  insufficient,  and  this  again  causes  a  reduction 
in  the  illuminating  power.  Correct  results  may,  however,  be 
obtained  with  this  burner  by  burning  the  gas  at  such  a  rate  that 
the  height  of  the  flame  in  the  chimney  is  about  three  inches,  as 
it  is  with  16-candle  gas  at  5  feet  per  hour,  and  then  making  a 
correction  for  the  consumption  of  gas.  Thus,  in  one  case  a 
poor  quality  of  gas  gave  an  illuminating  power  of  17'30  candles 
with  a  consumption  of  6'5  feet  per  hour,  the  true  illuminating- 
power  calculated  for  the  5-feet  rate  being  therefore  13'19 
candles,  whilst  when  burnt  at  the  rate  of  5  feet  per  hour  the 
same  gas  only  showed  an  illuminating  power  of  9*35  candles.1 

Another  means  of  testing  the  quality  of  coal-gas  is 
Lowe's  jet  photometer,  which  depends  upon  the  fact  that  the 
pressure  required  to  give  a  fixed  height  of  flame  in  gas  issuing 
from  a  small  jet  varies  inversely  as  the  illuminating  power. 
This  test  is  only  of  an  approximate  nature,  but  is  very  valuable 
in  the  gas  manufacture,  as  by  its  means  the  quality  of  the  gas 
passing  through  the  apparatus  can  be  roughly  ascertained  at 
any  moment. 

460  Composition  of  Coal-gas. — Purified  coal-gas  is  a  mixture 
of  combustible  gases  and  vapours,  which  may  be  divided  into 
two  classes,  viz. :  (1)  Those  which  burn  with  a  luminous  flame 
and  are  termed  the  illuminating  constituents  ;  (2)  those  which 
burn  with  a  non-luminous  or  scarcely  luminous  flame,  and  are 
termed  the  diluents. 

The  most  important  of  the  first  class,  which  are  also  termed 
heavy  hydrocarbons,  are  : — 

Ethylene,    C2H4,  Acetylene,  C.2H2, 

Propylene,  C3H6,  Allylene,  C3H4, 

Butylene,     C4H8,  Benzene-vapour,  C6H6. 

To  the  second  class  belong  hydrogen,  carbon  monoxide,  and 
marsh  gas.  In  addition  to  these  constituents,  coal-gas  usually 
contains  small  quantities  of  atmospheric  nitrogen  and  oxygen  as 
well  as  the  small  quantities  of  sulphur  compounds,  which,  as 
already  mentioned,  cannot  be  altogether  removed. 

The  influence  of  the  composition  of  the  coal  upon  that  of  the 
gas  obtained  by  its  distillation  is  seen  in  the  following  table 
showing  the  composition  of  gas  from  bituminous  and  from 
cannel  coal. 

1  A  Board  of  Trade  Committee  is  at  present  (1894)  sitting  to  consider  the 
question  of  the  standards  of  light ;  their  report  is  shortly  expected. 


784 


THE  NON-METALLIC  ELEMENTS 


COAL-GAS  FROM  CANNEL  COAL. 


Manchester  Manchester 

Gas. 

(Eunsen  <fe 
Roscoe.) 


Hydrogen 45*58 

Marsh  gas 

Carbon  monoxide    .    .    . 

defines 

Nitrogen       

Carbon  dioxide    .        .    . 

Oxygen 

Sulphuretted   hydrogen 


34-90 
6-64 
6-46 
2-46 
3-67 


Gas. 
(C.  R.  A. 

Wright.) 

52-71 
31-05 

4-47 
11-19 


Rochdale 

Gas. 

(C.  R.  A. 
Wright.) 

53-44 

29-87 

5-86 

10-83 


0-58        — 


London 
Gas. 

(Frank- 
land.) 

35-94 
41-99 

10-07 
10-81 

1-19 


0-29        —         —          — 


100-00  100-00  100-00  100-00 


COAL-GAS  FROM  BITUMINOUS  COAL. 


London  Gas. 
(Frankland.) 


e Kiel  berg  Gas.     London. 
(Landolt.)          S.  Metro- 
politan Gas. 
(Lewes.) 


Hydrogen 5005     51'24     44'00     57'08 

Marsh  gas 32'87     35'28     38'40     33991 

Carbon  monoxide    .    .    . 

Olefmes 

Nitrogen       ...... 

Carbon  dioxide    .... 

Oxygen 

Carbon  bisulphide  .    .    . 


12-89 
3-87 

0-32 


7-40 
3-56 
2-24 
0-28 


4-73 

7-27 

4-23 

0-37 


263 
4-38 
0-15 
0-79 
0-96 
0-02 


100-00  100-00  100-00  100-00 


461  The  influence  of  the  temperature  of  the  retorts  on  the 
composition  of  coal-gas  is  clearly  shown  by  experiments  made 
by  Dr.  Henry,  who  collected  and  analysed  samples  of  coal-gas 
at  various  stages  of  the  process  of  manufacture.  The  following 
table  gives  the  results  of  these  experiments  made  on  a  large 
scale,  in  five  different  gas-works,  showing  that  during  the  first 
hour  the  specific  gravity  of  the  issuing  gas  was  high,  whilst 
that  of  the  gas  coming  off  during  the  fifth  hour  became  much 
lower,  and  at  the  end  of  the  operation  fell  to  a  still  lower 

1  Saturated  hydrocarbons,  i.e.  marsh  gas  and  its  homologues. 


COMPOSITION  OF  COAL-GAS 


785 


point.  This  diminution  of  density  is  accompanied  by  an  altera- 
tion in  the  percentage  chemical  composition  as  is  shown  in  the 
table  :— 


Specific 

Hydro- 

Marsh 

Ole- 

Carbon 

Nitro- 

Gravity. 

gen. 

Gas. 

fines. 

monoxide 

gen. 

1st  hour  .     . 

0-633 

8-3 

70-8 

12-3 

5-8 

2-7 

5th  hour.     . 

0-500 

21-3 

56-0 

7-0 

11-0 

4*7 

10th  hour    . 

0-345 

60-0 

20-0 

o-o 

10-0 

10-0 

Similar  experients  made  by  Erdmann  and  Kornhardt  gave 
the  following  numbers  for  the  specific  gravity  of  the  gas  issuing 
at  different  periods  of  the  manufacture  : — 


Hours  after  commencement  — 

l 

2 

3 

4 

5 

6 

Erdmann    . 

0-6 

0-52 

0-43 

0-37 

0-37 

0-30 

Sp.  Gr. 

Kornhardt  . 

0-416 

0-397 

0-353 

0-282 

0-240 

— 

»    » 

The  following  table  shows  the  composition  of  the  gas  evolved 
from  a  Derbyshire  gas  coal  at  different  periods  after  charging. 
The  samples  were  aspirated  from  the  ascension  pipe  and  drawn 
through  dilute  sulphuric  acid  to  remove  tarry  matters  and 
ammonia.  The  temperature  of  the  retort  was  about  950°  C. 


Hours  after  commencement 

hour. 

i* 

hours. 

2£ 
hours. 

.«i 

hours. 

5 

hours. 

Sulphuretted  hydrogen  .  . 
Carbon  dioxide  
Unsaturated  hydrocarbons  . 
Oxvgen  

3'8 
3-0 
8-65 

o-o 

3-1 

2-8 
5-2 

o-o 

2-8 
2-6 
3-6 

o-o 

2-1 
2-3 
2-4 

o-o 

1-2 
1-7 

o-o 

trace 

Carbon  monoxide  .... 
Hydrogen 

4-35 

29'8 

5-0 
37'5 

4-9 
42-2 

4-5 
46-2 

3-8 
60-8 

Saturated  hydrocarbons  .  . 
Nitrogen  (by  difference)  .  . 

49-7 
07 

42-05 
4'35 

39-4 
4-5 

37-5 
5-0 

26-3 
6-2 

51 


786  THE  NON-METALLIC  ELEMENTS 


The  volume  of  gas  yielded  by  a  given  sample  of  coal  depends- 
also  upon  the  temperature  at  which  the  distillation  occurs, 
If  the  distillation  be  conducted  at  too  low  a  temperature, 
the  volume  of  gaseous  products  obtained  is  small  and  the 
quantity  of  liquid  products  known  by  the  name  of  gas-tar  is 
large,  whereas  if  the  temperature  be  raised  too  high,  or  if  the 
distillation  be  conducted  too  slowly  and  continued  for  too  long  a 
time,  a  gas  having  little  or  no  illuminating  power  and  containing 
large  quantities  of  non-combustible  nitrogen  is  given  off.  In 
practice  the  retorts  are  usually  maintained  at  a  cherry-red  heat, 
the  temperature  being  about  900—980  CC.  or  1650°—  1800°  F.  ; 
the  different  varieties  of  coal  yield  under  these  conditions  8,000 
—  12,000  cubic  feet  of  gas  per  ton. 

462  Chemical  analysis  of  Coal  gas.  —  To  carry  out  the  analysis 
of  coal-gas  a  measured  volume  of  the  gas  is  subjected  to  the  action 
of  various  absorbents  in  one  of  the  many  forms  of  apparatus 
which  have  been  devised  for  this  purpose,  and  the  amount 
absorbed  in  each  case  measured.  Carbonic  acid  is  absorbed  by 
treatment  with  caustic  potash  solution,  and  the  illuminat- 
ing hydrocarbons  by  bromine  'or  fuming  sulphuric  acid,1  the 
oxygen  by  phosphorus  or  pyrogallic  acid,  and  the  carbonic 
oxide  by  a  hydrochloric  acid  or  ammoniacal  solution  of  cuprous- 
chloride.  After  absorbing  these,  the  residue  consists  of  methane, 
hydrogen,  and  nitrogen,  which  are  determined  by  exploding  a 
mixture  of  a  known  volume  of  the  residual  gas  with  air  or 
oxygen.  The  contraction  observed  after  the  explosion  is  due  to 
the  condensation  of  the  water  formed  from  combustion  of  the 
hydrogen  and  marsh  gas.  After  the  explosion  the  gases  are 
treated  with  caustic  potash  which  absorbs  the  carbon  dioxide 
formed  by  the  combustion  of  the  marsh  gas.  From  these 
observations  the  amount  of  hydrogen  and  marsh  gas  can  be 
ascertained  as  follows  :  — 

Marsh  gas  in  combustion  yields  its  own  volume  of  carbon 
dioxide  : 


hence  the  volume  of  carbon  dioxide    observed    represents   the 
amount  of  marsh  gas  in  the  volume    taken.     The  contraction 

1  Fuming  sulphuric  acid  cannot  be  used  for  absorbing  the  unsaturated  hydro- 
carbons, unless  the  oxygen  has  been  previously  removed,  as  some  of  the  latter  is 
simultaneously  absorbed.  When  phosphorus  is  used  for  absorbing  the  oxygen,  the 
unsaturated  hydrocarbons  must  be  first  removed,  as  they  render  the  phosphorus 
inactive,  so  that  in  their  presence  bromine  must  be  used  as  the  absorbent. 


WOOD-GAS  787 


due  to  the  formation  of  water  is  due  partly  to  the  hydrogen  and 
partly  to  the  marsh  gas.  The  latter  in  combustion  combines 
with  twice  its  volume  of  oxygen,  which  must  therefore  be 
deducted  from  the  total  contraction  after  the  explosion  in  order 
to  obtain  the  amount  of  contraction  due  to  combustion  of  the 
hydrogen.  Two  volumes  of  hydrogen  unite  with  one  volume  of 
oxygen,  and  therefore  the  total  amount  of  hydrogen  present  is 
equal  to  two-thirds  of  the  contraction  thus  obtained.  The 
nitrogen  is  found  by  difference. 

In  making  the  analysis  in  this  manner  it  is  assumed  that  the 
only  hydrocarbon  present  in  the  gas  after  treatment  with  the 
various  absorbents  is  methane.  Traces  of  the  higher  homo- 
logues  of  methane  are  however  frequently  present,  and  these 
yield  more  than  their  own  volume  of  carbon  dioxide  on  explo- 
sion, which  renders  the  calculation  of  the  percentages  of 
methane,  hydrogen,  and  nitrogen  incorrect.  In  the 'case  of  coal- 
gas  the  amount  of  higher  saturated  hydrocarbons  is  usually  so 
small  that  the  error  may  for  ordinary  purposes  be  disregarded, 
but  in  the  case  of  oil-gas  (p.  788)  the  quantity  of  these  is 
much  greater,  and  the  above  method  cannot  be  adopted.  In 
order  to  absorb  these  hydrocarbons  the  gas  is  treated  with 
paraffin  oil,  which  has  been  previously  heated  on  the  water-bath 
for  an  hour.  This  absorbs  all  the  higher  homologues  of  methane 
and  some  of  the  methane  itself ;  the  residue  is  then  analysed  in 
the  manner  described  above,  and  the  volume  of  methane  found, 
added  to  that  absorbed  by  the  paraffin,  gives  the  total  volume  of 
saturated  hydrocarbons. 

In  carrying  out  the  analysis  of  illuminating  gases  a  large 
number  of  precautions  must  be  taken  to  obtain  reliable  results. 
For  these  and  other  details  reference  must  be  made  to  works  on 
gas  analysis.1 

WOOD-GAS. 

463  In  countries  where  wood  is  cheap  and  coal  dear  the 
former  material  is  distilled  for  the  purpose  of  yielding  an 
illuminating  gas.  The  first  to  propose  the  use  of  wood  for 
this  purpose  was  the  French  engineer,  Le  Bon,  at  the  end  of  last 
century.  His  proposal  was,  however,  not  carried  out,  inasmuch 

1  Bunsen's  Ga^ometry  (Roscoe's  Translation)  ;  Heinpel's  Methods  of  Gas  Analysis 
(Macmillan  &  Co.)  ;  Winkler  and  Lunge's  Technical  Gas  Analysis  (Gurney  & 
Jackson)  ;  see  also  Lewes,  Journ.  Soc.  Chem.  Ind.  1891,  407. 


788  THE  NON-METALLIC  ELEMENTS 

as  the  gas  thus  obtained  did  not  possess  a  sufficient  illuminating 
power,  consisting  of  a  mixture  of  hydrogen,  carbon  monoxide, 
carbon  dioxide,  and  a  little  marsh  gas.  With  these  gaseous 
products  a  large  quantity  of  easily  condensable  liquid  oils  distils 
over,  and  in  1849  Pettenkofer  showed  that  if  these  oils  are 
exposed  to  a  high  temperature  they  are  partially  decomposed 
into  heavy  gaseous  hydrocarbons.  This  observation  was  practi- 
cally applied  by  Rudinger,  who  in  this  way  succeeded  in  pre- 
paring a  gas  from  wood  which  could  be  used  for  illuminating 
purposes,  and  many  towns  in  Germany  and  in  Switzerland  are 
now  lighted  with  gas  thus  made  from  wood. 

The  retorts  used  in  this  manufacture  are  made  of  cast  iron, 
and  when  their  temperature  is  raised  to  a  red  heat,  they  are 
completely  filled  with  wood.  The  products  of  decomposition 
are  rapidly  evolved,  and  the  vapours  of  the  oils  coming  into 
contact  with  the  red-hot  sides  of  the  retort  are  decomposed  with 
formation  of  the  necessary  quantity  of  the  heavy  hydrocarbons. 
Wood-gas  contains  no  volatile  sulphur  compounds,  but  large 
quantities  of  carbon  dioxide,  which  is  got  rid  of  by  purification 
by  lime. 

The  following  numbers  give  the  average  composition  of  wood- 
gas  :— 

Per  cent. 

Heavy  hydrocarbons  fethylene  and  its  homologues)  10*6  to  6 '5 
Light  „  (marsh  gas  „  „  )  35 '3  to  9'4 

Hydrogen 417  to  187 

Carbon  monoxide 61'8  to  22'3 

Wood-tar  consists  of  a  mixture  of  a  variety  of  substances, 
amongst  which  creosote  is  the  most  important.  The  watery 
distillate  does  not  contain  much  ammonia  in  solution,  but  a 
number  of  organic  products,  such  as  acetone,  wood-spirit  (methyl 
alcohol),  and  acetic  acid  are  present  and  are  recovered  on  the 
large  scale. 

OIL-GAS. 

464  The  preparation  of  illuminating  gas  by  the  dry  distillation 
of  oil  was  first  carried  out  about  the  year  1815,  the  gas  being 
compressed  in  strong  reservoirs,  so  as  to  allow  of  its  being  moved 
from  place  to  place,  being  therefore  known  as  "  portable  gas." 
The  compressed  gas  always  deposited  a  considerable  quantity  of 


OIL-GAS  789 


liquid  hydrocarbons,  which  were  investigated  about  the  year  1820 
by  Faraday,  who  was  thus  led  to  the  discovery  of  benzene.  The 
manufacture  of  oil-gas  however  was  soon  discontinued  as  it  was 
unable  to  compete  with  the  cheaper  coal-gas.  Since  about  the 
year  1870  the  manufacture  has  been  revived,  owing  chiefly  to- 
the  demand  for  gas  of  high  illuminating  power  for  railway 
trains,  where  a  small  volume  of  the  gas  is  required  to  last  for 
a  long  period.  It  is  also  used  to  a  considerable  extent  for 
lighthouses. 

The  oil  employed  is  either  a  shale  oil  or  a  mineral  oil  having 
a  specific  gravity  of  0'82 — 0*88,  and  its  distillation  is  usually 
carried  out  by  one  of  two  processes,  known  as  the  Pintsch  and 
Pope  processes  respectively.  In  the  former  two  cast-iron  retorts 
one  above  the  other  are  employed,  both  being  maintained  at  a 
bright  cherry-red  heat ;  the  oil  is  allowed  to  drop  at  the  rate 
of  twelve  gallons  per  hour  on  to  a  plate  in  the  upper  retort, 
where  the  non-volatile  impurities  remain,  whilst  the  volatile 
portions  pass  through  the  lower  retort  where  they  are  further 
carbonised.  The  purification  of  the  gas  is  carried  out  in  the 
same  manner  as  with  coal-gas,  but  is  of  a  simpler  nature  inas- 
much as  the  amount  of  sulphur  compounds  in  the  gas  is  small. 
Details  of  the  Pope  process  have  not  been  published  ;  it  is  stated 
that  the  oil  drops  first  into  the  lower  retort,  and  the  vapours 
then  pass  through  the  upper  retort,  but  in  other  respects  the 
two  processes  are  probably  similar.  For  lighting  trains,  etc., 
the  gas  is  stored  in  reservoirs  beneath  the  carriages  under  a 
pressure  of  about  five  atmospheres.  The  tar  obtained  from  oil 
contains  a  comparatively  small  amount  of  benzene ;  it  is  there- 
fore of  little  or  no  commercial  value,  and  is  usually  employed 
for  heating  the  retorts.1 

Owing  to  the  rise  in  the  price  of  cannel  coal,  oil-gas  is  now 
frequently  used  in  its  place  for  enriching  coal-gas.  In  what  is 
known  as  the  "  Peebles ''  process  the  retorts  are  heated  only  to 
a  low  redness,  and  the  oil  added  in  a  fairly  large  stream ;  the  gas 
evolved  contains  large  quantities  of  undecomposed  oil  vapours, 
which  separate  in  the  liquid  state  in  the  condensers  and  after 
mixing  with  a  certain  proportion  of  fresh  oil  again  pass  back 
into  the  retort.2  In  this  manner  the  formation  of  any  liquid 
residuals  is  avoided,  the  sole  products  being  a  permanent  gas  of 
very  high  illuminating  power  and  a  dense  black  coke  which 

1  Ayres,  Proc.  Inst.  Civil  Eng.  93,  298  ;  see  also  Thorpe's  Diet.  vol.  ii.  p.  213. 

2  Journ.  Gas  Lighting,  1894,  i.  1050. 


790  THE  NON-METALLIC  ELEMENTS 

remains  behind  in  the  retort.  The  gas  is  then  mixed  with  the 
coal-gas  in  the  proportion  requisite  to  bring  the  latter  up  to  the 
standard  illuminating  power. 

WATER-GAS. 

465  When  steam  is  passed  over  coke  or  charcoal  at  a  tempera- 
ture of  about  500°  it  is  converted  into  hydrogen  and  carbon 
dioxide — 

2H20  +  C  =  C02  +  2H2. 

whilst  at  1000°  the  products  are  hydrogen  and  carbon  mon- 
oxide. 

C  +  H20  =  CO  +  H2. 

At  intermediate  temperatures  both  reactions  take  place,  but 
above  600°  the  product  consists  chiefly  of  carbon  monoxide  and 
hydrogen. 

The  mixture  of  gases  thus  prepared  is  known  as  water-gas,  and 
is  at  the  present  time  manufactured  on  the  large  scale  especially 
in  the  United  States,  where  it  is  used  for  both  lighting  and 
heating  purposes.  In  the  former  case  it  is  usually  "  carburetted  " 
by  mixing  it  with  the  vapour  from  hydrocarbon  oils,  and  raising 
the  whole  to  a  high  temperature.  Carburetted  water-gas  is  also 
used  in  this  country  to  some  extent  for  enriching  coal-gas,  but 
in  the  United  States  it  forms  two-thirds  of  the  whole  supply  of 
illuminating  gas. 

For  the  manufacture  of  uncarburetted  water-gas  two  cupolas 
are  usually  employed,  one  of  which  forms  the  "  generator  "  and 
the  other  the  "  superheater."  The  former  is  filled  with  coke  or 
anthracite,  and  after  the  fuel  has  been  fired,  is  raised  to  in- 
candescence by  an  air  blast.  In  this  process  a  gas  is  evolved, 
consisting  of  nitrogen,  hydrogen,  carbon  monoxide,  and  dioxide, 
and  known  as  generator  or  producer  gas.  This  passes  through  the 
superheater,  which  is  filled  with  firebrick  checker-work,  where 
it  parts  with  its  sensible  heat,  and  the  generator  gas  may  then 
be  employed  for  heating  boilers,  etc.  As  soon  as  the  generator 
has  attained  a  sufficiently  high  temperature  the  air  blast  is 
stopped  and  replaced  by  one  of  steam,  which  is  raised  to  a  high 
temperature  by  passing  through  the  superheater.  The  water- 
gas  then  comes  off  rapidly,  but  as  heat  is  absorbed  in  the  re- 
action the  temperature  of  the  generator  gradually  becomes 
lower.  When  the  temperature  is  so  low  that  the  proportion  of 


WATER-GAS  791 


-carbon  dioxide  becomes  considerable,  the  steam  supply  is  shut 
off  and  the  air  blast  substituted,  which,  when  the  temperature 
is  sufficiently  high,  is  again  replaced  by  steam,  this  cycle  of 
operations  being  maintained,  with  the  necessary  intervals  for 
charging  with  fresh  fuel  and  clearing  out  the  non-volatile  pro- 
ducts from  the  generator. 

It  will  be  seen  that  the  process  as  described  is  intermittent, 
and  various  forms  of  apparatus  have  been  devised  in  which  the 
coke  or  anthrac  ite  is  heated  by  an  external  furnace,  but  hitherto 
it  has  been  found  that  the  first  named  is  the  cheaper  and  more 
reliable  plan. 

Water-gas  has  about  two-fifths  of  the  heat  of  combustion  of 
London  coal-gas,  but  as  it  contains  such  a  small  proportion  of 
hydrocarbons,  it  requires  a  very  much  smaller  volume  of  air  for 
its  complete  combustion,  and,  therefore,  in  burning  gives  rise  to 
a  very  high  temperature.  It  has  on  this  account  been  used  in 
steel  smelting.  The  gas  known  as  "  Dowson  gas,"  which  is 
largely  used  for  driving  gas  engines,  is  prepared  by  passing  air 
and  steam  simultaneously  into  a  generator  filled  with  coke  or 
anthracite,  and  consists,  therefore,  of  a  mixture  of  producer  and 
water-gas. 

For  the  preparation  of  carburetted  water-gas,  two  cupolas  are 
employed  in  addition  to  the  generator,  both  of  which  are  filled 
with  firebrick  checker  work,  the  additional  cupola  being  termed 
the  "carburetter."  The  generator  is  worked  in  the  usual 
manner,  and  the  producer  gas  passed  through  the  carburetter 
and  superheater,  air  being  added  so  as  to  bring  about  its  com- 
bustion within  the  apparatus,  and  raise  the  brickwork  to  a  very 
high  temperature.  The  air  blast  of  the  generator  is  then  re- 
placed by  one  of  steam,  and  the  water-gas  passed  through  the 
'Carburetter  into  which  the  requisite  quantity  of  oil  is  allowed  to 
flow  ;  the  oil  vapours  mix  with  the  hot  water-gas,  and  both  pass 
together  through  the  superheater  which  renders  the  gas  per- 
manent. As  soon  as  the  temperature  falls  too  low  the  steam  is 
replaced  by  air,  the  flow  of  oil  stopped,  and  the  whole  apparatus 
raised  to  a  high  temperature  as  before,  and  this  cycle  of  opera- 
tions repeated.  The  purification  is  carried  out  in  the  same 
manner  as  with  coal-gas.1 

1  Journ.  Gas  Lighting,  1894,  i.  908  ;  Thorpe's  Diet.  vol.  ii.  p.  217. 


792  THE  NON-METALLIC  ELEMENTS 


THE  NATURE  OF  FLAME. 

466  The  first  statement  of  the  nature  of  flame  with  which 
We  are  acquainted  occurs  in  the  works  of  van  Helmont,  who 
regarded  it  as  burning  smoke,  but  scarcely  recognised  the  part 
played  by  the  atmosphere  in  the  phenomenon.  Hooke  shortly 
afterwards  speaks  of  "  that  transient  shining  body  which  we  call 
flame  "  as  "  nothing  but  the  parts  of  the  oyl  rarified  and  raised 
by  heat  into  the  form  of  a  vapour  or  smoak,  the  free  air  that 
encompasseth  this  vapour  keepeth  it  into  a  cylindrical  form,  and 
by  its  dissolving  property  preyeth  upon  those  parts  of  it  that 
are  outwards  .  .  .  producing  the  light  which  we  observe ;  but 
those  parts  which  rise  from  the  wick  which  are  in  the  middle 


FIG.  216. 

are  not  turned  to  shining  flame  till  they  rise  towards  the  top  of 
the  cone,  where  the  free  air  can  reach  and  so  dissolve  them. 
With  the  help  of  a  piece  of  glass  anyone  will  plainly  perceive 
that  all  the  middle  of  the  cone  of  flame  neither  shines  nor 
burns,  but  only  the  outward  superficies  thereof  that  is  con- 
tiguous to  the  free  and  urisatiated  air." 1  A  century  later 
Lavoisier  confirmed  and  extended  Hooke's  views,  and  showed 
that  flame  was  due  to  the  combination  of  the  components  of 
gaseous  substances  with  the  oxygen  of  the  air,  the  gases  being 
raised  to  incandescence  by  the  intensity  of  the  action. 

The  simplest  flames  with  which  we  are  acquainted  are  those 
of  hydrogen  and  carbonic  oxide  burning  in  air  or  oxygen.     The 

1  Lampas,  published  1677. 


STRUCTURE  OF  FLAME  793 

flame  of  either  of  these  gases  burning  from  the  end  of  a  tube 
appears  as  an  incandescent  cone,  which  on  examination  proves 
to  be  hollow,  the  incandescence  only  taking  place  when  the  gas 
has  mixed  by  diffusion  with  air.  The  hollow  nature  of  these, 
and  indeed  of  all  flames,  may  be  readily  shown  in  various  ways. 
(1)  A  bent  glass  tube  may  be  brought  into  the  centre  of  the 
flame,  when  the  unburnt  gases  will  pass  up  the  tube  and  may  be 
ignited  at  the  other  end  (Fig.  216).  (2)  The  head  of  a  match 
may  be  thrust  quickly  into  the  centre  of  the  flame  and  held 
there  for  some  time  without  the  phosphorus  catching  fire,  whilst 
the  wood  will  be  charred  and  may  even  take  fire  where  it  is  in 
contact  with  the  hot  outer  sheath  of  the  cone.  (3)  A  thin 
platinum  wire  held  horizontally  in  the  flame  is  seen  to  glow  at 


FIG.  217. 

two  points  where  it  comes  in  contact  with  the  outer  zones  in 
which  the  combustion  is  going  on,  whilst  between  them  it 
remains  cool. 

With  gases  which  yield  more  than  one  product  of  combustion 
the  phenomena  become  more  complex.  Thus,  for  example,  the 
flames  of  cyanogen  and  sulphuretted  hydrogen  burning  in  air 
are  found  to  consist  of  two  sharply  defined  cones,  possessing 
different  colours.  Matters  become  still  more  complex  in  the 
case  of  the  flames  with  which  we  are  most  familiar,  namely, 
those  obtained  from  a  burning  candle  or  from  gaseous  hydro- 
carbons. In  these  flames  four  distinct  regions,  first  defined  by 
Berzelius,  are  usually  distinguished :  (a]  the  dark  central 
region,  (b)  the  yellow  region,  (c)  the  blue  region,  (d)  the 


794  THE  NON-METALLIC  ELEMENTS 

faintly  luminous  region,  which  are  clearly  shown  in  Fig. 
217. 

The  dark  region  consists  of  a  zone  of  unburnt  gases,  in  which, 
however,  chemical  changes  are  going  on  owing  to  the  action  of 
the  heated  sheath  of  gases  surrounding  it. 

The  yellow  region,  which  is  as  a  rule  the  largest,  and  gives  off 
by  far  the  greatest  amount  of  light,  is  known  in  common  parlance 
as  the  luminous  portion.  The  first  theory  as  to  the  cause  of  the 
great  luminosity  of  this  zone  is  due  to  Davy,1  who  believed  it 
to  be  caused  by  the  decomposition  of  the  hydrocarbons  with 
separation  of  solid  particles  of  carbon,  which  increase  in  a  high 
degree  the  intensity  of  the  light,  being  raised  to  a  very  high 
temperature,  first  by  the  strongly  heated  gases  around  it  and 
then  by  its  own  combustion.  Numerous  facts  appear  to  favour 
this  view.  Thus  we  know  that  the  light  emitted  by  glowing 
solids  is  much  more  intense  than  that  given  by  glowing  gases 
at  the  same  temperature,  as  may  readily  be  shown  by  holding 
a  piece  of  platinum  wire  in  the  faintly  luminous  hydrogen  flame, 
when  it  becomes  heated  to  whiteness.  Further,  if  we  hold  a 
sheet  of  paper  a  short  time  horizontally  in  a  candle  flame  a 
black  ring  of  soot  is  deposited.  Davy's  theory,  therefore,  rapidly 
obtained  universal  acceptance,  and  remained  unchallenged  until 
about  1868. 

467  About  this  time  Frank  land  noticed  that  the  flame  of  a 
candle  burning  on  the  summit  of  Mont  Blanc  emits  much  less 
light  than  when  burning  in  the  valley  at  Chamounix,  although 
the  rate  of  combustion  is  the  same  in  both  cases,  and  was  led  to 
examine  the  effect  of  pressure  on  the  luminosity  of  flames.  He 
found  that  hydrogen  burns  in  oxygen  with  a  luminous  flame 
under  a  pressure  of  20  atmospheres,  and  from  these  and  other 
experiments 2  he  concludes  that  dense  gases  and  vapours  become 
luminous  at  a  much  lower  temperature  than  the  same  gases  in 
a  more  rarefied  condition.  The  luminosity  of  the  yellow  region 
of  a  hydrocarbon  flame  he  regards  as  due  not  to  the  separation 
of  carbon  but  to  the  formation  of  dense  hydrocarbons,  which 
then  burn  with  a  luminous  flame. 

Further  researches  3  have  shown,  however,  that  solid  particles 

1  On  the  Safety  Lamp  for  Miners   ivith  some  Researches  on  Flame,  1818  ;  Phil. 
Trans.  1817,  pp.  45  and  47. 

2  Journ.  Gas  Lighting,  March,  1861;  Phil.  Trans.  1861,  p.  629. 

3  Heumann,    Phil.    Mag.    1877,    1,    89,    366  ;  Hilgard,    Annalcn,    92,   129  ; 
Landolt,  Pogg.  Ann.  99,  389  ;  Soret,  Phil.   Mag.  1875,  50  ;  Burch,  Nature,  31, 
272;  Stokes,  Nature,  44,  263  ;  45,  133  ;  Proc.  Chcm.  Soc.  1892,  22. 


NATURE  OF  FLAME  795 

are  undoubtedly  present  in  the  flame,  and  these  must  of  ne- 
cessity emit  a  large  quantity  of  light  at  such  a  high  tempera- 
ture. Frankland's  views  cannot  therefore  be  taken  as  a  complete 
explanation  of  the  cause  of  the  luminosity,  but  it  is  probable 
that  both  the  incandescence  of  the  solid  particles  of  carbon  and 
•of  dense  hydrocarbons  are  together  the  chief  causes  of  the 
phenomenon. 

The  manner  in  which  the  separation  of  carbon  is  brought 
.about  is  still  a  matter  of  discussion.  It  was  first  supposed  that 
the  separation  was  due  to  the  fact  that  in  the  presence  of  an 
insufficient  amount  of  air  for  complete  combustion  the  hydrogen 
•only  of  the  hydrocarbons  combines  with  the  oxygen,  leaving  the 
carbon  in  the  elementary  condition.  This  theory  is  usually 
supposed  to  have  been  due  to  Davy,  but  it  is  nowhere  definitely 
stated  in  his  published  works.  The  experiments  of  other  ob- 
servers1 have  shown  that  when  hydrocarbons  are  burnt  with 
-an  insufficient  supply  of  oxygen,  the  chief  product  is  carbon 
monoxide,  and  that  considerable  quantities  of  hydrogen  are 
liberated.  Hence  the  conclusion  has  been  drawn  that  the 
.separation  of  carbon  in  the  flame  is  brought  about  not  by  the 
preferential  combustion  of  hydrogen,  but  by  the  action  of  the 
heat  from  the  outer  sheath  of  burning  gases  on  the  hydrocar- 
bons, which  causes  them  to  split  up  into  their  constituent 
elements'.  Armstrong  2  has,  however,  pointed  out  that  the  final 
state  of  affairs,  as  indicated  by  analysis  of  the  products,  is 
probably  the  resultant  of  several  successive  or  simultaneous 
chemical  actions,  and  that  the  data  at  present  at  our  disposal 
are  not  sufficient  to  enable  us  to  draw  the  conclusion  that  the 
oxidation  of  the  hydrocarbons  or  the  separation  of  carbon  or 
of  hydrogen  from  them  takes  place  in  any  one  way. 

Lewes  has  shown  by  the  analysis  of  the  gases  taken  from  a 
coal-gas  flame  at  different  heights  that  the  unsaturated  hydro- 
carbons decrease  very  slowly  in  the  dark  portion  of  the  flame, 
but  quickly  disappear  in  the  luminous  zone.  The  nature  of  the 
unsaturated  hydrocarbons,  however,  undergoes  a  considerable 
alteration  in  the  non-luminous  zone,  the  amount  of  acetylene 
increasing  very  rapidly  and  forming  70  per  cent,  of  the  total 

1  Dalton,  New  System,  Part  II.   442  (1810)  ;  Kersten,  J   Pr.  Chem.  (1),  84, 
310  ;  Smithells  and  Ingle,  Journ.   Chem.   Soc.   1892,   i.   204  ;    Lean  and   Bone, 
.Journ.  Chem.  Soc.  1892,  i.  873. 

2  Proc.  Chem,  Soc.  1892,  23. 


796  THE  NON-METALLIC  ELEMENTS 

unsaturated  hydrocarbons  when  the  top  of  the  non-luminous 
cone  is  reached.  He,  therefore,  concludes  that  the  first  action 
of  heat  on  the  mixed  hydrocarbons  is  to  convert  them  into 
acetylene,  which,  when  the  temperature  is  sufficiently  high,  is 
dissociated  into  its  elements.  If,  however,  the  flame  be  cooled 
by  allowing  it  to  impinge  against  a  cold  platinum  dish,  the 
temperature  does  not  become  sufficiently  high  to  bring  about  the 
decomposition  of  the  acetylene,  and  this  burns  as  such,  with  a 
non-luminous  flame,  whilst  on  heating  the  platinum  dish  on  the 
inside  the  luminosity  reappears  (Heumann).  In  confirmation 
of  this  supposition,  Lewes,  as  well  as  previous  investigators, 
has  shown  that  large  quantities  of  acetylene  are  formed  by  the 
action  of  heat  on  hydrocarbons  such  as  methane  and  ethylene.1 


FIG.  218.  FIG.  219. 

468  The  blue  portion  of  the  flame  is  the  least  in  extent  of 
any  of  the  four  divisions,  and  is  probably  caused  by  the  com- 
bustion of  hydrocarbons  which  have  become  mixed  with  a 
sufficient  quantity  of  air  to  allow  them  to  burn  with  a  scarcely 
luminous  flame ;  it  is  probable  that  the  combustion  is  incom- 
plete, and  that  the  reaction  going  on  here  corresponds  with  that 
taking  place  in  the  inner  cone  of  the  flame  of  the  Bunsen 
burner  (Smithells).  The  faintly  luminous  sheath  is  the  region 
of  complete  combustion  in  which  those  substances  which  have 
been  incompletely  oxidised  in  the  other  portions  of  the  flarne, 
chiefly  hydrogen  and  carbonic  oxide,  are  finally  converted  into 

1  Journ.  Chem.  Soc.  1892,  i.  322  ;  Proc.  Roy.  Soc.  55,  90. 


NATURE  OF  FLAME  797 


water  and  carbon  dioxide.     This  may  be  regarded  as  correspond- 
ing with  the  outer  cone  of  the  Bunsen  burner  flame. 

469  When  a  certain  amount  of  air  is  mixed  with  coal-gas  or 
any  other  hydrocarbon  before  burning,  the  flame  becomes  non- 
luminous,  and  burns  with  a  pale  blue  perfectly  smokeless  flame, 
which  has  a  two-coned  structure,  the  inner  cone  having  a  pale 
blue  colour  and  the  outer  a  still  paler  shade  of  the  same  colour. 
The  most  familiar  example  of  this  flame  is  seen  in  the  Bunsen 
gas-lamp,  now  so  universally  employed  for  heating  purposes. 
In  this  burner  the  gas  emerges  from  a  central  jet  (Fig.  219),  and 
passing  unburnt  up  the  tube  (e,  e,  Fig.  218),  draws  air  with  it 
through  the  holes  (c,  d),  the  mixture  burning  with  a  pale  blue  or . 
bluish  smokeless  flame.  If  the  airholes  be  closed,  the  gas  burns 
with  the  ordinary  smoky  flame.  The  first  explanation  given  of 
these  facts  was,  that  owing  to  the  presence  of  a  considerable 
quantity  of  oxygen  in  the  interior  of  the  flame  the  whole  of  the  car- 
bon is  enabled  to  burn  at  once,  so  that  no  solid  particles  separate 
in  the  flame.  It  was,  however,  soon  found  that  a  non-luminous 
flame  is  also  obtained  by  mixing  the  gas  with  indifferent  gases 
such  as  nitrogen  and  carbon  dioxide,  or  with  combustible  gases 
such  as  hydrogen  and  carbonic  oxide.  The  volume  of  these 
gases  required  to  bring  about  the  change  to  a  non-luminous 
flame  varies  considerably  in  each  case,  as  is  shown  by  the 
experiments  of  Lewes,1  embodied  in  the  following  table,  coal- 
gas  of  16-candle  power  being  employed  as  the  illuminating 


Volume  of  gas  required  to  render  1  vol.  of  coal-gas  non-luminous. 

1  vol.  of  gas  requires  0'5    vol.  of  oxygen. 

„  „          „        1*26       „      carbon  dioxide. 

„  „          „        2'27       „      air. 

„  „          „        2'30       „      nitrogen. 

„  „          „        511       „      carbonic  oxide. 

„      12-4         „      hydrogen. 

The  temperature  of  the  flames  in  each  case  shows  considerable 
variation,  as  is  seen  in  the  subjoined  table,  the  temperatures 
being  ascertained  by  means  of  the  platinum  and  platinum- 
rhodium  couple  devised  by  Le  Chatelier,  one  end  of  which 

1  Journ.  Chem.  Soc.  1892,  i.  332. 


798 


THE  NON-METALLIC  ELEMENTS 


was  introduced  into  the  different  parts  of  the  flame  through  the 
tube  from  which  the  gas  was  burning : 


Flame  rendered  non-luminous  by 

Luminous 

Point  in  flame. 

flame  from 

Air. 

Nitrogen. 

Carbon 
dioxide. 

Bunsen. 

Half  inch  above  burner  . 

54° 

30° 

35° 

135° 

1J  inch  above  burner 

175 

111 

70 

421 

Tip  of  inner  cone  . 

1090 

444 

393 

913 

Centre  of  outer  cone  . 

1533 

999 

770 

1328 

Tip  of  outer  cone  .     .     . 

1175 

1151 

951 

728  l 

Side  of  outer  cone,  level 

with  tip  of  inner  cone 

1333 

1236 

970 

1236 

From  these  results,  as  well  as  those  of  Heumann,2  it  appears 
that  oxidation,  dilution,  and  cooling  all  help  to  bring  about  the 
destruction  of  luminosity  in  the  flame.  The  conclusions  from 
these  and  other  experiments  are  summed  up  by  Lewes  as. 
follows : — 

The  various  actions  which    lead   to  the    loss  of  luminosity 


are- 


(1)  The  chemical  activity  of  the  oxygen  introduced  in  the  air, 
which  causes  loss  of  luminosity  by  burning  up  the  molecules  of 
hydrocarbons  before   in  their  diluted  condition  they  can  form 
acetylene. 

(2)  The  diluting  influence  of  the  nitrogen,  which  increases 
the  temperature  necessary  for  the  formation  of  acetylene  from 
the  hydrocarbons,  whilst  if  any  be  formed  a  higher  temperature 
is  necessary  for  its  decomposition.     In  this  way  diluents  alone 
will  render  a  flame  non-luminous,  and  in  the  normal  Bunsen 
flame  nitrogen  acts  in  this  way  until  the  hydrocarbons  have 
been  destroyed  by  oxidation. 

(3)  The  cooling  influence  of  the  air  introduced,  which  is  able 
to  add  to  the  general  result,  although  the  cooling  is  less  than 
the  increase  of  temperature  brought  about  by  the  more  rapid 
oxidation. 

(4)  In  a  normal  Bunsen  flame  the  nitrogen  and  the  oxygen 
are  of  about  equal  importance  in  bringing  about  non-luminosity, 

1  Probably  incorrect  owing  to  unsteadiness  of  flame  at  this  point. 

2  Annalen,  181,  129  ;   182,  1  ;  183,  102  ;  184,  206. 


NATURE  OF  FLAME 


799 


but  if  the  quantity  of  air  be  increased  the  oxidation  becomes 
the  principal  factor,  and  the  nitrogen  practically  ceases  to  exert 
any  influence. 

470  When  the  quantity  of  air  mixing  with  the  gas  in  a 
Bunsen  burner  is  greater  than  that  required  to  destroy  lumin- 
osity the  mixture  burns  much  less  quietly  and  the  inner  cone 
assumes  a  green  colour ;  if  more  air  be  gradually  added  a  point 
is  reached  at  which  the  mixture  becomes  so  explosive  that  the 
flame  passes  down  the  tube  and  ignites  the  gas  issuing  from  the 
central  jet  at  the  bottom  of  the  burner. 
This  continues  to  burn,  but  as  it  is 
unable  at  that  point  to  obtain  sufficient 
air  for  its  complete  combustion,  large 
quantities  of  acetylene  and  other  un- 
saturated  hydrocarbons  are  formed, 
giving  rise  to  the  well-known  un- 
pleasant odour. 

On  a  close  examination  of  the  phe- 
nomena which  take  place  when  air  is 
gradually  added  to  coal-gas  or  other 
hydrocarbon  burning  from  the  end  of 
a  glass  tube,  Smithells  observed  that 
the  inner  cone  becomes  greener  and 
smaller,  but  that  when  the  mixture 
becomes  sufficiently  explosive  the  flame 
does  not  pass  down  the  tube  as  a 
whole,  but  that  the  green  inner  cone 
detaches  itself  from  the  outer  one  and 
passes  down  the  tube.  If  the  tube 
be  constricted  lower  down,  the  des- 
cending cone  is  arrested  at  that  point, 
the  speed  of  the  gases  being  there 
greater,  and  it  continues  to  burn  there  whilst  the  outer  cone  re- 
mains in  its  former  position.  Smithells  has  devised  an  ingenious 
arrangement  in  which  this  separation  is  readily  effected,  repre- 
sented in  Fig.  220A  and  220s.  It  consists  of  two  concentric  tubes, 
a  and  b,  the  former  being  wider  than  the  other,  and  maintained  in 
a  concentric  position  by  the  india-rubber  collar  c,  and  the  brass 
guide  d.  The  outer  tube  can  be  slid  up  and  down  the  inner 
one  as  desired.  The  apparatus  is  placed  over  a  Bunsen  burner, 
and  on  turning  on  the  gas  and  applying  a  light,  the  usual 
Bunsen  flame  is  obtained  at  the  top  of  the  outer  tube.  If  the 


FIG.  220A.       FIG.  220s. 


800  THE  NON-METALLIC  ELEMENTS 

quantity  of  air  be  now  gradually  increased,  a  poir  '3  is  reached  at 
which  the  inner  portion  of  the  flame  descends  until  it  reaches 
the  orifice  of  the  inner  tube  where  it  is  arrested,  the  speed  of 
the  gas  being  greater  at  that  point.  By  sliding  the  outer  tube 
upwards,  the  two  cones  may  be  separated  any  desired  distance,  as 
shown  in  Fig.  220A,  or  the  outer  tube  may  be  lowered  until  the 
whole  flame  burns  in  the  usual  manner  from  the  inner  tube  as  seen 
in  Fig.  220B.  The  gases  in  the  space  between  the  cones  consist 
chiefly  of  carbon  dioxide,  water  vapour,  carbonic  oxide  and  hy- 
drogen, together  sometimes  with  small  quantities  of  hydrocarbons. 
The  flame  of  a  burning  mixture  of  cyanogen  and  air  can  also 
be  separated  into  two  cones  in  a  similar  manner,  the  inner  one 
having  a  cherry-red  and  the  outer  a  blue-gray  colour.  The 
interconal  gases  were  found  to  contain  nitrogen,  carbon  monoxide 
and  carbon  dioxide,  the  two  latter  gases  being  present  almost 
exactly  in  the  proportion  of  2  vols.  of  the  former  to  1  of  the 
latter.  Mixtures  of  air  with  sulphuretted  hydrogen  and  with 
carbon  bisulphide  can  also  be  separated  into  two  cones  in  the 
same  way,  and  with  care  the  same  may  be  done  when  air  is 
gradually  added  to  hydrogen  and  carbon  monoxide,  each  of 
which,  when  burning  alone,  yields,  as  already  stated,  a  single- 
coned  flame.1 


SILICON   OR  SILICIUM.     Si  =  28-2. 

471  This  element  is,  next  to  oxygen,  the  chief  constituent  of 
the  solid  earth's  crust.  It  always  occurs  combined  with  oxygen 
in  the  form  of  silicon  dioxide,  or  silica,  SiO2,  known  in  the 
crystalline  condition  as  tridymite,  quartz,  and  the  various  kinds 
of  sand  arid  sandstone,  and  in  the  amorphous  condition  as  opal, 
flint,  &c.  This  substance  combined  with  bases  forms  a  large  arid 
important  class  of  minerals  termed  the  silicates,  which  occur 
very  largely  in  many  geological  formations.  For  instance, 
granite  and  the  allied  primitive  rocks  contain  between  20  and 
36  per  cent  of  silicon. 

Minerals  rich  in  silica  were  used  in  ancient  times  by  reason 
of  their  hardness  for  the  purpose  of  glass-making,  and  Becher 
believed  that  they  contained  a  peculiar  kind  of  earth  which  he 
termed  terra  vitrescibilis.  In  the  seventeenth  century  it  was 
discovered  that  this  glassy  earth  undergoes  no  alteration  when 

1  Smithells,  Journ.  Chem.  Soc.  8921,  i.  204;  Nature,  49,  86,  149,  198; 
Armstrong,  Nature,  49,  100,  171  ;  Newth,  Nature,  49,  171. 


SILICON  80J 


heated  by  itself,  but  that  when  brought  in  contact  with 
certain  other  bodies,  it  can  be  made  to  form  a  fusible  glass. 
This  substance  was  for  a  long  time  supposed  to  be  the  essen- 
tial principle  of  all  earths,  but  it  was  found  that  it  differed  from 
them  inasmuch  as  it  has  no  power  of  neutralizing  acids  ;  and 
Tachenius,  in  the  year  1660,  noticed  that  it  possesses  acid 
rather  than  alkaline  properties,  since  it  combines  with  alkalis. 
The  true  nature  of  silica  was  then  unknown,  but  Lavoisier,  in 
his  chemical  nomenclature,  anticipates  that  the  time  may  pro- 
bably soon  arrive  when  this  substance  will  be  recognized  as 
a  compound  body.  After  Davy's  discovery  of  the  compound 
nature  of  the  alkalis  and  alkaline  earths  in  the  year  1808, 
silica,  which  was  then  classed  amongst  the  earths,  was  supposed 
to  possess  a  similar  constitution. 

Berzelius,  in  the  year  1810,  first  obtained  impure  silicon  by 
fusing  together  iron,  carbon,  and  silica;  and  in  1823  he  de- 
scribed the  following  method  for  obtaining  this  element1  in  the 
pure  state.  Ten  parts  of  dry  potassium  silico-fluoride  mixed 
with  eight  to  nine  parts  of  metallic  potassium  are  heated  to 
redness  in  an  iron  tube.  The  following  decomposition  takes 
place  :  — 

K2SiF6+4K 


Instead  of  potassium,  sodium  may  be  employed.  As  soon  as 
the  violent  reaction  which  takes  place  is  ended,  the  mass  is 
allowed  to  cool,  and  then  treated  with  cold,  and  afterwards  with 
hot  water,  until  all  the  potassium  fluoride  is  dissolved.  The 
residual  silicon  is  found  in  the  form  of  an  amorphous  brown 
powder,  which  may  likewise  be  obtained  by  passing  the  vapour 
of  silicon  tetrachloride  through  a  red-hot  tube  over  sodium,2  or 
silicon  tetrafluoride  over  heated  potassium. 

An  impure  form  of  silicon,  which  may  be  advantageously 
used  for  the  preparation  of  many  of  its  derivatives,  can  be 
rapidly  obtained  by  heating  40  grm.  of  dry  powdered  white  sand 
with  10  grm.  of  magnesium  powder  in  a  wide  test-tube.  The 
reaction  is  a  tolerably  vigorous  one,  and  if  carried  out  with 
precipitated  silica  is  accompanied  by  a  brilliant  flash  of  light.3 

Prepared  by  any  of  these  processes,  silicon  is  a  dark  brown 
amorphous  powder  which  when  heated  in  the  air  easily  takes  fire, 
burning  to  the  dioxide,  SiO2  ;  the  latter  frequently  fuses  round 
the  particles  of  the  silicon,  leaving  a  portion  of  this  substance 

1  Pogg.  Ann.  1,  169.  2  H.  Deville,  Ann.  Chim.  Phys.  [3],  49,  68. 

3  Gattermann,  Ber.  22,  186. 
52 


802  THE  NON-METALLIC  ELEMENTS 

unburnt  in  the  centre  of  the  mass.  Amorphous  silicon  is  not 
attacked  by  sulphuric  or  nitric  acid,  but  it  readily  dissolves  in 
aqueous  hydrofluoric  acid,  and  in  hot  concentrated  solutions  of 
the  alkalis.  When  it  is  heated  to  redness  in  absence  of  air,  it 
becomes  denser,  and  assumes  a  graphitic  appearance,  after  which 
it  oxidizes  much  less  readily  on  heating,  and  is  insoluble  in 
hydrofluoric  acid. 

Silicon  may  also  be  obtained  crystallized  either  in  six-sided 
plates  or  long  needles,  and  these  forms  are  sometimes  dis- 
tinguished as  graphitoidal  and  adamantine  silicon.  The  crystals 
of  both  forms  are  made  up  of  regular  octahedra,  and  they  do 
not  differ  markedly  in  either  physical  or  chemical  properties. 
The  crystalline  modification  of  silicon  is  prepared  by  heating 
metallic  aluminium  with  from  twenty  to  forty  times  its  weight 
of  potassium  silico-fluoride  in  a  Hessian  crucible  to  the  melting- 
point  of  silver.  A  regulus  is  thus  obtained,  and  this  when 
treated  with  hydrochloric  acid  and  then  with  hydrofluoric 
acid,  leaves  the  silicon  in  the  form  of  black  shining  six-sided 
tabular  crystals  resembling  graphite  in  their  appearance,  which 
have  a  specific  gravity  of  2'491  and  will  scratch  glass.  The 
reaction  is  thus  represented  :  — 

3K2SiF6  +  4A1  =  6KF  +  4A1F3  +  3Si. 

Crystallized  silicon  can  also  be  prepared  by  passing  a  slow  current 
of  silicon  tetrachloride  over  aluminium  previously  melted  in  an 
atmosphere  of  hydrogen  :  — 


The  silicon  formed  dissolves  in  the  excess  of  fused  aluminium 
until  the  metal  is  -saturated  with  it,  and  on  cooling,  the  silicon 
separates  out  in  the  form  of  long  needle-shaped  crystals  which 
consist  of  aggregations  of  octahedra  and  tetrahedra  of  an  iron- 
grey  colour  and  a  reddish  lustre. 

Crystalline  silicon  can  be  best  prepared  by  throwing  a  mixture 
of  thirty  parts  of  potassium  silico-fluoride,  forty  parts  of  granu- 
lated zinc,  and  eight  parts  of  finely-divided  sodium  into  a  red-hot 
crucible  which  is  kept  for  some  time  at  a  heat  just  below  the 
boiling-point  of  zinc.  The  regulus  is  then  treated  successively 
with  hydrochloric  acid,  boiling  nitric  acid,  and  hydrofluoric  acid, 
when  dark  glittering  octahedral  crystals  are  found  to  remain 

1  Wohler,  Annakn,  97,  266. 


PROPERTIES  OF  SILICON  803 

behind.  If  the  temperature  be  raised  above  the  boiling-point  of 
zinc,  the  silicon  melts  and  may  be  cast  in  sticks.1 

If  the  vapour  of  silicon  tetrachloride  be  passed  through  a 
porcelain  tube  heated  to  redness  and  containing  silicon,  that 
substance  becomes  denser  and  assumes  a  light  iron-grey  colour. 
In  this  process  a  portion  of  the  silicon  appears  to  be  volatilized, 
inasmuch  as  needle-shaped  crystals  are  found  in  the  further 
portion  of  the  tube,  the  formation  of  which  is  due  to  the  pro- 
duction and  decomposition  of  a  lower  chloride. 

Crystallized  silicon  is  not  attacked  by  any  acids  with  the 
exception  of  a  mixture  of  nitric  and  hydrofluoric  acids.  When 
strongly  heated  in  oxygen  it  oxidizes  only  slowly  and  super- 
ficially. Heated  however  to  redness  in  carbon  dioxide,  it  burns 
to  form  its  oxide,  whilst  carbon  monoxide  is  produced.  It 
dissolves  in  hot  caustic  potash  or  soda  with  evolution  of  hydrogen 
and  formation  of  the  corresponding  silicate  ;  thus — 

Si  +  2KOH  +  H20  =  K2SiO  +  2H. 

Crystalline  silicon  is  also  formed  when  the  amorphous  variety 
is  melted,  the  change  being  accompanied  by  the  evolution  of 
8.059  units  of  heat  per  atom.2  The  specific  heat  of  silicon 
varies  with  the  temperature  considerably,  but  becomes  constant 
at  about  232°,  and  is  then  equal  to  O203,  whilst  at  22°  it  is 
0*1697  (Weber).3  When  strongly  heated  in  an  electric  furnace 
silicon  volatilizes  and  condenses  in  small  spheres  mixed  with  a 
grey  powder  and  a  little  silica.4 

The  Atomic  Weight  of  Silicon. — Berzelius  determined  the 
atomic  weight  of  silicon  by  the  oxidation  of  the  element  and 
by  the  analysis  of  barium  silico-fluoride,  but  did  not  obtain 
satisfactory  results.  The  analysis  of  the  chloride  carried  out  by 
Dumas  and  Schiel  led  to  the  value  27*9,  whilst  Pelouze  obtained 
a  slightly  higher  number ;  finally,  the  conversion  of  the  bromide 
into  the  oxide  by  decomposition  with  water  has  given  the 
number  28' 2.5 

1  Ann.  -Chim.  Phys.  [3],  63,  2G  ;  67,  435. 

2  Troost  and  Hautefeuille,  Ann.  Chim.  Phys.  [5],  9,  76. 

3  Pogg.  Ann.  154,  367. 

4  Moissan,  Compt.  Rend.  H6,  1429. 

6  Thorpe  aud  Young,  Journ.  Chem.  Soc.  1887,  i.  576. 


804  THE  NON-METALLIC  ELEMENTS 


SILICON  AND  HYDROGEN. 

SILICON  HYDRIDE,  OR  SILICO-METHANE,  SiH4  =  32'2. 

472  This  substance,  which  was  discovered  by  Buff  and 
Wohler  in  1857,  is  obtained  by  acting  with  hydrochloric  acid 
upon  an  alloy  of  silicon  and  magnesium,  prepared  by  fusing 
together  forty  parts  of  anhydrous  magnesium  chloride  with  a 
mixture  of  thirty-five  parts  of  sodium  silico-fluoride,  ten  parts  of 
common  salt,  and  twenty  parts  of  sodium.  The  dark-coloured 
mass  which  is  thus  obtained  evolves  silicon  hydride  when  brought 
in  contact  with  acidulated  water.  For  this  purpose  the  magne- 
sium silicide  is  brought  into  the  bottle  (Fig!  221),  and  the  latter 
then  completely  filled  with  cold  water  from  which  all  the  air  has 


FIG.  221. 

been  expelled  by  boiling.  A  wide  gas-delivery  tube,  also  com- 
pletely filled  with  the  same  water,  is  connected  with  the  bottle, 
and  hydrochloric  acid  is  poured  down  a  funnel  tube  passing 
through  the  cork  to  the  bottom  of  the  bottle.  The  evolution 
soon  begins,  and  the  gas  must  be  collected  over  water  free  from 
air.  Every  bubble  of  the  gas  thus  obtained  takes  fire  sponta- 
neously when  brought  in  contact  with  the  air,  burning  brilliantly 
with  formation  of  a  cloud  of  silica  which  escapes  in  the  form  of 
a  ring  similar  to  that  produced  by  phosphuretted  hydrogen.  If 
a  jar  be  filled  with  the  gas  over  water,  and  then  opened  in  the  air, 
the  gas  also  takes  fire  spontaneously,  burning  with  a  luminous 
flame  and  depositing  a  brown  film  of  amorphous  silicon  in  con- 
sequence of  the  limited  supply  of  oxygen.  The  gas  thus  pre- 
pared always  contains  free  hydrogen,  and  this  depends,  according 
.to  Wohler,  upon  the  fact  that  the  black  mass  contains  two 


SILICON  HYDEIDE  805 


magnesium  compounds,  by  the  decomposition  of  one  of  which 
the  pure  silicon  hydride  is  evolved,  whilst  the  other  yields 
hydrogen  and  hydrated  silica. 

Pure  silico-methane  can  be  obtained  by  acting  with  sodium  on 
triethyl  silicoformate,  a  compound  which  will  be  subsequently 
described  (Friedel  and  Ladenburg).1  In  this  reaction  ethyl 
silicate  and  silico-methane  are  formed,  thus  :  — 

4Si  (OC2H5)3H  =  3Si(OC2H5)4  +  SiH4. 

The  sodium  employed  in  this  reaction  remains  unacted  on, 
and  the  part  it  plays  is  not  understood.  The  colourless  gas  thus 
obtained  does  not  take  fire  at  the  ordinary  temperature,  but  it 
does  so  when  slightly  warmed  or  when  mixed  with  hydrogen. 
If  the  gas  be  collected  over  mercury  and  the  bubbles  as  they 
emerge  from  the  surface  of  the  mercury  be  brought  in  contact 
with  a  heated  knife-blade,  they  take  fire,  and  the  mercury  soon 
becomes  sufficiently  heated  to  enable  the  bubbles  to  take  fire 
spontaneously.  Silicon  hydride  decomposes  at  a  red  heat  into 
amorphous  silicon  and  hydrogen,  the  volume  of  the  hydrogen 
gas  being  twice  that  of  the  compound  gas  taken,  and  when  the 
gas  is  passed  through  a  heated  narrow  tube  an  opaque  mirror 
of  silicon  is  deposited.  When  decomposed  with  caustic  potash, 
one  volume  of  the  gas  yields  four  volumes  of  hydrogen  :  — 


SiH4  +  H20  +  2KOH  =  K2Si03  +  4H 


2. 


The  gas  may  also  be  obtained  by  passing  silicon  fluoride  over 
heated  magnesium  and  then  treating  the  mass  with  acids.2 
Silicon  hydride  takes  fire  when  brought  into  chlorine  gas,  with 
formation  of  silicon  tetrachloride  and  hydrochloric  acid.  It  con- 
denses to  a  liquid  under  a  pressure  of  100  atmospheres  at  —  1°, 
of  70  atmospheres  at  -  7°,  and  of  50  atmospheres  at  -  11°;  its 
critical  temperature  is  probably  about  0°.3 

When  silicon  hydride  is  submitted  to  the  action  of  the  electric 
current  a  yellow  substance  of  the  formula  Si2H3  is  deposited, 
which  burns  when  heated  in  the  air  or  in  chlorine.4 

1  Annalen,  143,  124.  2  Warren,  Chem.  News,  58,  210. 

3  Ogier,  Compt.  Rend.  88,  236.  4  Ogier,  Compt.  Rend.  89,  1068. 


806  THE  NON-METALLIC  ELEMENTS 


SILICON  AND   FLUORINE. 

SILICON  TETRAFLUORIDE,  SiF4  =  103-8. 

473  This  gas  was  first  observed  by  Scheele  in  the  year  1771. 
It  was  also  obtained  by  Priestley,  and  was  afterwards  examined 
by  Gay-Lussac  and  Thenard  in  1808,  and  by  J.  Davy  in  1812. 
We  are  however  indebted  to   Berzelius  for  the  most  accurate 
investigation  of  this  compound,  carried  out  in  the  year  1823.1 

In  order  to  prepare  this  gas,  white  sand  or  powdered  glass  is 
heated  with  fluor-spar  and  concentrated  sulphuric  acid  : 

2CaF2  +  2H2S04  +  Si02  =  SiF4  +  2CaSO4  +  2H2O. 

In  this  operation  it  is  necessary  that  an  excess  of  sulphuric 
acid  should  be  employed  in  order  to  absorb  the  water  which  is 
formed  in  the  reaction  and  which  would  otherwise  decompose 
the  gas.2 

Silicon  tetrafluoride  is  a  colourless  gas,  fuming  strongly  in  the 
air,  possessing  a  highly  pungent  odour  like  that  of  hydrochloric 
acid,  and  condensing  to  a  colourless  liquid  under  a  pressure 
of  9  atmospheres,  or  when  exposed  to  a  temperature  of  —  160°. 
According  to  Olszewski3  the  tetrafluoride  freezes  at  —  102° 
and  volatilizes  without  liquefying.  The  specific  gravity  of  the 
gas  according  to  the  experiments  of  J.  Davy  is  3'57.  It  is 
incombustible,  and  is  decomposed  by  water  with  separation  of 
gelatinous  silica.  Fused  sodium  takes  fire  when  brought  into 
the  gas  and  burns  with  a  red  flame.  Dry  ammonia  combines 
with  the  gas,  forming  a  white  crystalline  body,  having  the 
composition  SiF42NH3,  which  is  decomposed  by  water.  Three 
volumes  of  the  gas  unite  with  two  of  phosphuretted  hydrogen 
at  —  22°  and  53  atmospheres  to  form  lustrous  crystals  of  an 
unstable  compound.4 

SlLICO-FLUORIC  ACID  OR  HYDROFLUOSILICIC  ACID,  H2SiF6. 

474  When  silicon  tetrafluoride  is  led  into  water  the  following 
decomposition  takes  place  : — 

3SiF4  +  4H20  =  2H2SiF6  +  Si(OH)4. 

1  Pogg.  Ann.  1,  169.  2  Faraday,  Phil.  Trans.  1845,  155. 

3  Monatsh.  5,  127.  4  Besson,  Comyt.  Rend.  HQ,  80. 


SILICON  TETRAFLUORIDE 


807 


The  silicic  acid  separates  out  in  theform  of  a  gelatinous  mass 
and  in  order  to  prevent  the  gas  delivery-tube,  by  which  the 
tetrafluoride  is  passed  into  the  water,  from  becoming  stopped 
up,  the  end  of  this  tube  is  allowed  to  dip  under  mercury  as  seen 
in  Fig.  222.  As  soon  as  the  mass  begins  to  become  thick  it 
must  be  frequently  stirred  up,  otherwise  channels  are  formed 
through  which  the  gas  can  escape  into  the  air  without  coming 
into  contact  with  the  liquid.  The  thick  jelly  is  pressed  through 
a  linen  filter  and  the  filtrate  concentrated  at  a  low  temperature. 


FIG.  222. 


The  same  acid  is  formed  when  silica  is  dissolved  in  hydro- 
fluoric acid,  and  also  when  silicon  fluoride  is  passed  into  concen- 
trated hydrofluoric  acid,  the  solution  in  this  case  depositing 
crystals  of  the  formula  H2SiF6  +  2H2O, l  which  melt  at  19°. 
The  saturated  solution  forms  a  very  acid,  fuming,  colourless 
liquid,  which  may  be  evaporated  down  in  platinum  vessels 
without  leaving  any  residue,  as,  on  boiling,  it  decomposes  into 
silicon  tetrafluoride  and  hydrofluoric  acid.  The  specific  gravity 
of  the  aqueous  hydrofluosilicic  acid  is  seen  in  the  following 
table.2 


1  Kessler,  Compt.  Rend.  90,  1285. 

2  Stolba,  J.  Pr.  Chcm.  [1],  90,  193. 


808  THE  NON-METALLIC  ELEMENTS 


Per  cent. 

0-5 

Specific  gravity. 

1-0040 

1-0 

1-0080 

1:5 

1-0120 

2-0 

1-0161 

5-0 

1  0407 

10-0 

1-0834 

15 

T1281 

20 

1-1748 

25 

1-2235 

30 

1-2742 

This  acid  forms  salts  which  are  termed  the  silico-fluorides. 
Most  of  these  are  soluble  in  water,  the  exceptions  being  the 
lithium  salt,  Li2SiF6,  the  sodium  salt,  Na2SiF6,  the  potas- 
sium salt,  K2SiF6,  the  barium  salt,  BaSiF6,  the  calcium  salt, 
CaSiFG,  and  the  yttrium  salt,  YSiF6,  which  are  more  or  less 
difficultly  soluble.  One  part  of  the  barium  salt  dissolves  in 
3,802  parts  of  cold  water.1  Hence  this  acid  is  used  as  a 
re-agent  for  barium  salts  and  for  the  separation  of  this  metal 
from  strontium.  The  soluble  silico-fluorides  possess  an  acid 
reaction,  and  a  bitter  taste.  They  all  decompose  on  heating 
into  a  fluoride  and  a  silicate. 

A  subfluoride  of  silicon,  the  exact  composition  of  which  is 
unknown,  is  said  to  be  formed  when  silicon  fluoride  is  passed 
over  melted  silicon  and  then  suddenly  cooled.  It  is  deposited 
as  a  white  volatile  powder,  which  contains  less  fluorine  than  the 
tetrafluoride  and  reduces  potassium  permanganate  solution.2 


SILICON  AND  CHLORINE. 

SILICON  TETRACHLORIDE,  SiCl4  =  168-96. 

475  This  compound,  discovered  by  Berzelius  in  the  year  1823, 
is  obtained  by  the  direct  union  of  its  elements  or  by  passing  a 
current  of  dry  chlorine  over  a  strongly  heated  mixture  of  silica 
and  charcoal,  obtained  by  mixing  silica  and  oil  in  the  form  of 
small  balls,  igniting  these  in  a  covered  crucible,  and  then  placing 
them  in  a  porcelain  tube  (a  b  Fig.  223)  heated  to  whiteness.  The 
reaction  which  takes  place  is  as  follows  :— 

SiO2  +  2C  +  2C12  =  SiCl4  +  2CO. 

1  Fresenius,  Annalen,  59,  120. 

2  Troost  and  Hautefeuille,  Ann.  Chim.  Phys.  [5],  7,  463. 


SILICON  TETKACHLORIDE 


809 


The  escaping  vapours  and  gases  pass  through  an  absorption 
tube  (c)  surrounded  by  a  freezing  mixture,  and  the  product  is 
separated  from  the  excess  of  absorbed  chlorine  by  shaking  it  up 
with  metallic  mercury  and  subsequent  distillation. 

It  may  be  readily  prepared  by  passing  chlorine  over  the  mass 
of  crude  silicon  obtained  by  Gattermann's  method,  heated  at 
300°— 310°  (p.  801). 

Silicon  tetrachloride  is  an  acrid  colourless  liquid,  fuming  in 
the  air,  having  a  specific  gravity  at  0°  of  1 '52408  and  boiling 
at  59'57°  (Thorpe).  The  specific  gravity  of  its  vapour,  according 
to  Dumas,  is  5*937.  When  thrown  into  water,  silicon  tetrachlo- 


FIG.  223. 

ride  is  at  once  decomposed,  hydrochloric  and  silicic  acids  being 
formed,  and  the  latter  deposited  as  a  gelatinous  mass. 

It  reacts  with  six  molecules  of  ammonia  to  form  a  white  amor- 
phous mass,  and  forms  an  unstable  crystalline  compound  when 
compressed  with  phosphuretted  hydrogen  at  a  low  temperature.1 

SILICON  TRICHLORIDE,  Si.2Cl6  =  267-54. 

476  Friedel  obtained  this  compound,  along  with  a  small 
quantity  of  a  spontaneously  inflammable  liquid 2  which  probably 

1  Besson,  Compt.  Rend.  HO,  240.  •  Annalen,  203,  254. 


810  THE  NON-METALLIC  ELEMENTS 

has  the  composition  SiCl.2,  by  gently  heating  the  corresponding 
tri-iodide  with  mercuric  chloride.1  The  same  compound  was 
obtained  by  Troost  and  Hautefeuille 2  by  passing  the  vapour 
of  silicon  tetrachloride  over  silicon  heated  in  a  porcelain  tube  to 
whiteness,  and  by  Gattermann  in  the  preparation  of  the  tetra- 
chloride from  crude  silicon.3  It  is  a  colourless  liquid  solidifying 
at  —1°  and  boiling  at  146°,  having  a  specific  gravity  at  0°  of 
1*58.  The  specific  gravity  of  its  vapour  is  9'7.  It  fumes 
strongly  in  the  air,  and  when  heated  it  takes  fire.  At  a  tem- 
perature of  350°  it  begins  to  decompose,  and  the  amount  of 
decomposition  increases  with  the  temperature  up  to  800°,  when 
it  is  completely  dissociated  into  the  tetrachloride  and  silicon. 

If  the  temperature  of  the  vapour  be  quickly  raised  beyond 
1000°  no  such  dissociation  is,  on  the  contrary,  observed.  We 
have  therefore  here  to  do  with  a  substance  possessing  the  re- 
markable property  of  being  stable  at  temperatures  below  350° 
and  above  1000°,  and  dissociating  at  intermediate  temperatures. 
This  explains  the  singular  fact  observed  by  Troost  and  Haute- 
feuille, that  if  the  vapour  of  silicon  tetrachloride  be  passed 
over  silicon  heated  to  above  1000°  in  a  porcelain  tube,  the  silicon 
is  transported  from  the  heated  to  the  cooled  part  of  the  tube. 
This  is  not  due  to  the  volatilization  of  the  silicon,  for  no  such 
change  is  observed  when  this  body  is  heated  in  an  atmosphere 
of  hydrogen,  but  is  to  be  explained  by  the  alternate  formation 
(at  a  high  temperature)  and  dissociation  (at  a  lower  temperature) 
of  the  trichloride. 

It  unites  with  ten  molecules  of  ammonia  to  form  a  white 
substance  which  loses  ammonia  at  100°.  Phosphuretted 
hydrogen  is  immediately  reduced  by  it  with  formation  of  solid 
hydrogen  phosphide.4 

A  chloride  of  the  formula  Si8Cl8,  boiling  at  210°— 215°,  is 
also  formed  by  the  action  of  chlorine  on  crude  silicon.  Water 
decomposes  it  with  formation  of  a  white  powder  H2Si305. 


SlLICO-CHLOROFORM   OR    TRICHLORS1LTCO-METHANE, 

SiHCl3=  134-77. 

477  This  body  was  first  obtained  in  the  impure  state  by 
Wohler  and  Buff  by  heating  silicon  in  a  current  of  dry  hydro- 
chloric acid  gas  at  a  temperature  just  below  red  heat,  and  may 

1  Compt.  Rend.  73,  1011.  2  Ann.  Chim.  Phys.  [5],  7,  461. 

3  Bcr.  27,  1945.  4  Besson,  Compt.  Rend.  110,  516. 


SILICON  TETRABROMIDE  811 

readily  be  prepared  in  this  way  at  a  temperature  of  450° — 500° 
from  the  crude  silicon  obtained  by  Gattermann's  method 
(p.  801).  In  order  to  prepare  the  pure  compound,  the  crude 
product  thus  obtained  is  condensed  in  a  tube  surrounded  by  a 
freezing  mixture,  and  the  silico -chloroform  separated  by  sub- 
sequent distillation  from  the  tetiachloride  formed  at  the  same 
time.  It  is  a  colourless,  mobile,  strongly-smelling  liquid  which 
fumes  on  exposure  to  the  air  and  boils  at  34°.  It  is  very 
inflammable  and  burns  with  a  green  mantled  flame,  evolving 
white  clouds  of  silica.  When  a  hot  glass  rod  is  brought  into  a 
mixture  of  this  body  and  air,  the  mixture  burns  with  explosion 
(Gatterinann).  Water  readily  decomposes  this  substance  in  the 
cold,  and  a  white  powder  is  precipitated  to  which  the  name 
silico-formic  anhydride,  Si2H2O3,  has  been  given  (Friedel  and 
Ladenburg) : — 

2SiHCl3  +  3H20  =  6HC1  +  Si2H203. 

This  body  is  very  unstable,  and  is  decomposed  by  dilute  ammonia 
into  silicic  acid  and  hydrogen. 

SILICON  TETKABROMIDE,  SiBr4  =  345'64. 

478  This  compound,  discovered  in  the  year  1831  by  Serullas,1 
is  obtained  by  a  reaction  similar  to  that  employed  for  the  pre- 
paration of  the  chloride.  On  leading  bromine  vapour  over  a 
heated  mixture  of  carbon  and  silica,  best  obtained  by  mixing 
gelatinous  silica  with  lamp-black  and  syrup  of  cane-sugar, 
drying  and  igniting,2  a  volatile  distillate  is  obtained,  and  this  is 
freed  from  an  excess  of  bromine  by  shaking  up  with  mercury 
and  subsequent  distillation.  It  may  also  be  prepared  directly 
from  crude  silicon,  which  is  heated  in  a  current  of  bromine 
vapour  (Gattermann). 

The  tetrabromide  is  a  colourless  heavy  liquid,  having  a  specific 
gravity  of  2  813,  boiling  at  153°,  and  solidifying  at  13°  to  a 
crystalline  mass.  When  brought  in  contact  with  water  it  is 
decomposed  into  hydrobromic  and  silicic  acids. 

It  forms  a  white  amorphous  compound,  SiBr3  +  7NH3,  with 
ammonia,  which  is  decomposed  by  water ;  and  seems  to  form 
an  unstable  compound  with  phosphuretted  hydrogen  at  a  high 

pressure.3 

1  Ann.  Chim.  Phys.  [2],  48,  87. 

2  Reynolds,  Journ.  Chem.  Soc.  1887,  590. 

3  Besson,  Compt,  Rend.  HO,  240. 


812  THE  NON-METALLIC  ELEMENTS 


SILICON  TRIBROMIDE,  Si2Br6  =  532-86. 

This  colourless  and  crystalline  compound  is  formed  when  the 
corresponding  iodine  compound  is  treated  with  bromine  in  the 
presence  of  carbon  bisulphide.  Large  crystalline  tablets  are 
formed,  which  melt  on  heating,  and  may  be  distilled  at  240° 
without  undergoing  decomposition.1 

SILICON  BROMOFORM,  SiHBr3  =  267-28. 

This  substance  is  formed  by  the  action  of  hydrogen  bromide 
on  silicon,2  and  may  be  prepared  in  this  way  from  crude  silicon 
(Gattermann). 

It  boils  at  115°— 117°  (Gattermann),3  109°— 111°  (Besson),4 
and  is  spontaneously  inflammable  in  the  air.  In  its  chemical 
relationships  it  resembles  silico-chloroform. 


SILICON  AND  IODINE. 

SILICON  TETRA-IODIDE,  SiI4  =  531-84. 

479  Friedel  obtained  the  tetra-iodide  by  the  direct  com- 
bination of  the  two  elements.  For  this  purpose  he  volatilized 
iodine  in  a  stream  of  dry  carbon  dioxide,  and  led  the  mixed 
gases  over  heated  silicon ; 5  the  iodide  may  also  be  prepared  in 
this  way  from  crude  silicon  (Gattermann). 

The  tetra-iodide  is  a  colourless  crystalline  mass  which  is 
deposited  in  the  form  of  regular  octahedra  from  solution  in 
carbon  bisulphide.  Its  melting-point  is  120'5°  and  its  boiling- 
point  290°.  Heated  in  the  air  it  takes  fire  and  burns  with  a 
reddish  flame.  It  is  decomposed  in  presence  of  water  into 
hydriodic  and  silicic  acids. 

SILICON  TRI-IODIDE,  Si2I6  =  811-86. 

This  compound  was  likewise  first  obtained  by  Friedel  and 
Ladenburg6  by  heating  the  tetra-iodide  with  finely-divided 
silver  to  a  temperature  of  280° : 

2SiI4  +  2Ag  =  Si2I6  +  2AgI. 

1  Annalen,  203,  253.  2  Wohler  and  Buff,  Annalen,  104,  99. 

8  Ber.  22,  193.  4  Compt.  Rend.  H2,  530. 

5  Annalen,  149,  96.  6  Annalen,  203,  247. 


SILICON  AND  IODINE  813 

It  crystallizes  from  carbon  bisulphide  in  splendid  colourless 
hexagonal  prisms  or  rhombohedra,  which  fume  on  exposure  to 
the  air,  and,  owing  to  the  absorption  of  moisture,  change  to  a 
white  mass  with  formation  of  silicic  and  hydriodic  acids.  It 
melts  when  heated,  but  decomposes  with  the  formation  of 
SiI4,  and  a  lower  iodide.  Ice-cold  water  decomposes  it,  and  also 
the  trichloride,  without  evolution  of  hydrogen,  but  with  formation 
of  a  white  substance  having  the  composition  Si2O4H2,  to  which 
the  name  of  silica-oxalic  acid  has  been  given  because  it  possesses 
a  composition  corresponding  with  that  of  oxalic  acid,  C2O4H2. 
This  substance  is,  however,  decomposed  even  by  weak  bases 
with  evolution  of  hydrogen  and  formation  of  silicic  acid  accord- 
ing to  the  equation  : — 

H2Si204  +  4KOH  =  2K2Si03  +  2H20  +  H2. 
When  heated  or  rubbed  it  decomposes  with  a  feeble  explosion.1 

SILICO-IODOFORM,  SiHI3  =  406'93. 

For  the  purpose  of  preparing  this  compound,  a  mixture  of 
hydrogen  a,nd  hydriodic  acid  is  passed  over  silicon  heated  just 
to  redness.  It  is  a  colourless  strongly-refractive  liquid  boiling 
at  220°  and  having  a  specific  gravity  at  0°  of  3'362.  It  is 
decomposed  by  water  in  a  manner  corresponding  to  the  chlorine 
compound  (Friedel). 


MIXED  HALOGEN  DERIVATIVES  OF  SILICON. 

480  When  a  mixture  of  hydrogen  bromide  with  silicon  chlo- 
ride vapour  is  passed  through  a  red-hot  glass  tube,  a  mixture  of 
the  chlorobromides  of  silicon  is  formed.2  Bromotrichlorosilicon, 
SiCl3Br,  is  also  formed  by  the  action  of  bromine  on  silicon 
chloroform  at  100°,  or  on  silicon  chlorosulphide  at  the  ordinary 
temperature.3 

The  chloro-iodides  and  bromo-iodides  of  silicon  are  obtained 
by  similar  reactions.  They  are  all  decomposed  by  water  and 
form  solid  compounds  with  ammonia.  Their  physical  properties 
.are  given  in  the  annexed  table  : — 

1  Gattermann,  Ber.  27,  1243. 

2  Besson,  Compt.  Rend.  112,  788. 

3  Friedel,  Annalen,  143,  118  ;  145,  185. 


814 


THE  NON-METALLIC  ELEMENTS 


Formula. 

Boiling-point. 

Melting- 
point. 

Combines  with 

SiCl3Br    .    .    . 

80° 

_ 

5'5NH3 

SiCl2Br2  .    .    . 

103°—  105° 

— 

5NH3 

^iClBrg     .    . 

/  126°—  128°  (B)  ) 
}  140°—  141°  (R)  j 

-30° 

11  NH3 

2  SiCLI 

113°—  114° 

— 

5-5  NH3 

2  SiCLL  .    .    . 
2SiClI,    .    .    . 

172° 
234°—  237° 

2° 

5NH3 

3SiBr3I    .    .    . 

192° 

14° 

— 

3  SiBr2I2  .    .    . 
3SiBrI3   .    .    . 

230°—  231° 
255° 

38° 
53° 

— 

SILICON  AND  OXYGEN. 
SILICON  DIOXIDE  OR  SILICA,  SiO2  =  59'96. 

481  Silicon  forms  only  this  one  oxide,  which  is  an  extremely 
important  constituent  of  our  planet.  It  is  found  not  only  in 
the  mineral  but  also  in  the  vegetable  and  animal  kingdoms, 
existing  in  large  quantity  in  the  glassy  straw  of  the  cereals 
and  of  bamboos,  and  in  the  feathers  of  certain  birds,  which 
have  been  found  to  contain  as  much  as  40  per  cent,  of  this 
substance.  Vast  deposits  of  pure  silica  in  a  very  fine  state 
of  division  occur  in  various  parts  of  Germany,  especially  in 
Hanover  and  near  Berlin.  This  consists  of  the  scales  of 
extinct  diatomaceaB,  and  is  termed  kiesel-guhr.  Large  quanti- 
ties of  this  substance  are  now  used  for  a  variety  of  purposes, 
especially  for  the  preparation  of  dynamite,  for  filtering,  and  as  a 
non-conducting  medium  for  packing  steam-pipes.  The  minute 
and  beautifully-formed  spicules  of  the  spongidse  and  radiolariae 
also  consist  of  pure  silica.4  In  the  mineral  kingdom  it  is  found 
in  three  distinct  forms  : — 

(a)  Quartz  is  the  most  important  form  of  silica.  It  crystal- 
lizes in  the  hexagonal  system  (trapezohedrally  tetartohedral) 
forming  usually  combinations  of  the  prism  with  the  rhombo- 

1  Reynolds,  Journ.  Chem.  Soc.  1887,  590. 

2  Besson,  Compt.  Rend.  112,  611,  1314. 

3  Friedel,  Ber.  2,  60. 

*  Besson,  Compt.  Rend.  Hjj    1447. 


SILICON  AND  OXYGEN 


815 


hedron ;  three  of  the  most  usual  forms  are  shown  in  Figs.  224, 
225,  and  226.  The  relation  of  its  axes  is  1  :  T0999.  It  has  a 
specific  gravity  of  2  65  and  a  hardness  of  7.  The  purest  form  of 
quartz  is  known  as  rock  crystal.  It  is  usually  colourless  and 
transparent.  Sometimes,  however,  quartz  is  coloured  by  the 
presence  of  traces  of  oxides  of  manganese,  which  give  it  a  violet 
tint,  and  it  is  then  termed  amethyst  quartz.  Other  varieties 
containing  more  or  less  impurity  are  known  as  milk  quartz, 


FIG.  224. 


FIG.  225. 


FIG.  226. 


rose  quartz,  and  smoky  quartz.  Crystalline  quartz  also  occurs 
in  large  masses,  forming  whole  mountain  ranges  as  quartzose 
rock.  It  forms  one  of  the  chief  constituents  of  granite,  gneiss, 
and  syenite,  whilst  sand  and  sandstone  consist  of  an  impure 
variety  of  quartz. 


FIG.  227. 

(&)  Tridymite  is  a  second  crystalline  variety  of  silica  dis- 
covered by  G.  v.  Rath1  in  the  trachytic  porphyry  found  at 
Pachuca  in  Mexico.  This  form  of  silica  occurs  in  many  other 
similar  rocks,  and  is  found  in  considerable  quantity  in  the 
trachyte  of  Stenzelberg  in  the  Siebengebirge.  Tridymite  cry- 
stallizes in  the  asymmetric 2  system,  and  forms  six-sided  tablets, 
which  are  combinations  of  a  prism  with  pyramids,  Fig.  227.  The 

1  Pogg.  Ann.  133,  507,  and  135,  437. 

2  Lasaulx,  Jahrb.  Min.  1878,  p.  408. 


816 


THE  NON-METALLIC  ELEMENTS 


ratio  of  the  axes  is  0*5812  : 1  : 1-1040,  and  the  whole  form 
approaches  very  closely  to  one  belonging  to  the  rhombic  system. 
It  has  the  specific  gravity  2*3,  and  the  same  hardness  as  quartz. 
A  very  characteristic  property  of  this  mineral  is  that  it  generally 
occurs  in  trillings  (Fig.  228),  from  which  its  name  has  been 


FIG.  228. 


derived.     These  are  generally  found  grown  together  in  twins, 
as  in  Fig.  229. 

Identical  with  tridymite  is  the  mineral  asmanite  which  has 
been  found  in  certain  meteorites. 


FIG.  229. 

(c)  Amorphous  Silica  occurs  in  nature  containing  more  or  less 
water  as  opal.  This  substance  is  either  colourless  or  variously 
coloured,  possesses  a  vitreous  fracture,  and  has  a  specific  gravity 
of  2'2.  It  not  unfrequently  happens  that  crystals  of  tridymite 
are  found  in  the  non-transparent  portions  of  this  opal,  and 
these  remain  as  an  insoluble  residue  when  the  opal  is  treated 


SILICA  817 


with  caustic  potash  (G.  Kose).  Chalcedony,  agate,  and  flint 
are  intimate  mixtures  of  amorphous  silica  with  quartz  or 
tridymite. 

Hydrated  amorphous  silica  is  formed  by  passing  silicon  tetra- 
fluoride  into  water.  When  the  gelatinous  mass  is  washed  with 
water,  dried,  and  ignited,  pure  finely  divided  amorphous  silica 
remains  behind  as  a  white  very  mobile  powder.  Amorphous 
silica  may  be  obtained  in  the  same  form  by  decomposing  a 
silicate  of  an  alkali-metal  with  an  acid.  For  this  purpose  one 
part  of  quartz,  flint,  or  white  sand,  is  fused  with  four  parts 
of  sodium  carbonate.  If  flint  or  quartz  be  employed  it  must 
previously  be  reduced  to  powder,  and  this  is  easily  effected  by 
heating  the  mass  to  redness  and  then  quickly  plunging  it  into 
cold  water,  after  which  it  becomes  friable  and  can  readily  be 
powdered.  The  fused  mass  is  then  heated  with  hydrochloric 
acid,  when  it  becomes  gelatinous,  an  alkali  chloride  and 
silicic  acid  being  formed.  The  gelatinous  mass  is  afterwards 
evaporated  to  dryness  on  the  water- bath,  and  the  residue 
moistened  with  strong  hydrochloric  acid,  in  order  to  dissolve 
any  oxide  of  iron  or  other  oxides,  whilst  the  silica  remains 
undissolved  in  the  anhydrous  state  as  a  light  white  powder, 
which  only  requires  washing  and  drying  to  render  it  perfectly 
pure.  Amorphous  silica  has  a  specific  gravity  of  2'2.  If,  how- 
ever, it  be  heated  strongly  for  a  long  time  its  specific  gravity 
increases,  inasmuch  as  it  is  then  transformed  into  tridymite.1 
Rock  crystal  undergoes  the  same  change  when  the  perfectly 
pure  material  is  finely  powdered  and  heated  to  2,000°,  its  specific 
gravity  diminishing  from  2'6  to  2'3.2 

Silica  melts  in  the  oxy-hydrogen  flame  to  a  colourless  glass 
which  may  be  drawn  out  in  threads.  It  slowly  volatilizes  when 
maintained  at  a  high  temperature  in  a  wind  furnace  3  for  some 
time,  and  can  readily  be  made  to  boil  when  heated  in  the 
electric  furnace.4 

In  all  three  conditions  silica  is  insoluble  in  water,  and  also  in  all 
acids  except  hydrofluoric,  in  which  it  readily  dissolves.  Crystal- 
lized silica  is  as  a  rule  scarcely  attacked  by  alkalis,  whilst  the  amor- 
phous form  is  soluble,  especially  when  in  a  fine  state  of  division,  but 
loses  this  property  to  a  large  extent  when  ignited.  The  amor- 
phous variety,  especially  if  it  contains  water,  also  dissolves  in 

1  Rammelsberg,  Ber.  5,  1006.  2  G.  Rose,  Ber.  2,  338. 

3  Cramer,  Zeit.  Angew.  Chem.  1892,  484. 

4  Moissan,  Compt.  Rend.  116,  1122. 


818  THE  NON-METALLIC  ELEMENTS 

alkaline  carbonates.  Hence  silica  is  found  in  many  spring  waters, 
especially  those  of  the  hot  Icelandic  springs,  which  on  exposure 
to  air  deposit  this  substance,  the  alkali  silicates  being  decom- 
posed by  the  atmospheric  carbonic  acid,  while  silica  and  an 
alkaline  carbonate  are  formed.  The  occurrence  of  silica  in 
these  springs  was  observed  so  long  ago  as  1794  by  Black,  but 
even  before  that  date  Bergman  had  shown  that  small  quantities 
of  silica  were  contained  in  solution  in  the  water  of  many 
springs. 

When  an  alkaline  solution  of  silica  is  heated  in  a  sealed  tube 
the  glass  is  attacked  and  an  acid  silicate  is  formed  from  which 
silica  separates  out  on  cooling.  If  the  temperature  at  which 
the  deposition  occurs  be  above  180°,  the  silica  separates  out  as 
quartz;  if  below  this  point,  it  crystallizes  out  as  tridymite; 
whilst  at  the  ordinary  temperature  of  the  air  it  separates  in 
the  form  of  a  hydrated  amorphous  mass.1 

The  various  forms  of  silica  are  employed  in  a  variety  of 
technical  processes,  many  of  which  will  be  hereafter  described, 
especially  its  application  to  the  manufacture  of  glass  and 
porcelain.  The  coloured  varieties  of  silica  are  also  largely  used 
as  gems  and  for  other  ornamental  purposes.  It  has  recently 
been  found  possible  to  colour  the  natural  agates  artificially. 
Thus  brown  or  yellow  agates  or  chalcedonies  when  strongly 
heated  are  changed  into  ruby  carnelians,  the  yellow  oxide  of 
iron  being  thereby  changed  into  the  red  anhydrous  oxide.  Many 
agates  and  chalcedonies  are  permeable  to  liquids,  and  in  this  way 
they  may  be  artificially  coloured.  This  fact  was  known  to  the 
ancients  and  made  use  of  in  darkening  the  colour  of  agates. 
Thus  Pliny  describes  that  in  Arabia,  agates  occur  which  having 
been  boiled  in  honey  for  seven  days  and  nights  become  marked 
by  veins,  striaB  or  spots,  and  are  thus  rendered  much  more 
valuable  as  ornaments,  the  boiling  in  honey  having  the  object 
of  freeing  them  from  all  earthy  and  impure  materials.  Pliny 
was  evidently  only  acquainted  with  half  the  mode  of  procedure, 
the  object  being  to  saturate  the  stone  with  honey,  which  by 
heating  was  carbonized,  and  thus  brown  or  black  streaks  were 
produced.  This  process  was  kept  very  secret  by  the  Roman 
lapidaries,  and  for  centuries  they  came  to  the  valley  of  the  Nahe 
(where  formerly  rich  agate-quarries  existed,  and  which  is  still 
the  chief  seat  of  the  agate  industry)  to  buy  up  badly-coloured 
stones  and  to  colour  them,  at  home.  Not  very  long  ago  a  trades- 
1  Maschke,  Pogg.  Ann.  145,  549,  and  146,  90. 


SILICIC  ACID  819 


man  from  Idar  was  imprisoned  in  Paris  for  debt,  and  met  there 
a  "Romaner,"  who  told  him  the  secret,  which  consists  of  soak- 
ing the  stones  in  sulphuric  acid  after  they  have  been  boiled  in 
honey,  the  sugar  being  thus  carbonized  and  the  agate  coloured 
dark.1  Agate  is  also  largely  used  for  making  the  agate  mortars 
so  necessary  for  the  chemist,  and  rock  crystal  is  employed  for 
preparing  the  unalterable  weights  which  he  uses  in  his  most 
accurate  investigations. 

SILICIC  ACID. 

482  Silica  belongs  to  the  class  of  acid-forming  oxides,  and  we 
should  therefore  expect  the  hydrated  acid  to  have  either  the 
formula  Si(OH)4  or  SiO(OH)2. 

Orthosilicic  acid,  Si(OH)4,  is  just  as  little  known  as  is  sulphurous 
acid,  H2SO3,  or  carbonic  acid,  H2CO3,  since,  like  these  acids,  it 
appears  to  have  a  great  tendency  to  split  up  into  water  and  the 
acid-forming  oxide.  If  a  solution  of  an  alkaline  silicate,  termed 
soluble  glass,  be  acidified  with  hydrochloric  acid,  a  portion  of 
the  silicic  acid  separates  out  as  a  gelatinous  mass,  whilst  another 
portion  remains  in  solution.  If,  on  the  other  hand,  the  solu- 
tion is  sufficiently  dilute,  no  precipitate  will  occur,  all  the  silicic 
acid  remaining  dissolved.  This  liquid  contains  silicic  acid, 
hydrochloric  acid,  and  common  salt  in  solution.  In  order  to 
separate  the  two  latter  compounds  from  the  former,  the  liquid  is 
brought  into  a  flat  drum,  the  bottom  of  which  consists  of  parch- 
ment paper,  and  this  "  dialyser  "  containing  the  liquid  is  allowed 
to  swim  on  the  surface  of  a  large  volume  of  water.  The  sodium 
chloride  and  the  excess  of  hydrochloric  acid  pass  through  the 
membrane,  whilst  a  clear  aqueous  solution  of  silicic  acid  remains 
behind. 

This  mode  of  separation  was  termed  "  dialysis "  by  its  dis- 
coverer, Graham.2  This  chemist  first  pointed  out  that  sub- 
stances which  crystallize,  hence  termed  "  crystalloids,"  have  the 
power  of  passing  in  solution  through  a  porous  membrane,  whilst 
substances,  such  as  gum  and  glue,  which  form  jellies  and 
are  termed  "  colloids,"  are  unable  to  pass  through  a  porous 
diaphragm  or  septum  such  as  parchment  paper.  In  this 
way  an  aqueous  solution  of  pure  silicic  acid  may  be  obtained 
which  contains  5  per  cent,  of  silica,  and  this  may  be  concen- 

1  Ber.  ilber  die  Entw.  Chem.  2nd.  p.  293. 

2  Phil.  Trans.  1861,  p.  204. 


820  THE  NON-METALLIC  ELEMENTS 


trated  by  boiling  it  in  a  flask  until  it  reaches  a  strength  of  14 
per  cent.  When  heated  in  an  open  vessel,  such  as  an  evaporat- 
ing basin,  it  is  apt  to  gelatinize  round  the  edge,  after  which  the 
whole  solidifies.  The  solution  of  silicic  acid  thus  prepared  has  a 
feebly  acid  reaction  and  is  colourless,  limpid,  and  tasteless.  On 
standing  for  a  few  days  the  solution  gelatinizes  to  a  transparent 
jelly.  This  coagulation  is  retarded  by  the  presence  of  a  few 
drops  of  hydrochloric  acid,  or  of  caustic  alkali,  but  it  is  brought 
about  by  even  the  smallest  traces  of  an  alkaline  carbonate. 

If  the  clear  solution  be  allowed  to  evaporate  in  a  vacuum 
at  15°  a  transparent  glasslike  mass  remains  behind,  which, 
when  dried  over  sulphuric  acid,  possesses  approximately  the 
formula,  H2SiO3  =  SiO2  +  H20.  This  has  been  termed  meta- 
silicic  acid.  By  drying  the  gelatinous  silicic  acid  at  the 
ordinary  temperature,  it  was  at  one  time  supposed  that 
hydrates  of  a  constant  composition  could  be  obtained,  and  it 
was  believed  that  the  different  kinds  of  opals,  which  usually 
contain  water,  consisted  of  hydrates  of  well-defined  composition 
corresponding  with  the  different  hydrates  of  phosphoric  acid. 
Further  investigation,  however,  has  shown  that  the  quantity 
of  water  contained  in  the  artificial,  as  well  as  in  the  natural 
amorphous  silica,  varies  within  very  considerable  limits,  whilst 
the  water  can  be  partially  driven  off  at  low  temperatures,  so 
that  it  is  now  supposed  to  be  mechanically  and  not  chemically 
combined.  In  other  words,  these  hydrates  must  be  considered 
to  be  very  loose  compounds  of  silica  and  water. 

THE  SILICATES. 

483  Although  no  solid  hydrate  of  silicic  acid  possessing  a 
constant  composition  is  known,  we  are  acquainted  with  an  ex- 
tremely large  number  of  the  salts  of  silicic  acid  termed  silicates, 
of  which  by  far  the  largest  proportion  occur  in  nature  as  distinct 
mineral  species.  Of  great  theoretical  interest  also  are  the  volatile 
organic  silicates  or  silicic  ethers,  which  correspond  in  formula 
with  the  hypothetical  modifications  of  silicic  acid,  the  hydrogen 
atoms  of  the  latter  being  replaced  by  monovalent  organic 
radicals.  Our  knowledge  of  the  chemical  nature  of  the  mineral 
silicates  is  very  limited,  and  does  not,  as  a  rule,  extend  beyond 
the  empirical  or  simplest  formula  of  the  compound.  Many  of 
the  naturally  occurring  silicates  contain  water,  which  may  be 
either  present  as  water  of  crystallization,  which  is  lost  at  a 


THE  SILICATES 


comparatively  low  temperature,  or  as  water  of  constitution,  in 
which  case  it  is  only  lost  on  strong  ignition. 

According  to  Groth,1  nearly  all  the  silicates  fall  into  one  or 
other  of  the  classes  contained  in  the  following  table,  a  few 
examples  of  each  class  being  given.  The  small  Roman 
numerals  placed  over  the  symbols  denote  the  number  of  atoms 
of  hydrogen  replaced  by  the  metal  or  group  in  question.  In 
many  minerals  some  of  the  metals  are  partially  displaced  by 
equivalent  amounts  of  other  isomorphous  elements. 

I.  Derivatives  of  orthosilicic  acid,  H4SiO4. 

Ethyl  orthosilicate     .        .        .  (02H5)4Si04. 
Sodium  silicate          .        .        .  Na4Si04 

Olivine Mg2SiO4. 

Phenakite Be2Si04. 

Dioptase    •  H2CuSiO4. 

iii 

Muscovite  (potash  mica)  .        .  KH2Al3(SiO4)3. 

iii       ii 

Common  garnet         .        .        .  Al2Ca3(SiO4)3. 

II.  Derivatives  of  metasilicic  acid,  H9SiO3. 

Ethyl  metasilicate     .        .        .  (C2H5)2SiO3. 
Potassium  metasilicate     .        .  K2Si03. 
Wollastonite      ....  CaSiO3. 
Enstatite MgSiO3. 

Augite CaMg(Si03)2. 

i     iii 

Leucite KAl(SiO3)2. 

Talc H2Mg3(Si03)4. 

ii       iii 

Emerald Be3Al2(SiO3)6. 

III.  Derivatives  of  disilicic  acid,  H2Si205,  and  of  the  polysilicic 
acid,  H4Si3O8,  derived  from  this  and  metasilicic  acid. 

Orthoclase  (felspar)  .        .  (K ;  Na)  AlSi3O8. 

i          iii 

Petalite      .  ...  (Li ;  Na)6Al8(Si206)15. 

IV.  Basic  silicates,  derived  from  ortho-  and  metasilicic  acids. 

iii         i 

Andalusite         ....  Al(AlO)Si04. 

Calamine (ZnOH)2SiO3. 

Serpentine         ....  Mg2(MgOH)H3(SiO4)2. 

1  Tabellarische  Uebersicht  der  Mineralien  (Vienna,  1882). 


822  THE  NON-METALLIC  ELEMENTS 

V.  Hydrated  silicates. 

(a)  Zeolites,  containing  water  of  crystallization  : 

Natrolite          ....  Na2Al(Alb)(SiO3)3+2H2O. 
Analcime         .  .        .  NaAl(SiO3)2  +  H2O. 

(b)  Containing  varying  amounts  of  water  loosely  combined  : 
Allophane        ....  Al2SiO5  +  5H.2O. 
Halloysite        ....  Al2Si2O7  +  4H2O. 

Many  silicates  have  been  prepared  artificially  either  by  the 
fusion  of  the  constituents  in  the  proper  proportions  with  or 
without  a  flux,  or  by  heating  them  together  with  water  at  a  high 
temperature  under  pressure.  Thus  felspar,  mica,  olivine,  and 
garnet  have  all  been  prepared  artificially,  and  these  researches 
have  thrown  much  light  on  the  study  of  the  igneous  rocks  in 
which  these  minerals  occur.1 

The  silica,tes  of  the  alkali-metals  are  the  only  ones  soluble 
in  water.  These  form  the  so-called  soluble  glass,  which  dis- 
solves the  more  readily  the  larger  the  quantity  of  alkali  it 
contains.  Helmont  was  acquainted  with  this  property  of  the 
silicates,  and  was  aware  that  acids  precipitated  silicic  acid  from 
such  an  alkaline  solution. 

Of  the  silicates  insoluble  in  water,  some,  and  especially  those 
which  are  hydrated  silicates  containing  water  in  combination, 
are  easily  attacked  by  hydrochloric  acid.  In  this  case  the  silicic 
acid  separates  out  as  a  gelatinous  mass. 

In  order  to  decompose  the  silicates  which  are  unattacked 
by  acids,  it  is  necessary  to  fuse  them  with  an  alkali  carbonate 
and  afterwards  to  decompose  the  fused  mass  with  hydrochloric 
acid. 

SILICON  OXYCHLORIDE,  Si2Cl60  =  283'42. 

484  When  the  vapour  of  silicon  tetrachloride  is  passed  over 
felspar,  AlKSi3O8,  heated  in  a  porcelain  tube  to  whiteness, 
potassium  chloride  and  silicon  oxychloride  are  formed,  and  this 
compound  is  also  obtained  when  a  heated  mixture  of  chlorine 
with  one  fifth  to  one  half  of  its  volume  of  oxygen  is  passed 
over  crystallized  silicon  at  about  8000.2  It  is  a  colourless 

1  For  further  information  on  this  point  see  Fuchs,  Die  kiinstlichen  Mineralien 
(Harlem,  1870).     Fouque  and  Levy,  Synthese  des  Mineraux  ct  des  Roches  (Paris, 
1882).     Bourgeois,  Reproduction  artificielle  des  Mineraux  (Paris,  1884).     Traube, 
Ber.  26,  2735. 

2  Troost  and  Hautefeuille,  Bull.  Soc.  [2],  35,  360. 


SILICON  AND  SULPHUR  823 


fuming  liquid  boiling  at  137°,  and  is  easily  decomposed  by 
water  into  silicic  and  hydrochloric  acids.  Its  vapour  density 
is  IQ'05  corresponding  with  the  formula  given.  By  the 
further  action  of  this  oxy chloride  on  heated  felspar,  or,  better, 
by  repeatedly  passing  a  mixture  of  oxygen  and  the  oxy  chloride 
through  a  red-hot  tube  filled  with  pieces  of  porcelain,  the 
following  series  of  oxychlorides  has  been  obtained  by  Troost 
and  Hautefeuille  : — l 

Boiling  point. 

Si4O3Cl10  152°  to  154° 

Si4O4Cl8  198°  to  202° 

Si8O10Cl12  about  300° 

(Si2O3Cl2)n  above  400° 

(Si407Cl2)n  „      440°. 

The  vapour  density  of  the  first  of  these  corresponds  with  half 
the  molecular  weight  required  for  the  formula  given.  The 
molecular  weights  of  the  second  and  third  have  been  ascertained 
by  a  determination  of  their  vapour  densities,  whilst  those  of  the 
last  two  compounds  are  unknown,  but  are  doubtless  multiples  of 
the  simplest  ratio. 


SILICON  AND  SULPHUR. 

SILICON  BISULPHIDE,  SiS2. 

485  A  mixture  of  silica  and  carbon,  such  as  is  used  in  the 
preparation  of  silicon  tetrachloride,  is  employed  for  the  prepara- 
tion of  this  compound.  The  vapour  of  carbon  bisulphide  being 
led  over  this  mixture  heated  to  whiteness,  long  silky  needles 
are  formed,  which  burn  in  the  air  to  form  silica  and  sulphur 
dioxide  and  are  decomposed  by  water  into  sulphuretted  hydro- 
gen and  silicic  acid.  This  last  substance  is  thus  obtained  in 
the  form  of  bright  shining  crystals  when  the  sulphide  is  exposed 
to  the  action  of  moist  air.2 

An  orange-yellow  coloured  volatile  sub-sulphide,  SiS,  and  an 
oxysulphide  of  silicon,  SiSO,  are  said  to  be  formed  by  the  action 
of  sulphuretted  hydrogen  on  silicon  at  a  white  heat.3 

1  Ann.  Chim.  Phys.  [5],  7,  453. 

2  Fremy,  Ann.  Chim.  Phys.  [3],  38,  314. 

3  Colson,  Bull.  Soc.  Chim.  [2],  38,  56. 


THE  NON-METALLIC  ELEMENTS 


SILICON  CHLOROHYDROSULPHIDE,  SiCl3SH. 

This  compound  was  discovered  by  Pierre,1  but  its  exact 
composition  was  determined  by  Friedel  and  Ladenburg.2  In 
order  to  prepare  it  a  mixture  of  sulphuretted  hydrogen  and 
the  vapour  of  silicon  tetrachloride  is  passed  through  a  red- 
hot  porcelain  tube : — 

SiCl4  +  H2S  -  SiCl3HS  +  HC1. 

It  is  a  colourless  liquid  which  boils  at  96°,  and  fumes  in  tne 
air,  water  decomposing  the  compound  into  hydrochloric  and 
silicic  acids,  sulphuretted  hydrogen  and  sulphur.  Its  vapour 
possesses  a  specific  gravity  of  5 '78. 


SILICON  AND  NITROGEN. 
SILICON  NITRIDE. 

486  When  silicon  is  strongly  heated  in  nitrogen  gas,  a  white 
amorphous  substance,  known  as  silicon  nitride,  is  produced.  If 
pure  nitrogen  is  employed,  the  nitride  formed  has,  according  to 
Schiitzenberger,3  the  composition  Si2N3,  and  is  insoluble  in 
hydrofluoric  acid.  When  the  experiment  is  carried  out,  as  it 
was  by  Deville  and  Wohler,4  by  heating  silicon  in  a  crucible 
placed  inside  another  which  was  packed  with  carbon  to  pre- 
vent the  access  of  oxygen,  the  product  contains  silicon  carbo- 
nitride,  Si2C2N,  silicon  carbide,  SiC,  silicon  carboxide,  SiCO, 
and  a  nitride  of  the  formula  SisN4  which  dissolves  in  hydro- 
fluoric acid  with  formation  of  ammonium  silicofluoride.5 

A  white  amorphous  powder  was  also  obtained  by  Deville  and 
Wohler  6  by  the  action  of  ammonia  on  silicon  chloride,  which, 
according  to  Gatterman,7  contains  a  substance  of  the  compo- 
sition SiN2H2.  According  to  Schiitzenberger  and  Colson  the 
product  of  the  reaction,  after  ignition  in  a  current  of  hydrogen, 
has  the  composition  Si5N6Cl2,  and  when  ignited  in  a  current  of 
ammonia  yields  a  substance  of  the  formula  HSi2N3,  which  is 
soluble  in  hydrofluoric  acid  and  evolves  ammonia  when  treated 
with  caustic  potash.8 

1  Ann.  Chim.  Phys   [3],  24,  286.  2  Ann.  Chim.  Phys.  [4],  27,  416. 

3  Compt.  Rend.  89,  644  ;  92,  1508.  4  Annalen,  HQ,  248. 

6  Compt.  Rend.  H4,  1089.  «  Annalen,  104,  256. 

7  Ber.  22,  1,  94.  «  Compt    Rend.  92,  1511. 


SILICON  AND  CARBON  825 

SILICON  AND  CARBON. 

SILICON  CARBIDE,  SiC. 

487  Silicon  carbide  is  formed  when  a  mixture  of  coke,  sand 
and  salt  is  fused  in  an  electric  furnace  by  means  of  a  carbon 
terminal,1  and  may  be  obtained  pure  by  fusing  silicon  with  the 
requisite  amount  of  carbon  in  the  electric  furnace.2  It  is  also 
formed  by  the  action  of  carbon  on  iron  silicide  and  by  the 
union  of  the  vapours  of  carbon  and  silicon  in  the  electric 
furnace,  needles  of  the  carbide  being  deposited  (Moissan). 
The  crystals  when  pure  are  colourless  or  sapphire  blue  and  are 
usually  found  as  six-sided  plates,  but  when  formed  by  the  com- 
bination of  the  two  vapours  they  are  prismatic.  Their  specific 
gravity  is  3*12  and  they  are  hard  enough  to  scratch  rubies. 
Silicon  carbide  is  not  oxidized  by  oxygen  at  1000°  and  is  not 
attacked  by  sulphur  vapour,  by  fused  potassium  nitrate,  or  by 
any  acid.  It  is  completely  decomposed  by  chlorine  at  1200° 
and  on  fusion  with  lead  chromate  it  is  gradually  oxidized,  whilst 
fused  caustic  potash  slowly  converts  it  into  potassium  carbonate 
and  silicate. 

The  crude  material,  obtained  by  the  method  first  described, 
is  known  as  carborundum  and  is  used  as  a  cutting  and  polishing 
agent. 

SILICON  CARBOXIDE,  SiCO. 

When  silicon  is  heated  to  whiteness  in  an  atmosphere  of 
carbon  dioxide,  a  greenish  white  mass  is  left,  which  after  treat- 
ment with  hydrofluoric  acid  has  the  composition  (SiCO)x. 

3Si  +  2C02  =  Si02  +  2SiCO. 

This  compound  is  not  acted  on  by  alkalis  and  is  not  affected  by 
being  heated  in  oxygen,  but  is  oxidized  by  a  mixture  of  oxide 
and  chromate  of  lead.  At  a  lower  temperature  silicon  carbide 
is  formed.3 

Amorphous,  substances  of  the  empirical  formula,  SiCO3, 
Si2C2O,  and  Si2C3O2  have  also  been  obtained  by  analogous 
methods.4 

1  Miihlhauser,  Zeit.  Anorg.  Chem.  5,  105. 

2  Moissan,  Compt.  Rend.  117,  425. 

3  Schiitzenberger  and  Colson,  Compt.  Rend,  92,  1508  ;  114,  1087. 

4  Colson,  Bull.  Soc.  Chem.  [2].  38,  56.  .    . 


326  THE  NON-METALLIC  ELEMENTS 


CONSTITUTION  OF  THE  VOLATILE  SILICON  COMPOUNDS. 

488  Silicon  forms  a  series  of  compounds  which  are  volatile,  and 
the  molecular  weight  of  which  can,  therefore,  be  readily  ascer- 
tained. When  this  has  been  determined,  it  appears  that  the 
chemical  constitution  of  these  bodies,  that  is  to  say,  the  mode 
in  which  the  different  atoms  are  arranged  in  the  molecule,  can 
be  most  simply  expressed  on  the  assumption  that  silicon  is  a 
tetravalent  element.  The  following  graphic  formulae  show  the 
constitution  of  the  most  important  of  these  compounds  : — 

Silicon  tetrachloride,  Silicon  oxychloride, 

Cl  Cl 


Cl— Si— Cl  Cl— Si— Cl 

i  j, 

Cl-  Si— Cl 

i 

Silicon  trichloride,  Silicon  chlorhydrosulphide, 

Cl  Cl 


Cl— Si— Cl  Cl— Si— S— H 

Cl— Si— Cl  Cl 


CRYSTALLOGRAPHY  827 


CRYSTALLOGRAPHY. 

489  When  a  chemical  substance  passes  from  the  gaseous  or 
the  liquid  state  into  that  of  a  solid,  it  generally  assumes  a 
definite  geometrical  form,  and  is  said  to  crystallize. 

A  crystal  is  a  solid  body,  formed  in  this  way,  and  bounded  by 
plane  surfaces.  As  a  rule,  every  chemical  substance  in  the 
solid  state  possesses  a  distinct  form  in  which  it  crystallizes,  and 
by  which  it  can  be  distinguished. 

The  occurrence  of  various  mineral  substances  in  distinct 
crystalline  forms  was  noticed  by  the  ancients,  and  they  gave  the 
name  crystal  (/c/ouo-raXXo?,  ice)  to  one  of  these,  viz.,  to  quartz  or 
rock-crystal,  because  they  believed  that  this  body  owed  its  for- 
mation to  the  effect  of  cold.  Geber  was  aware  that  crystals 
can  be  obtained  artificially  by  the  evaporation  of  a  solution  of 
a,  salt.  He  describes  the  production  of  several  chemical  com- 
pounds in  the  crystalline  condition,  and  shows  how  they  may 
be  purified  by  recrystallization.  Many  years,  however,  elapsed 
before  this  property  of  matter  was  regarded  as  anything  more 
than  an  unimportant  one.  Libavius,  it  is  true,  asserted  in  the 
year  1597,  that  the  nature  of  the  saline  components  of  a  mineral- 
water  could  be  ascertained  by  an  examination  of  the  crystalline 
forms  of  the  salts  left  on  the  evaporation  of  the  water.  But 
so  imperfect  were  the  views  regarding  the  formation  of  crystals 
that  Lemery  classed  crystals  simply  according  to  their  thickness, 
believing  that  this  is  dependent  upon  the  size  of  the  ultimate 
particles  of  the  acids  contained  in  the  salts.  In  the  year  1703, 
Stahl  pointed  out  that  the  compounds  obtained  by  the  action 
of  acids  upon  sea-salt  crystallized  in  forms  different  frohi  those 
assumed  by  the  corresponding  compounds  of  potash,  and  hence 
he  concluded  that  sea-salt  contains  a  peculiar  alkali,  distinct 
from  the  common  alkali  potash.  Gulielmini  appears  to  have  held 
much  more  rational  views  than  Lemery  concerning  the  formation 
of  crystals.  In  his  Dissertatio  de  Salibws,  published  in  1707, 
he  asserts  that  the  smallest  particles  possess  definite  crystalline 


828  CRYSTALLOGRAPHY 

forms,  and  that  the  differences  in  form  which  we  observe  between 
the  crystals  of  alum,  nitre,  and  sea-salt,  are  caused  by  similar 
differences  existing  in  the  forms  of  the  smallest  particles. 

It  is,  however,  to  Haiiy  (1743—1822)  that  we  are  indebted 
for  the  foundations  of  the  science  of  crystallography.  Haiiy  was 
the  first  to  point  out  the  fact  that  every  crystalline  substance 
possesses  certain  definite  and  characteristic  forms  in  which  it 
crystallizes,  and  that  all  these  different  forms  can  be  derived  from 
one  fundamental  form  which  can  be  ascertained  by  measure- 
ment of  the  angles  of  the  crystal.  Upon  this  principle  he 
founded  in  1801  his  celebrated  system  of  the  classification  of 
minerals. 

General  Characteristics  of  Crystals. — A  crystal  of  any  sub- 
stance is  characterised,  in  the  first  place,  by  its  special 
geometrical  form,  and  in  the  second  place,  by  its  physical 
properties,  both  of  which  we  must  suppose  to  be  due  to  the 
special  mode  of  arrangement  of  the  particles  of  which  the  crys- 
tal is  built  up.  In  an  amorphous,  or  non-crystalline,  substance 
the  physical  properties  are  as  a  rule  the  same  in  every  direction 
throughout  its  mass,  whilst  in  a  crystalline  substance  this  may 
not  be,  and  usually  is  not,  the  case.  Thus,  for  example,  under 
ordinary  conditions  light  travels  through  a  piece  of  glass  at  the 
same  rate  in  whatever  direction  it  may  happen  to  pass,  whilst 
in  a  crystal  of  quartz  or  calc-spar  the  rate  is  different  in  different 
directions.  The  physical  properties  are  even  more  character- 
istic of  a  crystalline  substance  than  the  geometrical  form  which 
it  assumes,  and  can  be  recognised  in  fragments  of  the  substance 
in  which  the  characteristic  crystalline  form  may  be  quite  un- 
recognisable. 

Symmetry  of  Crystals. — Any  plane  which  divides  a  crystal 
into  two  parts  which  are  geometrically  similar,  so  that  one  part 
bears  the  same  relation  to  the  other  as  it  would  to  its  image  in 
a  plane  mirror  placed  in  the  same  position  as  the  dividing  plane, 
is  called  a  plane  of  symmetry,  whilst  a  direction  at  right  angles 
to  this  plane  is  known  as  an  axis  of  symmetry.  Thus  in  Fig.  230, 
representing  a  double  pyramid  with  a  square  base,  there  are 
shown  three  planes  of  symmetry,  ABCD,  AECF,  BEDF,  each  of 
which  divides  the  figure  into  two  symmetrical  halves,  and  three 
axes  of  symmetry  EF,  AC,  and  BD.  These  three  planes  have 
not  however  the  same  degree  of  symmetry.  If  the  whole  figure 
be  rotated  about  the  axis  EF  through  an  angle  of  90°,  the  axis 
AC  will  take  the  position  BD,  and  a  figure  precisely  similar  to 


SYMMETRY  OF  CRYSTALS 


829 


the  original  will  be  obtained.  If,  however,  the  rotation  be 
made  about  BD  this  will  not  be  the  case ;  the  axis  EF  will  take 
the  direction  AC,  but,  since  it  is  not  equal  to  AC,  the  resulting 
figure  will  be  different  from  the  original,  and  in  order  to  obtain 
a  similar  figure  the  rotation  must  be  continued  through  an 
additional  angle  of  90°.  The  axes  AC  and  BD  are  said  to  be 
equivalent,  and  the  plane  ABCD  which  contains  two  equivalent 
axes  is  called  a  principal  plane  of  symmetry,  whilst  the  planes 
AECF  and  BEDF  are  termed  secondary  planes  of  symmetry. 
The  corresponding  axes  are  similarly  distinguished.  It  has, 
moreover,  been  found  that  every  plane  of  geometrical  symmetry 
of  a  crystal  is  also  a  plane  of  physical  symmetry,  the  two  parts 


into  which  the  crystal  is  divided  being  not  only  geometrical  but 
also  physical  counterparts. 

Classification  of  Crystals. — The  classification  of  crystals  is 
based  on  the  degree  of  symmetry  which  they  possess,  this  being 
determined  by  the  number  of  principal  and  secondary  planes  of 
symmetry  which  characterise  them.  All  the  possible  forms  of 
crystals  are  comprised  in  the  following  six  systems  : — 


Degree  of  Symmetry. 

Three  principal  planes,  cut- 
ting one  another  at  90°. 

Six  secondary  planes,  cutting 
one  another  at  60°. 


Name  of  System. 
Regular  or  Cubic. 


830 


CRYSTALLOGRAPHY 


Degree  of  Symmetry. 

2.  One  principal  plane. 

(a)  Six  secondary  planes,  cut- 
ting one  another  at  30°. 

(b)  Four    secondary    planes, 
cutting    one     another    at 
45°. 

3.  No  principal  plane. 

(a)  Three   secondary   planes, 
cutting    one    another    at 
90°. 

(b)  One  secondary  plane. 

(c)  No  secondary  plane. 


Name  of  System. 

Hexagonal. 
Quadratic. 


Rhombic. 

Monosymmetric. 

Asymmetric. 


Parts  of  a  Crystal,  Faces,  Edges,  and  Angles. — The  plane  sur- 
faces by  which  crystals  are  bounded  are  termed  faces.  The 
straight  lines  formed  by  the  intersection  of  two  contiguous 


ccOx 

COO® 

,„.-"•-  

*~-^p 

FIG.  231. 


FIG.  232. 


FIG   233. 


faces  are  termed  edges.  Angles  are  made  by  the  incidence  of 
the  bounding  surfaces  or  faces,  and  are  sometimes  termed  solid 
angles  or  summits,  and  are  distinguished  as  three-faced,  four- 
faced,  &c.,  according  to  the  number  of  faces  by  which  they  are 
formed. 

Similar  faces  are  those  which  resemble  each  other  in  form 
and  have  a  similar  relative  position.  Dissimilar  faces  are  those 
which  are  unequal,  and  occupy  different  positions.  The  crystals 
represented  by  Figs.  231  and  232  are  bounded  by  similar  faces; 
that  shown  in  Fig.  233  is  bounded  by  dissimilar  faces.  In  Fig. 
233  the  four-sided  faces  lying  between  four  trianglar  faces  are 
similar,  and  these  latter  are  also  similar. 

Simple  and  Complex  Forms. — A  crystal    which    is   bounded 


PARTS  OF  A  CRYSTAL 


831 


entirely  by  similar  faces  is  termed  a  simple  form.  A  form  in 
which  dissimilar  faces  occur  is  termed  a  complex  form,  or  a 
combination.  Every  complex  form  is  made  up  of  two  or  more 
simple  forms.  This  will  be  understood  if  we  imagine  one 
set  of  its  similar  faces  to  be  extended  until  all  dissimilar 
faces  have  disappeared.  Thus,  if  the  triangular  faces  in  Fig. 


FIG.  234. 


FIG.  235. 


233  be  extended  until  they  inclose  space,  the  form  will  become 
that  of  the  octohedron  (Fig.  231) ;  whereas  a  similar  extension  in 
the  square  faces  gives  rise  to  a  cube  (Fig.  232). 

Faces  are  said  to  be  dominant  when  they  are  the  largest 
present  in  a  given  combination,  or  when  their  simplest  form 
most  closely  resembles  that  of  the  combination.  Subordinate  or 


71  J 

r"j-\ 

! 

\ 

; 

.P! 

1 

: 

jfc* 

OOP 


\ 

oq'P 


FIG.  237. 


secondary  faces,  on  the  other  hand,  are  the  smallest  ones  seen  in 
a  combination,  or  those  which  are  relatively  unimportant,  and  do 
not  determine  the  form  of  the  combination.  Figs.  233,  234,  and 
235  represent  combinations  of  the  octohedron  and  the  cube.  In 
Fig.  233  the  faces  of  both  forms  are  equally  developed  ;  in  Fig. 
234  the  octohedral  faces  are  dominant ;  in  Fig.  235  those  of  the 
cube. 


832  CRYSTALLOGRAPHY 

The  simple  dominant  form  is  termed  the  fundamental  form  ; 
simple  forms  are  again  distinguished  as  closed  and  unclosed 
forms.  The  first  of  these  classes  includes  all  those  whose  faces, 
or  faces  produced,  inclose  space.  Such  a  form  is  shown  for  the 
faces  marked  P,  Fig.  236,  which  will,  if  produced,  give  rise  to  the 
double  six-sided  pyramid.  The  second  class  is  made  up  of  forms 
which  cannot  thus  inclose  space,  and  an  example  of  such  a 
form  is  seen  in  Fig.  237,  in  which  the  faces  marked  oo  P  are 
arranged  in  directions  parallel  to  each  other,  and  cannot  inclose 
space.  Fig.  237  is  a  crystal  made  up  of  two  open  forms,  the 
faces  oo  P,  and  the  faces  0  P. 

Crystallographic  Axes  and  Symbols. — For  the  sake  of  conveni- 
ence it  is  usual  to  select  three  (or  in  the  hexagonal  system  four) 
lines  intersecting  in  a  point  as  axes  about  which  the  faces  of 
the  crystal  are  disposed.  Whenever  possible,  axes  of  symmetry 
are  chosen  for  this  purpose,  and  are  termed  the  Crystallographic 
axes.  Thus,  in  the  regular  system,  which  is  characterised  by 
three  principal  planes  of  symmetry,  the  three  principal  axes  of 
symmetry  are  taken  as  the  Crystallographic  axes,  these  being  all 
at  right  angles  to  each  other ;  whilst  in  the  rhombic  system, 
which  has  no  principal  but  three  secondary  planes  of  symmetry, 
the  secondary  axes,  which  are  also  at  right  angles,  are  chosen. 
In  order  to  briefly  express  the  position  of  any  face  with  refer- 
ence to  these  axes,  the  lengths  cut  off  by  this  face  from  the 
three  axes  are  expressed  in  terms  of  the  lengths,  cut  off  by  a 
face  of  one  of  the  simple  forms  of  the  crystal,  the  simplest 
pyramid,  each  face  of  which  cuts  all  the  three  axes,  being 
generally  taken.  The  ratio  of  the  lengths  thus  cut  off  from  the 
axes  by  this  fundamental  face  is  known  as  the  axial  ratio. 

In  the  regular  system  the  simplest  pyramid,  the  regular 
octohedron  (Fig.  231)  cuts  all  the  axes  at  equal  distances  from 
their  point  of  intersection,  since  the  three  axes  are  interchange- 
able, and  hence  the  crystal  made  up  of  such  faces  is  known  by 
the  formula  a:a:a  (Weiss),  or  still  more  shortly  0  (contracted 
from  octohedron  ;  Naumann).  A  crystal  of  which  each  face  cuts 
one  of  these  equal  axes  at  the  distance  a,  a  second  at  4a,  and  the 
third  at  2a,  is  similarly  known  as,  a :  4a :  2a  or  more  shortly 
402,  the  numbers  4  and  2  being  known  as  the  parameters  of 
the  face. 

Another  system  of  symbols,  known  as  Miller's  index  system, 
is  also  largely  used.  The  fundamental  face  (0)  is  represented 
by  the  formula  (111),  whilst  the  position  of  any  other  face  is 


HEMIHEDRAL  FORMS 


833 


given  by  indices  which  are  the  reciprocals  of  the  parameters. 
Thus  in  the  case  of  the  face  a  :  4a :  2a,  the  parameters  are 
1:4:2,  and  the  indices  therefore  1 : 1/4 : 1/2,  or,  removing 
fractions  (412).  In  the  rhombic  system  the  face  of  the  simplest 
pyramid  cuts  the  three  axes  at  different  distances  a  :  b  :  c,  con- 
tracted to  P  (pyramid) ;  a  face  cutting  the  axis  a  at  the  distance 
2a,  the  axis  b  as  before,  and  the  axis  c  at  2c.  would  be  expressed 
as  2a  :  b  :  2c  (Weiss)  ;  2P2  (Naumann)  ;  121  (Miller). 

If  a  face  is  parallel  to  one  of  the  axes  its  parameter  is  infinity 
(•oo),  and  its  index  the  reciprocal  of  this  or  zero. 

It  has  been  found  experimentally  that  only  faces  of  which  the 
indices  may  be  expressed  by  rational  numbers  occur  on  any 
crystal. 

Holohedral,  Hemihedral,  Tetartohedral,  and  Hemimorphous 
Forms. — A  crystal  which  is  bounded  by  all  the  faces  required  by 


FIG.  238. 


FIG.  239. 


the  symmetry  of  the  system  to  which  it  belongs  is  said  to  be 
holohedral.  Thus  in  the  regular  system,  the  octohedron  is  a 
holohedral  form,  because,  starting  with  one  face  cutting  all 
three  axes  at  equal  distances,  the  symmetry  of  the  system 
requires  seven  other  faces  fulfilling  the  same  condition,  making 
in  all  the  eight  faces  of  the  octohedron.  In  many  cases,  how- 
ever, only  one  half  of  these  faces  is  present  on  a  crystal,  which 
is  then  said  to  be  hemihedral.  Thus,  if  the  four  faces  afd,  fbc, 
bga,  and  gcd  of  the  octohedron  in  Fig.  239  be  extended,  a  closed 
form  (Fig.  238),  known  as  the  regular  tetrahedron,  is  obtained, 
whilst  the  extension  of  the  remaining  four  faces  gives  rise  to  a 
similar  figure  (Fig.  240).  These  two  forms,  although  identical 
in  shape,  are  distinguished  as  positive  and  negative,  and  may 
occur  in  combination  with  other  forms  either  alone  or  together. 
Figs.  241  and  242  represent  combinations  of  the  cube  with  a 
positive  tetrahedron. 

54 


834 


CRYSTALLOGRAPHY 


When  the  two  tetrahedra  are  equally  developed  on  the  same 
crystal  the  form  produced  is  identical  with  the  original  octo- 
hedron. 

When  only  one  quarter  of  the  faces  of  the  holohedral  form 
occurs,  the  crystal  is  said  to  be  tetartohedral. 

Hemihedral  crystals  always  have  a  smaller  degree  of  geo- 


FIG.  240. 


FIG.  241. 


FIG.  242. 


metrical   symmetry   than    the   holohedral   forms   of  the   same 
system ;  the  physical  symmetry,  however,  remains  unaltered. 

In  other  cases  certain  faces  appear  at  one  end  of  an  axis  of 
symmetry  but  not  at  the  other,  the  crystal  being  thus  made  up 
of  two  dissimilar  portions  (Fig.  308).  Such  crystals  are  said  to 
be  hemimorphous,  and  exhibit  remarkable  physical  differences  at 
the  two  dissimilar  ends. 


FIG.  243. 


FIG.  244. 


Single  Crystals  and  Twin  Crystals,  or  Hades. — Holohedral 
crystals  having  their  faces  symmetrically  arranged  about 
their  axes  are  termed  single  crystals  in  contradistinction  to 
those  which,  although  formed  according  to  a  certain  law,  are  not 
subject  to  the  same  symmetrical  arrangement.  This  latter  class 
are  called  twin  crystals.  In  one  portion  of  such  crystals  the 
axes  lie  in  a  different  position  from  that  which  they  occupy  in 


TWIN  CRYSTALS 


835 


another  portion,  and  the  twin  may  be  supposed  to  be  derived 
from  the  single  or  normal  crystal  by  the  latter  having  been  cut 
into  two  portions  parallel  to  a  certain  plane,  usually  parallel  to 
a  possible  face  of  the  crystal,  and  one  part  having  been  turned 
round  on  the  other  through  a  given  angle.  Fig.  243  represents 


FIG.  245. 


FIG.  246. 


such  a  twin  crystal  or  hemitrope,  often  found  in  magnetic  iron 
ore,  and  obtained  by  turning  one  half  of  an  octohedron  (Fig. 
244)  through  an  angle  of  180°  on  the  other  half.  Fig.  245 
exhibits  a  twin  form  often  observed  in  gypsum. 

These  twin  forms  are  distinguished  from  single  crystals  by 


FIG.  247. 


FIG.  248. 


the  occurrence  in  them  of  re-entering  angles,  as  shown  in  the 
figures.  Intersecting  twin  crystals,  as  opposed  to  twins  by  contact, 
such  as  that  shown  in  Fig.  246,  frequently  occur  in  fluorspar 
and  in  sal-ammoniac. 

Perfect    Crystals    and   Imperfect    or    Distorted    Crystals. — It 
generally  happens  that  crystals  found  in  nature  are  distorted 


836 


CRYSTALLOGRAPHY 


or  irregularly  developed.  For  instance,  crystals  of  quartz  usually 
occur  in  the  forms  shown  in  Figs.  247,  248,  and  249,  which 
apparently  bear  no  relation  to  the  regular  and  much  rarer  form 
of  quartz  seen  in  Fig.  250.  There  is,  however,  no  difficulty 
in  ascertaining  the  simple  form  of  these  crystals  in  spite  of 


FIG.  249. 


FIG.  250. 


this  apparent  want  of  conformity  due  to  the  increase  of  certain 
faces  and   the  decrease  of  others,  and  in   spite   even   of  the 
fact  that  only  half  of  the  complete  crystal  is  often  seen,  as  in 
Fig.  248. 
%  If  the  angles  which  the  similar  faces  in  the  perfect  and  the 


FIG.  251. 


FIG.  252. 


imperfect  crystals  make  with  one  another  be  carefully  measured, 
*we  find  that  the  corresponding  angles  in  the  distorted  forms 
are  always  identical  with  those  in  the  perfect  crystal.  Thus, 
for  instance,  the  contiguous  faces  of  the  prism  cut  each  other 
at  a  constant  angle  of  120°,  whilst  each  face  of  the  pyramid  cuts 
the  next  face  at  an  angle  of  130°  44'. 


MEASUREMENT  OF  CRYSTALS  837 

Similar  distorted  growths  are  observed  in  the  case  of  many 
artificial  crystals.  These  are  produced  by  the  undue  develop- 
ment in  some  special  direction,  owing  to  the  alteration  of 
external  circumstances,  of  the  perfect  crystal  which  is  first 
formed.  Thus  crystals  of  alum,  which  are  octohedral,  appear 
not  unfrequently  in  the  form  shown  in  Fig.  251,  which  is 
derived,  as  is  seen  in  Fig.  252,  by  an  irregular  growth  of  the 
octohedron. 

In  describing  or  drawing  the  crystalline  form  of  any  chemical 
substance,  the  ideal  forms  only  are  considered.  All  irregulari- 
ties and  distortions  are  ignored,  and  the  corresponding  faces  of 
the  crystal  are  supposed  to  be  placed  at  equal  distances  from 
the  origin. 

Artificial  Growth  of  Crystals. — The  small  crystals  which  are 
first  deposited  from  a  solution  are  usually  perfect.  If  these 
are  placed  in  a  concentrated  solution  of  the  substance,  and 
carefully  turned  every  day  so  that  all  the  faces  of  the  crystal 
are  equally  exposed  to  the  action  of  the  solution,  large  and 
perfectly  developed  crystals  can  be  obtained.  The  same  end 
is  attained  by  hanging  a  small  and  perfect  crystal  in  the 
saturated  solution  suspended  by  a  fine  hair.  The  solution 
gradually  evaporates,  and  the  crystal  grows  symmetrically. 
Perfect  crystals  of  alum  of  large  dimensions  can  in  this  way 
readily  be  grown. 

Determination  of  Crystalline  Forms  and  Axial  Ratios. — The 
angles  which  the  faces  of  a  crystal  make  with  one  another 
serve  as  the  data  from  which  the  crystalline  form  is  determined, 
for  it  is  by  the  measurement  of  these  angles  that  the  relative 
length  and  the  mutual  inclination  of  the  axes  can  be  ascertained 
by  calculation. 

The  instruments  used  for  this  purpose  are  the  hand  gonio- 
meter (Fig.  253)  and  the  reflecting  goniometer  (Figs.  254  and 
255).  The  first  of  these  instruments  was  made  in  the  last 
century  by  Carangeot  of  Paris,  for  the  use  of  the  French 
crystallographer  Rom6  de  1'Isle.  It  can  only  be  used  for  the 
measurement  of  tolerably  large  crystals,  and  consists  of  a  divided 
semicircle,  a,  b,  d,  to  which  two  metallic  rules  are  adapted. 
One  of  these  (k  m)  is  fixed,  the  other  (g  Ji)  is  movable  round 
an  axis  (c)  placed  at  the  centre  of  the  semicircle.  The  crystal 
to  be  measured  is  placed  between  the  rules  (g  A)  and  (k  m),  so 
that  the  edges  of  these  rules  may  both  be  at  right  angles  to  the 
line  of  intersection  of  the  two  faces  whose  angular  distance  is  to 


838 


CRYSTALLOGRAPHY 


be  measured.  The  angle  is  then  read  off  on  the  divided  circle. 
This  instrument,  though  very  useful,  and  indeed  necessary  for 
large  crystals,  is  quite  inapplicable  to  small  ones,  and  in  any 
case  cannot  yield  very  accurate  measurements. 


FIG.  253. 


To  obviate  these  difficulties  the  reflecting  goniometer  was 
invented  by  Wollaston  in  the  year  1809.  It  is  arranged  on  the 
following  principle.  The  divided  circle  gh  (Fig.  254)  carries  a 
moveable  axis  or  arm  upon  which  the  crystal  to  be  examined  is 


FIG.  254. 


fixed.  This  crystal,  shown  in  section  in  abed,  fixed  on  the  arm 
by  means  of  some  wax,  must  now  be  placed  in  such  a  position 
that  the  edge  (c),  the  angle  over  which  (b  c  d)  has  to  be  measured, 
is  placed  exactly  parallel  with  the  axis  of  the  instrument. 


MEASUREMENT  OF  CRYSTALS 


839 


Having  placed  the  goniometer  opposite  a  window,  the  eye  of 
the  observer  being  at  (o),  the  reflection  of  one  rib  of  the  window 
in  one  bright  face  of  the  crystal  is  noticed.  The  divided  circle 
is  now  moved  round,  the  crystal  moving  with  it,  until  the 
second  face  (dc)  of  the  crystal  comes  round  to  the  position 
formerly  occupied  by  the  first  (cb),  as  ascertained  by  the  re- 
appearance of  the  rib  of  the  window  frame.  The  angle  through 
which  the  crystal  has  been  turned  is  evidently  the  supplement 


FIG.  255. 


of  the  required  angle,  so  that  if  the  pointer  (p)  stood  at  0°  to 
begin  with,  and  after  the  circle  was  turned  at  x°,  the  angle  of 
the  crystal  is  180°  —  x°.  A  simple  form  of  reflecting  goniometer 
is  seen  in  Fig.  255.  The  crystal  (a)  is  fastened  with  wax  upon 
the  end  of  the  moveable  rod  (oo),  which  can  be  bent  so  as 
to  enable  the  crystal  to  be  properly  adjusted.  The  screw  (G) 
serves  to  turn  the  divided  circle  (E),  so  that  180°  on  the  circle 
is  made  to  coincide  with  the  zero  point  on  the  vernier  (R). 


840 


CKYSTALLOGKAPHY 


When  this  is  done  the  screw  (u)  is  tightened,  and  thus  the 
circle  is  held  fast.1 

Cleavage  of  Crystals. — The  cohesion  of  a  crystal  is  as  a  rule 
less  in  one  direction  than  in  another.  This  direction  is  de- 
pendent upon  the  special  form  of  the  crystal,  and  is  termed 
the  cleavage.  This  facility  of  breaking  more  readily  in  one 
direction  than  another  is  well  seen  when  calc-spar  or  rock  salt 
is  broken.  The  planes  of  cleavage  in  a  crystal  can  be  ascertained 
by  means  of  a  chisel  and  hammer,  or  by  the  help  of  a  strong 
knife. 

I.  THE  REGULAR,  CUBIC,  OR  ISOMETRIC  SYSTEM. 

490  This  system  is  characterised  by  three  principal  planes 
of  symmetry,  all  at  right  angles,  and  six  secondary  planes  of 
symmetry  cutting  one  another  at  an  angle  of  60°. 

The  three  principal  axes  of  symmetry  are  chosen  as  the 
crystallographic  axes,  and  they  are  therefore  all  equal  and  at 
right  angles  to  each  other. 


ooOoo 


FIG.  256. 


FIG.  257. 


The  simplest  form  of  the  system  is  the  regular  octahedron 
(Fig.  256).  Each  of  its  eight  faces  cuts  the  three  axes  at  an 
equal  distance  from  the  origin.  This  distance  being  (a)  the 
general  formula  for  this  form  is  a  :  a :  a  (Weiss),  abbreviated  to 
the  symbol  0  (Naumann).  Many  substances  crystallize  in  octo- 
hedra,  amongst  others  spinelle,  mercury,  magnetic  oxide  of  iron, 
alum,  and  lead  nitrate. 

The  next  simple  form  is  the  cube  (Fig.  257).  Each  face  of 
the  cube  cuts  one  axis  at  the  distance  (a)  and  lies  parallel  to 

1  For  a  description  of  a  much  more  accurate  instrument  Groth's  Physikalische 
Krystallographie  may  be  consulted. 


THE  REGULAR  SYSTEM 


841 


the  other  two  axes.  The  formula  for  this  form  is  oo  a  :  a :  oo  a, 
or  x  0  oo .  The  combinations  of  octohedron  and  cube  have 
been  already  mentioned  (Figs.  233,  234,  and  235).  Sodium 
chloride,  potassium  iodide,  and  fluor-spar  are  substances  which 
crystallize  in  cubes,  whilst  galena,  sulphide  of  silver,  and  lead 
nitrate  occur  in  various  combinations  of  octohedron  and  cube, 


FIG.  258. 


FIG.  259. 


which  are  produced  by  a  truncation  of  the  six  summits  of  the 
octohedron  to  a  greater  or  less  degree. 

The  third  simple  form  is  the  rhombic  dodecahedron  (Fig.  258). 
The  formula  for  this  form  is  a :  a :  oo  a,  or  abbreviated  oo  0,  as 
each  face  of  this  crystal  cuts  two  of  the  axes  at  the  same  distance 
from  the  origin,  whilst  it  is  parallel  to  the  third  axis.  The 


^--             ooO« 

0,0. 

^ 

A                OOO 

X 

0 

s 

8             coOao 

X        ooO 

FIG.  261. 

FIG.  260. 


rhombic  dodecahedron  combines  with  the  octohedron  by  re- 
placing each  of  its  twelve  edges,  as  is  shown  in  Fig.  259.  The 
combination  in  which  the  dodecahedron  is  dominant  is  seen 
in  Fig.  260.  Fig.  261  indicates  the  form  obtained  by  the 
combination  of  cube  and  dodecahedron.  The  following  sub- 
stances crystallize  in  dodecahedra  and  its  combinations :  garnet, 
phosphorus,  cuprous  oxide  (Fig.  259),  magnetic  oxide  of  iron 


842 


CRYSTALLOGRAPHY 


(Fig.  260),  and  alum  when  deposited  from  an  alkaline  solution 
(Fig.  261). 

Each  of  the  simple  crystals  which  we  have  described  is  in- 
capable of  assuming  more  than  one  form.  Other  crystalline 
forms,  belonging  to  the  regular  system,  occur  which  can  exist  in 
several  modifications.  The  first  of  this  class  of  forms  is  the 


FIG.  262. 


FIG.  263. 


Icositetrahedron  (Fig.  262).  The  formula  of  this  form  is  a  :  ma :  ma, 
mOm.  This  signifies  that  if  a  face  cuts  one  axis  at  the  distance 
(a)  it  cuts  the  two  other  axes  at  a  different  distance,  m  times 
(a),  when  the  face  is  produced.  The  form  a :  2a :  2a,  or  2O2 
(Fig.  262)  occurs  frequently.  This  means  that  the  distance  at 
which  two  axes  cut  the  face  produced  is  twice  as  great  as  that 


FIG.  264. 


FIG.  265. 


FIG.  266. 


at  which  the  third  axis  cuts  it.  Fig.  263  has  the  formula 
a  :  3a :  3a,  and  other  forms  such  as  a  :  f a :  fa  are  also  found, 
though  not  so  frequently.  Combinations  of  the  icositetrahedron 
with  the  foregoing  forms  are  seen  in  Figs.  264,  265,  and  266. 
The  simple  form  (Fig.  262)  occurs  in  silver  glance.  The  com- 
bination (Fig.  264)  has  been  observed  in  spinelle ;  Fig.  265 
occurs  in  analcime,  and  Fig.  266  is  often  noticed  in  garnet. 


THE  EEGULAR  SYSTEM 


843 


The  TrwJcisoctohedron,  or  pyramid  octahedron  (Fig.  267), 
is  represented  by  the  formula  a :  a :  ma,  or  mO.  The  value  of 
m  is  generally  2.  The  simple  form  does  not  occur  frequently, 
but  it  is  often  seen  in  combination.  If  a  four-sided  pyramid 
replace  each  side  of  the  cube  we  obtain  the  Tetrakishexahedron 
(Fig.  268).  Each  face  in  this  form  cuts  an  axis  at  the  distance 


FIG.  267. 


FIG.  268. 


FIG.  269. 


(a)  from  the  centre ;  the  second  axis  at  the  distance  (ma) ;  and 
the  third  axis  at  the  distance  oc  .  Hence  the  general  formula 
for  this  is  a  :  ma :  ooa,  abbreviated  into  mO  oo.  The  more 
common  forms  are  2  0  oo  and  3  0  oc.  The  first  of  these  occurs 
in  gold  and  copper  (Fig.  268),  whilst  the  second  form  is  found 


ot>0« 


xOoo 


FIG.  270. 


FIG.  271. 


in  fluor-spar,  when  it  usually  occurs  with  the  cube,  as  seen  in 
Fig.  269. 

The  last  holohedral  form  of  the  regular  system  is  the  Hexa- 
kisoctohedron  (Fig.  270),  or  the  forty-eight  sided  figure.  This 
form  is  represented  by  a :  ma :  na,  or  m  0  n.  Of  these,  the  forms 
3  0  3/2  and  402  most  frequently  occur,  although  generally  met 
with  in  combinations.  The  form  3  0  3/2  is  found  in  garnet ;  4  0  2 


844 


CRYSTALLOGRAPHY 


in  fluor-spar.     A  combination  of  this  form  with  the  cube  is  seen 
in  Fig.  271. 

The  hexakisoctohedron,  m  0  n,  is  the  most  complicated  of  all 
the  holohedral  forms  of  the  regular  system,  and  the  other  six 
forms  may  be  represented  as  special  cases  of  this  more  general 
expression.  This  is  seen,  if  special  values  are  given  to  m  and  n  ; 
thus,  m  =  n;  m  =  1 ;  m  =  oo,  &c.  The  relations  between  this  and 
the  other  forms  of  the  regular  system  are  shown  by  the  following 
diagram  : — 


FIG.  272 


Hemihedral    forms    of    the     Regular    System. — The    tetra- 
hedron (Fig.    238)   is    one    of   the    most   important    of   these 


FIG.  273. 


FIG.  274. 


V 


FIG.  275. 


FIG.  276. 


forms.      This    is    derived    by   the    extension  of  the  alternate- 
faces  of  the  octohedron  until  the  other  faces  disappear.     The 

formula  of  the  tetrahedron  is   ~,  and  the  two  forms    (p.  833) 


THE  REGULAR  SYSTEM 


845 


are  further  distinguished  by  the  signs  +  (Fig.  238)  and  — 
(Fig.  240).  A  combination  of  the  two  tetrahedra  is  seen  in 
Fig.  273 ;  Figs.  274  and  275  are  combinations  of  the  tetrahedron 
and  cube,  and  Fig.  276  is  a  tetrahedron  on  which  the  faces  of 
the  rhombic  dodecahedron  appear.  The  following  substances 
crystallize  in  tetrahedra:  sodium  sulphantimoniate,  sodium 


D 


FIG.  277. 


FIG.  278. 


chlorate,  cuprous  chloride,  and  fahl-ore.  Combinations  of  two 
tetrahedra  are  frequently  seen  in  boracite.  In  this  case  the  two 
tetrahedra  are  equally  developed,  so  that  the  crystal  appears  to 
be  an  octohedron,  but  it  can  be  distinguished  from  this  by  the 
fact  that  four  of  its  faces  are  bright,  whilst  the  other  four 


FIG.  279. 


FIG.  280. 


are  non-reflecting.  The  triakistetrahedron  is  derived  from  the 
icositetrahedron,  and  therefore  exists  in  several  modifications. 

The   general    formula    is    m|m-   and   each   form   can    have   a 

positive  or  negative  position.  These  occur  as  complete  forms, 
and  also  in  combination,  as  in  the  case  of  fahl-ore  (Figs.  277 
and  278). 


846 


CRYSTALLOGRAPHY 


The  hemihedral  forms  of  the  triakisoctohedron  are  termed 
deltoid-dodecahedra  (Figs.  279  and  280).     Their  general  formula 

is  —  and  they  may  be  either  positive  or  negative. 


FIG.  281. 


FIG.  282. 


The   hexakisoctohedron   in  like   manner   yields    hemihedral 
forms,  termed  Hexakistetrahedra  (Figs.  281  and   282).     These 


FIG.  283. 


FIG.  284. 


forms  are  found  in  combination  in  fahl-ore  and  boracite,  and 
alone  in  diamond. 


FIG.  285. 


FIG.  286. 


FIG.  287, 


Another  class  of  hemihedral  forms  is  also  derived  from  the 
hexakisoctohedron.     These   are    termed    dyaJdsdodecahedra,  or 


THE  HEXAGONAL  SYSTEM 


847 


trapezoid  dicositetrahedra  (Figs.  283  and  284).  In  order  to  dis- 
tinguish these  forms  from  those  of  the  hexakistetrahedron  the 
faces  are  represented  by  the  same  general  formula,  but  bracketed 

thus,  ("~s~)i  They  occur  in  cobalt  glance  and  iron  pyrites. 
The  pentagonal  dodecahedron  is  the  hemihedral  form  of  the  tetra- 
kishexahedron,  and  therefore  has  the  formula  —  —  .  The  form 


occurs  most  frequently  in  pyrites  and  cobalt  glance  (Figs. 
286).     The  combination  shown   in  Fig.   287  is  also 


^—~ 

285   and 

found  in  the  same  substance. 

In  addition  to  the  forms  already  mentioned,  another  hemi- 
hedral form,  the  pentagonal  icositetrahedron,  and  a  tetartohedral 
form,  the  tetrahedral  pentagondodecakedron,  are  also  derived 
from  the  hexakisoctohedron, 


II.  THE  HEXAGONAL  SYSTEM. 

491  This  system  is  characterised  by  one  principal  plane  of 
symmetry  and  six  secondary  planes  of  symmetry  making  angles 
of  30°  with  one  another.  The  principal  axis  of  symmetry  is 
chosen  as  the  principal  crystallographic  axis,  and  is  placed 


OOP 


FIG.  289. 


jooP 


FIG.  290. 


vertically;  three  of  the  secondary  axes  of  symmetry,  which  lie 
in  a  plane  perpendicular  to  the  vertical  axis  being  taken  as 
secondary  crystallographic  axes.  These  are  all  equal,  but  are 
not  equal  to  the  vertical  axis  in  length,  and  meet  in  a  point 
making  angles  of  60°. 

Each  face  of  the  simplest  pyramid  (Fig.  289)  cuts  the  prin- 
cipal or  vertical  axis  and  two  of  the  secondary  axes,  but  runs 


848 


CRYSTALLOGRAPHY 


parallel  to  the  third.  The  formula  for  the  pyramid  is  there- 
fore a :  a :  oo  a :  c,  in  which  the  letter  c  refers  to  the  principal 
axis,  or  abbreviated  P.  The  vertical  axis  of  an  acute  pyramid 
is  of  course  longer  than  the  secondary  axes,  whilst  that  of  an 
obtuse  pyramid  is  shorter.  Thus,  for  example,  the  relative 
length  of  the  axes  in  the  case  of  the  acute  pyramid  of  quartz  is 
I'l  :  1,  whilst  in  the  case  of  the  obtuse  pyramid  of  beryl  it  is 
0-498 :  1. 

If,  in  the  pyramid,  the  length  of  the  principal  axis  be  reduced 
to  0,  a  face  known  as  OP  will  be  produced,  which  is  parallel  to 
the  plane  containing  the  three  secondary  axes,  whilst  if  the 
length  of  this  same  principal  axis  be  increased  indefinitely,  the 
pyramid  becomes  an  open  six-sided  prism  (Fig.  290),  the  symbol 
of  which  is  a :  a :  oo  a :  oo  c,  or  x  P.  This  prism  may  occur 


FIG.  291. 


along  with  the  face  OP  in  the  closed  combination   shown  in 
Fig.  290,  OP,  oo  P. 

The  prisms  and  pyramids,  of  which  each  face  cuts  two  of  the 
secondary  axes  at  equal  distances,  and  is  parallel  to  the  third, 
so  that  the  secondary  axes  pass  through  the  angles  (Figs.  289, 
290),  are  termed  forms  of  the  first  order.  A  second  series  of 
similar  forms  occurs  in  this  system,  which  may  be  derived  from 
the  forms  of  the  first  order  by  rotating  them  through  an  angle 
of  30°,  so  that  the  secondary  axes  cut  the  centre  of  the  six 
sides  of  the  base  of  the  pyramid.  Such  prisms  and  pyramids 
are  termed  forms  of  the  second  order.  Fig.  291  exhibits  the 
relation  between  these  two  series  of  forms.  It  will  be  seen  that 
in  the  pyramid  of  the  second  order  which  is  there  represented 
in  section,  each  face  cuts  one  of  the  secondary  axes  at  the 


THE  HEXAGONAL  SYSTEM  849 


distance  a,  and  the  other  two  axes  at  the  distance  2a,  The 
formula  of  the  pyramid  of  the  second  order  is  accordingly 
2a :  a  :  2a  :  c,  or  P2,  whilst  that  of  the  corresponding  prism  of 
the  second  order  is  2a  :  a  :  2a,  oo  c,  or  oo  P2. 

In  the  same  way  faces  may  occur  which  cut  the  vertical 
axis  at  a  greater  or  less  distance  than  c  from  the  centre,  so  that 
the  general  formula  for  the  pyramid  of  the  first  order  is  mP, 
whilst  that  of  pyramids  of  the  second  order  is  mP2.  In  the 
case  of  the  combination  of  several  pyramids  in  which  the  value 
of  m  varies,  these  values,  in  accordance  with  the  general  law 
(p.  833),  exhibit  a  simple  ratio,  such  as  1 :  2,  1  :  3,  1  :  5,  etc. 
Combinations  of  pyramids  of  the  first  and  second  orders  also 
occur ;  when  they  are  equally  developed,  the  combination  assumes 


FIG.  292. 

the  form  of  a  symmetrical  twelve-sided  pyramid,  the  formula 
for  which  is  a  :  na  :  pa  :  me,  abbreviated  into  mPn  (Fig.  292). 
This  is  in  reality  the  most  general  form  possible  in  the  system 
from  which  all  the  other  forms-  may  be  derived.  Thus  when 
n  =  l  and  p=  oo  ,  it  becomes  an  hexagonal  pyramid  of  the  first 
order,  whilst  when  p  =  oo  it  becomes  a  pyramid  of  the  second 
order. 

The  closed  holohedral  forms  of  the  hexagonal  system  do  not 
occur  frequently.  On  the  other  hand,  combinations  such  as 
shown  in  Fig.  293  are  found  in  quartz;  Fig.  294  exhibits  a 
form  found  in  calc-spar,  and  Fig.  295  a  combination  seen  in 
apatite. 

The  hemihedral  forms  in  the  hexagonal  system  are 
numerous  and  important,  occurring  even  more  frequently  than 

55 


850 


CRYSTALLOGRAPHY 


the  holohedral  forms.     The  most  important  of  these  are  the 
rhombohedra  (Fig.  296),  which  are  derived  from  the  hexagonal 


oP 


OOP 


FIG.  294. 


FIG.  295. 


FIG.  293. 

pyramid  by  extending  the  alternate  faces  until  they  produce  a 

p 
closed  form  (Fig.  297).     These  faces  have  the  symbol  £  or  R 


FIG.  296. 


FIG.  297. 


Gale-spar,  iron-spar,  and  sodium  nitrate  occur  in  the  form  of 
simple  rhombohedra,     The  rhombohedron,  like  the  tetrahedron, 


FIG.  298. 


FIG.  299. 


FIG.  300. 


may  occupy  two  positions  according  as  the  one  or  the  other  set 
of  alternate  faces  of  the  six-sided  pyramid  is  extended.     These 


THE  HEXAGONAL  SYSTEM 


851 


are  shown  in  Figs.  298  and  299 ;  the  two  rhombohedra  are 
distinguished  by  the  signs  +  and  — .  Combinations  of  the  rhom- 
bohedron  also  frequently  occur.  One  of  these  is  represented  by 
Fig.  300,  which  exhibits  a  combination  of  several  positive  and 


FIG.  301. 


FIG.  302. 


FIG.  303. 


negative  rhombohedra,  occurring  in  chabasite.     Fig.  301  repre- 
sents  a    combination    of    two    rhombohedra   which   are    both 
positive,  but  differ  in  axial  ratio,  a  form  which  occurs  in  calc- 
spar.  A  combination  of  the  rhombohedron  with 
a  prism  of  the  first  order  is  seen  in  Fig.  302  ; 
this  form  has  also  been  observed  in  calc-spar. 
A    combination    of  a   rhombohedron    with   a 
prism  of  the  second  order  is  shown  in  Fig.  303. 
This  has  been  observed  in  dioptase. 

A  second  hemihedral  form  of  this  system  is 
the  scalenohedron.  This  is  obtained  from  the 
symmetrical  twelve-sided  pyramid,  by  extend- 
ing the  alternate  pairs  of  planes,  thus  giving 
rise  to  a  positive  ( -f )  and  negative  ( — ) 
scalenohedron.  If  we  suppose  a  rhombohedron 
to  be  placed  within  a  scalenohedron,  it  is 
termed  the  inscribed  rhombohedron,  and  the 
lateral  edges  of  both  forms  will  be  seen  to 
have  a  similar  position  with  regard  to  the 
axes.  It  is  clear  that  the  scalenohedron  may 
be  supposed  to  be  derived  from  such  a  rhom- 
bohedron by  an  elongation  of  the  axis  to  the 
distance  n,  lines  being  drawn  from  the  end  of 
this  axis  to  the  lateral  solid  angles  of  the  rhombohedron,  as  seen  in 
Fig.  304.  Although  in  this  way  an  infinite  number  of  scaleno- 
hedra  may  be  formed,  we  find  that  only  those  forms  actually 


FIG.  304. 


352 


CRYSTALLOGRAPHY 


exist  in  which  the  length  of  the  primary  axis  bears  some  simple 
ratio  to  that  of  the  inscribed  rhombohedron.  The  general 
symbol  for  the  scalenohedron  is  +  mB,n.  The  axis  in  the  case 
of  the  commonest  scalenohedron  occurring  in  calcite  is  three 


FIG.  305. 


FIG.  306. 


FIG.  307. 


times  as  long  as  that  of  the  inscribed  rhombohedron  ,  hence 
the  symbol  for  this  scalenohedron  is  +  R3. 

Tetartohedral  Forms. — In  addition  to  some  other  forms  of 
hemihedry,  a  number  of  tetartohedral  forms  occur  in  this 
system.  Such  forms,  shown  in  Figs.  306  and  307,  occur  in  the 


FIG.  308. 


case  of  quartz.  Two  forms,  known  as  trigonal  trapezohedra,  are 
possible,  both  derived  from  the  twelve-sided  pyramid,  and  these 
two  are  not  congruent,  so  that  when  they  occur  as  in  Figs.  306 
and  307  on  different  crystals  one  of  these  appears  as  the 


THE  QUADRATIC  SYSTEM 


853 


reflected  image  of  the  other,  or  one  is  right-handed  and  the 
other  left-handed. 

Some  cases  of  hemimorphism  also  occur  in  this  system, 
a  well-known  instance  of  which  is  seen  in  crystals  of  tourmaline, 
seen  in  Fig.  308. 


III.  THE  QUADRATIC,  TETRAGONAL,  OR  PRISMATIC  SYSTEM. 

492  As  in  the  hexagonal  system,  the  principal  axis  of  sym- 
metry is  chosen  as  the  principal  crystallographic  axis,  whilst  two 


FIG.  309. 


of  the  secondary  axes  of  symmetry  serve  as  the  other  two  crys- 
tallographic axes.     These  two  are  in  one  plane,  at  right  angles 


FIG.  310. 


FIG.  312. 

to  each  other  and  to  the  principal  axis, 
and  equal  to  each  other  but  not  to 
the  principal  axis.  This  system  differs 
from  the  hexagonal  in  only  possessing 
four  secondary  planes  of  symmetry 
making  angles  of  45°. 

The  most  general  form  of  the  system 


854 


CRYSTALLOGRAPHY 


is  the  ditetragonal  pyramid  (Fig.  309),  which  has  the  symbol 
nPm,  and  from  which  all  the  other  forms  may  be  derived.     It 


coP 


coP 


oP 


«Pco 


FIG.  313. 


FIG.  314. 


FIG.  315. 


has  an  octagonal  base,  and  is  bounded  by  sixteen  triangular 
faces.  The  various  forms  are  related  much  in  the  same  way  as 
in  the  hexagonal  system. 


FIG.  316. 


FIG.  317. 


FIG.  318. 


FIG.  319. 


The  simplest  form  of  this  system  is  that  of  the  four-sided 
pyramid  with  a  square  base.  This  may  be  either  of  the  first 
order,  a  :  a :  c  or  P,  or  of  the  second  order,  a :  oo  a  :  c  or  P  x  , 


FIG.  320. 


FIG.  322. 


derived  from  the  former  by  rotating  it  through  45°.  These 
square  pyramids  are  distinguished  as  acute  and  obtuse  (Figs. 
310  and  311).  When  several  of  these  square  pyramids  occur 


THE  QUADRATIC  SYSTEM 


855 


together  in  a  combination,  the  lengths  of  the  various  principal 
axes,  in  accordance  with  the  general  law  (p.  833),  stand  in 
a  simple  ratio  to  one  another.  This  is  seen  in  Fig.  312, 
which  represents  a  crystal  of  nickel  sulphate.  From  this  it 
will  be  seen  that  the  principal  axis  of  the  pyramid  P  is 
exactly  twice  as  long  as  that  of  the  pyramid  J  P.  A  combina- 


7^7      °P 

17    ** 


FIG.  323. 


/ 


FIG.  324. 


FIG.  325. 


tion  of  pyramids  of  the  first  and  second  order  is  shown  in  Fig. 
313.  This  form  is  likewise  found  in  nickel  sulphate.  When 
the  principal  axis  equals  0,  the  basal  terminal  planes  OP  make 
their  appearance,  and  when  it  is  infinitely  prolonged  we  obtain 
the  faces  of  the  prism  of  the  first  order,  oo  P,  or  those  of  the 
secondary  prism,  oo  Poo  .  See  Figs.  314  and  315. 

Combinations  belonging  to  the  quadratic  system  are  seen  in 


FIG.  326. 


FIG.  327. 


FIG.  328. 


Figs.  316  and  317  ;  the  first  of  these  is  found  in  zircon,  the  second 
in  crystals  of  calcium  copper  acetate.  Fig.  320  represents  another 
common  form  of  this  system,  the  crystals  of  potassium  ferro- 
cyanide.  The  usual  form  of  tin-stone  (stannic  oxide)  is  seen  in 
Fig.  321,  that  of  mellite  in  Fig.  322.  Figs.  323  to  325  exhibit 
the  more  complicated  forms  of  nickel  sulphate. 

A  remarkable  form  of  the   quadratic  system  occurs  in  the 


856 


CRYSTALLOGRAPHY 


mineral  leucite.  This  is  a  combination  of  P  with  4P2,  their 
forms  being  equally  developed  (Fig.  326).  This  form  can  only 
with  difficulty  be  distinguished  from  that  of  the  regular 
icositetrahedron  (202).  So  close  indeed  is  the  resemblance  of 
these  forms  that  leucite  was  long  supposed  to  crystallize  in  this 
form  of  the  cubic  system,  and  the  name  leucitohedron  was  given 
to  it.  We  owe  the  discovery  of  the  true  crystalline  form  of 
the  mineral  to  Vom  Rath.1  The  combination,  Fig.  327,  clearly 
shows  the  quadratic  form,  inasmuch  as  small  faces  of  a  third 
pyramid  as  well  as  those  of  a  prism  occur.  The  twin  crystal 


i 
i 

ooP 

: 

f 

j  

FIG.  329. 


FIG.  331. 


(Fig.  328)  occurring  in  the  case  of  leucite  also  shows  that  the 
form  is  a  quadratic  one. 

Hemihedral  forms  can  be  derived  from  the  quadratic  prism  by 
the  same  process  as  that  by  which  the  tetrahedron  is  derived 
from  the  octohedron.  The  forms  thus  obtained  are  termed 
quadratic  sphenoids.  These  can  occur  in  two  positions,  shown 
in  Figs.  329  and  330;  and  they  are  distinguished  from  the 
regular  tetrahedra  inasmuch  as  each  face  is  a  scalene  and  not 
an  equilateral  triangle.  They  are  not  often  found  in  nature ; 
Fig.  331  shows  a  combination  which,  occurs  in  the  case  of 
Epsom  salts  and  of  zinc  vitriol. 

1  Pogg.  Ann.  Erganz.  Bd.  6,  198. 


THE  RHOMBIC  SYSTEM  857 


IV.  THE  RHOMBIC  SYSTEM. 

493  This  system  is  characterised  by  three  secondary  planes  of 
symmetry  all  at  right  angles.  The  three  secondary  axes  of 
symmetry  are  chosen  as  the  crystallographic  axes,  and  they  are 
all  unequal  and  at  right  angles.  It  is  usual  to  take  as  the 
principal  axis  that  which  lies  in  the  direction  in  which  the 
crystal  is  most  fully  developed  or  in  which  most  modifications 
are  observed.  This  is  called  (c),  and  placed  vertically.  In  the 
following  figures  the  longer  of  the  other  two  axes,  termed  the 
macrodiagonal  (&),  runs  from  back  to  front  of  the  observer,  and 
the  shorter,  the  lr  achy  diagonal  (c),  from  right  to  left.  The 
arrangement  of  the  horizontal  axes  is  purely  conventional,  and 
is  frequently  the  reverse  of  that  just  described. 

The  fundamental  form  of  this  system  is  the  rhombic  pyramid 
Fig.  332,  having  the  formula  a :  b  :  c  or  P.  Secondary  pyramids 
also  occur,  and  these  are  distinguished  by  the 
signs  ~  or  "  placed  over  the  P  according  as  the 
length  of  the  intercept  on  the  macro-  or  brachy- 
diagonal  is  increased.  The  amount  of  this  altera- 
tion is  represented  by  a  number  placed  after  the 
letter  P,  so  that  the  symbol  Pn  denotes  a  pyramid 
the  faces  of  which  cut  the  principal  axis  and 
the  brachydiagonal  at  the  normal  distances  c  and 
a,  and  the  macrodiagonal  at  a  distance  of  n  times 
b ;  whilst  Pn  signifies  that  the  brachydiagonal  is 
cut  at  a  distance  of  n  times  a. 

Faces  also  occur  which  would  cut  the  principal 
axis  at  a  distance  which  is  some  multiple  of  the  normal  length  cy 
and  this  is  represented  by  a  letter  placed  before  the  symbol 
P.  Thus  the  symbol  3P2  signifies  a  form  each  face  of  which 
cuts  the  macrodiagonal  at  the  distance  b,  the  principal  axis  at  a 
distance  of  3  times  c,  and  the  brachydiagonal  at  a  distance  of 
twice  a  from  the  point  of  intersection  of  the  axes. 

In  addition  to  these  series  of  pyramids,  a  number  of  prisms 
are  also  possible,  the  faces  of  which  cut  two  of  the  axes  and  are 
parallel  to  the  third.  If  the  faces  are  parallel  to  the  principal 
axis,  the  form  is  known  as  a  vertical  prism  oo  P,  if  to  the  macro- 
or  brachy-diagonal  as  a  macro-  or  brachy-dome,  Poo  or  ]Poo  ,  these 
last  two  being  nothing  more  than  horizontal  prisms  (Figs.  334 
and  338). 


858 


CRYSTALLOGRAPHY 


Faces  which  are  parallel  to  two  of  the  axes  but  cut  the 
third  are  known  as  terminal  planes  or  pinacoids.  Of  these 
there  are  three,  OP,  usually  called  the  basal  plane,  oo  Poo  or 
the  macropinacoid,  and  x  f  x  the  brachypinacoid  (Fig.  339). 


«P 


FIG.  333. 


FIG.   334. 


FIG.  335. 


A  form  termed  the  right  rhombic  prism  is  formed  by  the  com- 
bination of  the  three  terminal  planes,  and  is  found  in  nature  in 
crystals  of  anhydrite,  CaSO4. 

These  various  forms  are  summarised  with  their  symbols  in 
the  following  table,  in  which  a  =  brachy diagonal,  b  =  the  macro- 
diagonal,  and  c  =  the  vertical  axis. 


oP 


""^^ 


FIG.  336. 


FIG.  337.  FIG.  338.  FIG.  339. 


Rhombic  Pyramids. 

a :  b  :  me,  abbreviated  into  mP  =  primary  rhombic  pyramid, 
na :  b  :  me         „  „     mf  n  =  brachypyramid. 

a  :  nb  :  me         „  „     mPn  =  macropyramid. 

Vertical  Prisms. 

a  :  b  :  oc  c,  abbreviated  into  ocP  =  rhombic  prism  (primary), 
na  :  b  :  occ         „  ,,      ocPn  =  brachyprism. 

a :  nb  :  a  c          „  „      ex  Pn  =  macroprism. 


THE  RHOMBIC  SYSTEM 


859 


Domes  or  Horizontal  Prisms. 

oc  a  :  b  :  me,  abbreviated  into  mf  ex  =  brachydome. 
a  :  or  b  :  me  mPoc  =macrodome. 


Terminal  Planes. 

,  i       .  ,    i    •   ,  Ar>      f  basal    terminal    plane 

=  1 


a:ab:*c 


or  pinacoid. 

-5  f  brachy  terminal  plane 
»  aPK  or  pinacoid 

-fs  f  macro-terminal  plane 
»  °<P*  orpinacoidP 


Sulphur  is  one  of  the  most  important  substances  crystallizing 
in  this  system.  It  occurs  in  rhombic  pyramids  and  in  numerous 
combinations.  Fig.  332  exhibits  the  fundamental  form  of 


FIG.  340. 


FIG.  341. 


FIG.  342. 


sulphur,  in  which  the  ratio  of  the  axes  a :  b  :  c  —  0'811  :  1 :  1*898, 
and  Figs.  333  and  334  various  combinations  which  are  found  on 
crystals  of  this  element.  Zinc  sulphate  is  found  in  the  forms 
represented  in  Figs.  335,  336,  337.  Barium  formate,  crystal- 
lizing in  a  combination  of  a  prism  with  a  dome,  is  shown  in 
Fig.  338.  Fig.  339  represents  a  crystal  of  uranium  nitrate, 
possessing  two  basal  terminal  faces.  Potassium  sulphate  crystal- 
lizes in  several  interesting  forms.  Of  these,  Figs.  340  and  341 
may  at  once  be  recognized  as  belonging  to  the  rhombic  system, 
whilst  Figs.  342  and  343  might  at  first  sight  be  mistaken  for 
hexagonal  forms. 

The  most  important  hemihedral  forms  of  the  rhombic  system 
.are  the  rhombic  sphenoids.     The  faces  of  the  two  forms  of  these 


860 


CRYSTALLOGRAPHY 


sphenoids  (shown  in  combination  Figs.  344  and  345)  are  equal  in 
number,  and  exactly  similar  in  form,  but  when  the  +  and  —  forms 
occur  on  separate  crystals,  they  are  developed  on  opposite  sides,  so 
that  the  one  may  be  regarded  as  a  reflected  image  of  the  other. 
Fig.  344  is  the  double  sodium  and  ammonium  salt  of  ordinary 
tartaric  acid,  which  is  termed  dextro-tartaric  acid,  from  its  hav- 
ing the  power  of  turning  the  plane  of  polarization  to  the  right. 


iC"        °p 

* 

<:    .  op  .  -, 

.„ 

l 

«P 

y 

2Po> 

^J 

»P2 

Z^> 

\ 

coP 

9  Poo 

s* 

OOP 

\ 

ooPoo 

W 

OOPOD 

OOPOD 


FIG.  343. 


FIG.  344. 


FIG.  345. 


Fig.  345  is  a  salt  of  the  same  bases  combined  with  a  very  similar 
acid,  differing  only  from  ordinary  tartaric  acid  in  its  power  of 
rotating  the  plane  of  polarization  to  the  left.  Another  in- 
teresting example  of  a  rhombic  sphenoid  is  found  in  the  ordinary 
crystals  of  magnesium  sulphate,  MgSO4  +  7H2O.  In  these  the 
domes  at  the  summits  of  the  vertical  prism  are  placed  in  opposite 
directions. 


V.  THE  MONOCLINIC  OR  MONOSYMMETRIC  SYSTEM. 

494  This  system  has  only  one  plane  of  symmetry,  and  there- 
fore only  one  axis  of  symmetry.  This  is  chosen  as  one  of  the 
crystallographic  axes,  and  is  known  as  the  axis  of  symmetry,  or 
more  generally  as  the  orthodiagonal.  This  axis  is  supplemented 
by  two  others  which  lie  in  the  plane  of  symmetry,  and  are, 
therefore,  at  right  angles  to  the  orthodiagonal.  These 
intersect  at  an  angle  (/:?)  which  is  not  a  right  angle,  and  one 
of  them  is  usually  referred  to  as  the  primary  axis,  the  other 
being  known  as  the  clinodiagonal.  The  position  of  the  crystal, 
like  that  of  crystals  of  the  rhombic  system,  is  purely  a  matter  of 
convention.  In  the  following  figures  the  orthodiagonal  runs  from 
back  to  front  of  the  observer,  so  that  the  plane  of  symmetry 
is  parallel  to  the  plane  of  the  paper,  whilst  the  clinodiagonal 
runs  from  right  to  left.  Many  authors,  on  the  other  hand,  make 


THE  MONOSYMMETRIC  SYSTEM 


861 


the  orthodiagonal  run  from  left  to  right,  and  place  the  primary 
axis  vertical.  The  symbols  referring  to  the  clinodiagonal  are 
distinguished  by  being  enclosed  in  parentheses.  The  complete 
description  of  each  form  of  this  system  includes  the  axial  ratio 
and,  in  addition,  the  angle  of  intersection  of  the  primary  axis  with 
the  clinodiagonal.  Thus,  for  instance,  the  data  of  the  crystals  of 
felspar  are  a :  b  :  c  =  1'519  :  1 :  0*844  ;  @  =  63°  53'. 

The  fundamental  form  of  this  system,  since  there  is  only  one 
plane  of  symmetry,  consists  of  a  hemipyramid  made  up  of  four  faces. 
Thus  in  Fig.  346,  in  which  the  plane  of  symmetry  is  parallel  to 
the  plane  of  the  paper,  the  two  upper  right-hand  faces  and  the 
two  lower  left-hand  faces  (marked  -}-)  belong  to  one  of  these 
hemipyramids,  whilst  the  other  two  pairs  of  faces  (marked  — ) 
belong  to  the  other.  The  complete  octohedron  shown  in  the 
figure  is,  therefore,  made  up  of  two  crystallographically  distinct 
forms,  which  are  known  as  -f-P  and  —P.  Each  of  these  two 
hemipyramids  may  of  course  occur  by  itself  in  combination  with 
other  forms,  so  that  very  frequently  only  four  pyramid  faces  are 
found  on  complex  crystals  of  this  system  (Figs.  348,  349).  The 
pinacoids,  prisms,  and  domes,  which  are  known  as  the  ortho-  and 
clino-domes,  are  derived  from  the  pyramids  in  precisely  the  same 
way  as  in  the  rhombic  system.  The  prism  and  the  clinodome 
comprise  four  faces,  whilst  the  orthodome,  owing  to  the  presence 
of  only  one  plane  of  symmetry,  consists  of  only  two  faces,  and 
is  really  a  hemidome  +  P°°  or  —  Poo  .  The  following  figures 
represent  the  forms  of  a  variety  of  well-known  chemical  com- 
pounds crystallizing  in  this  system. 


FIG.  346. 


FIG.  347. 


FIG.  348. 


FIG.  349. 


Figs.  347,  348,  and  349  give  the  crystalline  form  of  sodium 
acetate;  Fig.  350  that  of  cane-sugar;  Fig.  351  that  of  nickel 
potassium  sulphate ;  Fig.  352,  ferrous  sulphate  (green  vitriol), 
FeSO4+  7H2O.  In  this  latter  form  the  primary  axis  cuts  the 


862 


CRYSTALLOGRAPHY 


clinodiagonal  at  an  angle  of  75C  40',  and  the  relation  of  the  axes 
a  :  b  :  c  =  0'8476  : 1  :  1-267.  Gypsum,  CaSO4  +  2H2O,  fre- 
quently occurs  in  fine  monosymmetric  crystals.  The  form  of 


FIG.  350. 


FIG.  351. 


FIG.  352. 


gypsum  is  shown  in  Figs.  353  and  354  ;  the  relation  of  these 
axes  is  as  a:  b  :c  =  l'4504  : 1  :0'6003,  with  an  inclination  of 
80°  57'. 


FIG.  353. 


FIG.  354. 


FIG.  355. 


Some  substances,  such  as  felspar,  crystallize  in  forms  which 
are  apparently  very  different  (Figs.  355  and  356)  owing  to  the 
greater  or  less  development  of  certain  faces.  Crystals  of  Glauber's 


FIG.  356. 


FIG.  357. 


FIG.  358. 


salt  and  oxalic  acid  are  often  found  developed  in  the  direction 
of  the  orthodiagonal  (Figs.  357  and  358). 

Hemimorphous  forms  are  not  uncommon  in  this  system,  only 


THE  ASYMMETRIC  SYSTEM 


two  faces  of  the  clinodome  being  found,  both  of  them  on  the 
same  side  of  the  plane  of  symmetry.  Dextro-tartaric  acid  not 
unfrequently  crystallizes  in  the  simple  form  Fig.  358.  More 
usually,  however,  the  four-faced  angles  at  the  front  are  replaced 
by  a  dome  (Fig.  360),  whilst  in  the  case  of  the  laevo-tartaric 
acid  the  corresponding  angles  at  the  back  are  similarly  replaced, 
as  shown  in  Fig.  361. 


FIG.  360. 


FIG.  361. 


FIG.  362. 


It  is  a  singular  fact  that  hemimorphous  forms  of  this  kind 
generally  occur  in  the  case  of  bodies  which  are  optically  active — 
that  is,  which  possess  the  power  of  rotating  the  plane  of  polari- 
zation of  light.  They  are  accordingly  found  (Fig.  362)  in  the 
case  of  cane-sugar,  which  also  possesses  this  optical  property. 


VI.  THE  TRICLINIC  OR  ASYMMETRIC  SYSTEM. 

495  This  system  is  entirely  without  any  plane  of  symmetry,  and 
the  choice  of  axes  is,  therefore,  quite  arbitrary.  Any  one  face  of 
the  crystal  is  chosen  as  the  pyramid  face,  and  three  unequal  axes 
cutting  one  another  at  angles  a,  0,  7  are  then  adopted.  One  of 
these  is  known  as  the  primary  axis,  the  other  two  being  dis- 
tinguished as  brachy-  and  macro-diagonals.  Five  values  have 
to  be  given  in  order  completely  to  determine  an  asymmetric  form, 
viz.  a :  b  :  c,  the  relative  length  of  the  axes,  and  the  three  acute 
angles  (a,  /3,  7)  which  they  form  with  each  other. 

The  possible  forms  belonging  to  this  system  correspond  closely 
with  the  forms  of  the  rhombic  system.  In  addition  to  the 
faces  P  of  the  fundamental  pyramid,  the  following  faces  of 
secondary  pyramids  occur,  m  P,  Pn,  Pn,  m  Pn,  m  Pn,  together 
with  the  surfaces  of  the  prisms,  ocP,  ocPn,  ocfn.  Then  we 


864 


CRYSTALLOGRAPHY 


have  the  domes,  in  P  a,  m  P  a,  and  the  basal  end-faces,  0  P, 
oc  Pa,  afroc,  a  P  a. 

Since  there  is  no  plane  of  symmetry,  each  crystallographically 
complete  form  consists  of  a  pair  of  parallel  faces,  so  that  an 
entire  octohedron  would  be  made  up  of  four  independent 
pyramids,  and  each  prism  or  dome  of  two  independent  forms. 


FIG.  363. 


oP 

FIG.  364. 


These  forms  having  similar  and  parallel  faces  are  represented  by 
accentuated  letters.  Thus  P'  represents  a  face  of  the  funda- 
mental form,  which  occurs  in  the  upper  front  of  the  crystal  to 
the  right,  whereas  the  similar  and  parallel  face  ,P  is  one  which 
occurs  in  the  lower  part  of  the  crystal  to  the  left.  Thus  the 
symbol  oc  /P  indicates  the  two  faces  of  the  prism  x>  P,  one  of 


FIG.  365. 


FIG.  366. 


FIG.  367. 


which  occurs  above  to  the  left ;  m  yP'  a  denotes  the  faces  of  a 
brachydiagonal  dome  which  lie  above  to  the  right  and  below  to 
the  left. 

The  direction  in  which  crystals  belonging  to  the  triclinic 
system  are  placed  is  a  purely  conventional  one.  Thus  Fig.  363 
may  in  the  first  place  be  regarded  as  a  combination  of  the  forms 
ocPx),  ocpoc,  OP;  or  secondly  as  oc/P,  ocP',,  OP. 


ISOMOEPHISM  865 


The  following  figures  exhibit  some  commonly  occurring 
asymmetric  forms : — 

Copper  sulphate,  CuSO4  +  oH2O,  Fig.  364.  Calcium  thio- 
sulphate,  Fig.  365.  Albite,  Figs.  366  and  367. 

Other  common  substances  crystallizing  in  this  system  are 
axinite,  boric  acid,  cupric  selenate,  manganese  sulphate,  potas- 
sium anhydrochromate,  and  sodium  succinate. 

ISOMORPHISM. 

496  In  his  celebrated  system  of  classifying  minerals  devised 
in  1801,  Haiiy  propounded  the  principle  that  a  difference  in  the 
primary  form  of  two  crystals  invariably  indicates  a  difference  of 
chemical  composition,  whilst  identity  of  form,  except  in  crystals 
of  the  regular  system,  equally  implies  identity  of  composition. 
It  was  however  soon  found  that  his  principle  could  not  be 
applied  in  all  its  generality,  for  Leblanc  had  shown  so  long  ago 
as  1787  that  crystals  of  an  identical  form  could  be  obtained 
from  solutions  containing  sulphate  of  copper  and  sulphate  of 
iron  mixed  in  very  different  proportions.  He  likewise  states 
that  crystals  of  alum,  a  sulphate  of  aluminium  and  potassium, 
may  frequently  be  found  to  contain  a  considerable  quantity 
of  iron,  although  no  alteration  in  the  crystalline  form  can  be 
noticed.  Vauquelin,  too,  showed  in  1797  that  common  potash 
alum  may  contain  large  quantities  of  ammonia,  and  yet  the 
crystalline  form  of  the  substance  does  not  undergo  any  change. 
It  was  also  well  known  that  many  minerals  which  are  identical 
in  crystalline  form  may  possess  a  very  different  chemical  com- 
position. Thus  in  certain  specimens  of  red  silver  ore,  arsenic  is 
present  as  an  essential  constituent,  whilst  in  others  antimony 
takes  its  place.  In  like  manner,  the  common  garnet,  crystalliz- 
ing in  the  regular  system,  sometimes  contains  much  iron  and 
little  aluminium,  and  sometimes  large  quantities  of  aluminium 
and  little  iron.  Berthollet  considered  facts  like  these  to  be 
in  accordance  with  his  views  on  chemical  combination,  but 
Proust  explained  them  by  supposing  that  we  have  here  to 
deal  not  with  chemical  compounds  but  rather  with  mechanical 
mixtures. 

In  the  year  1816  Gay-Lussac  made  the  remarkable  observa- 
tion that  when  a  crystal  of  common  potash  alum  is  hung  up  in 
a  saturated  solution  of  ammonia  alum  it  grows  exactly  as  if  it 
had  been  placed  in  the  solution  from  which  it  was  originally 

56 


866  CRYSTALLOGRAPHY 

obtained.  From  this  fact  he  drew  the  conclusion  that  the 
molecules  of  these  two  alums  possess  the  same  form.  Later  on, 
in  1819,  Beudant  noticed  that  if  solutions  containing  two  of  the 
following  salts,  sulphate  of  zinc,  sulphate  of  iron,  or  sulphate  of 
copper,  be  crystallized,  the  crystals  which  are  deposited  always 
possess  the  form  of  one  of  these  salts,  although  they  contain  a 
considerable  quantity  of  the  other  salt,  which,  when  crystallized 
by  itself,  possesses  a  totally  different  form. 

In  order  to  explain  these  and  similar  well-recognised  facts, 
Haiiy  threw  out  the  notion  that  certain  bodies  possess  the  power 
of  crystallization  to  such  a  degree  that  even  when  present  in 
small  quantities  they  compel  other  bodies  to  adopt  their  crystal- 
line form. 

Clear  light  was  thrown  on  this  subject  by  the  researches  of 
Mitscherlich,  the  results  of  which  were  communicated  to  the 
Berlin  Academy  in  1819.  Mitscherlich  showed  that  the  com- 
pounds of  various  elements  possessing  a  similar  constitution 
have  also  identical  crystalline  form,  as  ascertained  by  the 
measurement  of  their  angles.  The  first  substances  examined 
by  Mitscherlich  were  the  arsenates  and  phosphates  of  sodium 
potassium,  and  ammonium.  He  showed  that  the  crystals  of  the 
phosphate  and  arsenate  of  the  same  metal  riot  only  contain  the 
same  quantity  of  water  of  crystallization,  and  crystallize  in  the 
same  form,  but  that  when  the  two  salts  are  mixed  in  varying 
quantities  the  crystals  which  such  a  solution  deposits  are  of  the 
same  form  as  those  obtained  from  solutions  of  the  pure  salts, 
whilst  the  proportion  of  each  ingredient  found  in  the  crystal 
deposited  from  the  mixed  solution  varies  according  to  the  propor- 
tions in  which  the  ingredients  were  mixed.  Hence  Mitscherlich 
concluded  that  analogous  elements  or  groups  of  elements  can 
replace  one  another  in  compounds  without  any  alteration  of 
crystalline  form.  Such  substances  are  said  to  be  isomorphous 
(t'0-0?,  equal  to,  fioptyij,  shape).  •  A  large  number  of  other  com- 
pounds were  shown  by  Mitscherlich  to  conform  to  the  above 
law.  Subsequent  investigations  have,  however,  proved  that  the 
angles  of  isomorphous  crystals  are  not  absolutely  identical,  but 
that  each  substance  differs  slightly  from  the  other  in  this 
respect.  When  several  of  such  substances  are  crystallized  from 
solution  together,  the  angles  of  the  crystal  deposited  are  the 
mean  of  those  of  the  pure  substances. 

497  The  following  minerals  serve  as  an  excellent  illustration 
of  the  law  of  Isomorphism  : — 


ISOMORPHISM 


867 


Apatite,  Ca3(PO4)2  +  Ca2 1  |°* 

Pyromorphite,  Pb8(PO4)2  +  Pb2 j  ^ 
Mimetesite,  Pb3(As04)2  +  Pb2 j  Sj3* 
Vanadinite,  Pb3(V04)2  +  Pb2  j  ^°4 

The  isomorphous  chemical  elements  in  these  minerals  are : — 
(1)  Phosphorus,  Arsenic,  Vanadium.     (2)  Calcium  and  Lead. 
(3)  Chlorine  and  Fluorine. 


FIG.  368. 


They  all  crystallize  in  hexagonal  prisms,  seen  in  Figs.  369  to 
371,  derived  from  the  fundamental  form  P.  (Fig.  368).  The 
values  of  the  terminal  solid  angle  a,  the  lateral  solid  angle  ft 


ooP 


FIG.  369. 


FIG.  370. 


and  the  length  of  the  primary  axis  (c),  that  of  the  secondary 
axes  being  taken  as  the  unit,  are  found  to  be : — 


c.  a. 

Apatite     .    .    .     07321  142°  20' 

Pyromorphite   .     07362  142°  15' 

Mimetesite    .    .     07392  142°  70' 

Vanadinite    .         07270  142°  30' 


0. 

80°  25' 
80°  44' 

80°  58' 
80°     0' 


Not  urifrequently  both  chlorine  and  fluorine  occur  in  the 
same  specimen  of  apatite ;  and  phosphorus  and  arsenic,  not 
unfrequently,  replace  one  another  in  pyromorphite. 


-868  CK  YST  ALLOG  R  APH  Y 


Another  well-marked  case  is  that  of  the  rhombohedral 
carbonates  of  the  magnesium  class  of  metals,  calcium,  mag- 
nesium, iron,  zinc,  and  manganese.  These  minerals  all  crys- 
tallize in  similar  rhombohedra,  and  the  several  metals  can 
replace  one  another  in  varying  proportions  without  any  change 
of  form  occurring.  It  has,  however,  been  observed  that  the 
angles  of  the  different  rhombohedra  are  not  exactly  equal,  but 
that  the  different  members  of  the  series  vary  in  this  respect  by 
one  or  two  degrees ;  thus  : — 

Angle  on 
termnl.  edge. 

Calcium  carbonate,  or  calc-spar,  CaCO3.     .  105°     5' 

Calcium  magnesium  carbonate,  or  dolomite,  (CaMg)CO3106°  15' 
Magnesium  carbonate,  or  magnesite,  MgCO3.  .  107°  25' 

Ferrous  carbonate,  or  spathic  iron  ore,  FeCO3  .     .  107°     0' 

Zinc  carbonate,  or  calamine,  ZnCO3.     .  107°  40' 

Manganese  carbonate,  or  diallogite,  MnC03.     .  106°     5' 

The  replacement  in  minerals  of  varying  proportions  of 
the  different  isomorphous  compounds  is  well  illustrated  by 
the  following  percentage  analysis  of  spathic  iron  ore : — 

Ferrous  oxide,  FeO 45'55 

Manganous  oxide,  MnO 12*50 

Lime,  CaO T57 

Magnesia,  MgO T80 

Carbonic  acid,  CO2 38'58 

100-00 

If  we  now  divide  the  percentage  of  oxide  by  its  combining 

45*55 

weight  thus     '         for  ferrous  oxide,  etc.,  etc.,  we  obtain  the 

following  numbers  respecting  the  proportion  between  the  number 
of  equivalents  of  the  several  constituents  present  :— 

FeO  =  0-6372,  MnO-0'1773,  CaO  =  0'0282,  MgO  =  0'0448,  or 
summing  these  we  have  0'8875.  This  is,  however,  very  nearly 
the  proportion  which  must  be  present  in  order  to  unite  with 

OQ.ro 

\^=  =  0'8833  equivalents  of  CO2,  the  difference  being  owing  to 

4o'o7 

.errors  of  experiment.  In  other  words,  the  number  of  molecules 
of  the  basic  oxides  present  is  the  same  as  that  of  the  carbon 
-dioxide ;  hence  the  formula  for  this  spathic  iron  ore  is  (Fe,  Mn, 


ISOMORPHISM  869 


Ca,  Mg)CO3,  signifying  that  the  relative  quantities  of  the 
metals  in  question  present  are  indeterminate,  but  are  in  the 
aggregate  such  as  are  needed  to  combine  with  CO2.  Hence  we 
have  the  following  as  the  composition  of  the  mineral : — 

Ferrous  carbonate,         FeCO3  .  .  .  73'39 

Manganese  carbonate,   MnCO3  .  .  .  20*24 

Calcium  carbonate,        CaCO3  .  .  .  2*80 

Magnesium  carbonate,  MgCO3  .  .  .  3'95 


100-38 

Fahl-ore  offers  another  striking  example  of  isomorphism. 
The  simple  formula  for  this  mineral  is.  2Cu2S -f  Sb2S3,  but 
usually  part  of  the  copper  is  replaced  by  iron,  zinc,  silver,  or 
mercury,  and  some  of  the  antimony  by  arsenic.  This  is  seen 
by  the  following  analysis  : — 

Sulphur,     S.    .     25-48-s-   31'82  =  0'801 

^  0-304 


0-651 


100-00 

If  the  several  percentage  weights  be  divided  by  the  corre- 
sponding atomic  weights,  and  if  the  resulting  quotients  of  the 
isomorphous  constituents  be  added  together,  the  following 
result  is  obtained  : — 

8  =  0-801 

(Sb,  As)  =  0-304 

(Cu,  Fe,  Zn,  Ag)  =  0-651 

These  numbers  have  the  ratio  5'2  :  2  : 4'2,  or  allowing  for 
experimental  error,  5:2:4,  and  the  formula  of  Fahl-ore  is 
therefore  : — 

2[(Cu,Fe,Zn,Ag)2S]  +  (Sb,As)2S3. 


Antimony,  Sb    . 

17-76  +119-4   =0-149) 

Arsenic, 

As    . 

11-55-s-   74-4    =0-155  J 

Copper, 

Cu    . 

30-73-h   62-8    =  0-489  \ 

Iron, 

Fe     . 

1-42-s-   55-6    =  0-025  / 

Zinc, 

Zn    . 

2-53-T-   65       =  0-039  f 

Silver, 

Ag    . 

10-53-s-  107-13-  0-098  J 

870  CRYSTALLOGRAPHY 


DIMORPHISM  AND  TRIMORPHISM. 

498  Even  before  Haiiy  started  the  idea  that  bodies  which 
crystallize  in  different  forms  differ  also  in  chemical  composition, 
Vauquelin  had  noticed  that  titanic  acid  occurs  as  rutile  and 
anatase,  two  minerals  possessing  distinct  crystalline  forms.  In 
like  manner,  Klaproth  pointed  out  that  hexagonal  calc-spar  is 
the  same  chemical  compound  as  rhombic  arragonite.  These 
exceptions  to  Haiiy's  law  were  then  explained  by  the  presence 
in  the  compound  of  some  impurity  which  has  the  power  of 
altering  the  crystalline  form.  Thus,  arragonite  was  found  to 
contain  small  quantities  of  strontium  carbonate,  a  mineral 
which  is  found  crystallized  in  the  same  form,  and  this  small 
proportion  was  supposed  to  exert  so  powerful  an  action  as  to 
compel  the  calcium  carbonate  to  assume  a  rhombic  form. 

In  1821,  Mitscherlich  proved  that  this  property  of  crystallizing 
in  two  distinct  forms  is  common  to  many  bodies  both  elementary 
and  compound,  and  he  termed  such  bodies  Dimorphous. 

Other  substances,  again,  are  capable  of  existing  in  three 
distinct  crystalline  forms.  These  Mitscherlich  termed  Tri- 
morphous  substances.  If,  lastly,  two  substances  exhibit  a  double 
isomorphism,  they  are  said  to  be  Isodimorphous.  The  trioxides 
of  arsenic  and  antimony  serve  as  a  striking  example  of  isodi- 
morphism.  For  a  long  time  these  compounds  were  only  known 
to  occur  in  two  forms  which  were  not  isomorphous,  and  this 
was  the  more  remarkable  as  their  elementary  constituents 
exhibit  such  a  close  analogy.  It  was  afterwards  found  that 
arsenious  oxide,  As4O6,  which  usually  crystallizes  in  regular 
octohedra,  is  occasionally  met  with  in  rhombic  crystals,  exactly 
identical  in  form  with  those  in  which  antimonious  oxide,  Sb4O6, 
commonly  occurs  in  nature.  A  mineral  consisting  of  this 
latter  oxide,  and  called  senarmontite,  was  next  discovered. 
The  crystals  of  this  are  octohedral,  so  that  the  isodimorphism 
of  these  substances  is  now  completely  proved,  especially  since 
it  has  been  found  possible  to  produce  the  octohedral  crystals  of 
antimonious  oxide  artificially. 

Dimorphous  and  trimorphous  bodies  are  not  only  distinguished 
by  their  difference  in  crystalline  form.  The  other  physical 
properties,  such  as  specific  gravity,  hardness,  refractive  power, 
etc.,  are  all  different.  One  of  the  best  examples  of  a  trimorphous 
compound  is  titanium  dioxide,  TiO2.  The  commonest  form  of 
this  compound  is  rutile.  This  mineral  crystallizes  in  the  quad- 


DIMORPHISM  AND  TRIMORPHISM  871 

ratic  system  with  the  combinations  P,  oc  P,  P  30,  and  x  P  x  ;  the 
relations  of  the  axes  are  a  :  b  =  1  :  0*6442  ;  its  specific  gravity 
is  4*2494.  The  second  form  of  titanium  dioxide  is  anatase, 
Fig.  372 ;  it  likewise  crystallizes  in  the  quadratic  system, 
but  the  relation  of  the  axes  is  in  this  case  quite  different 


FIG.  372. 

from  that  in  the  case  of  rutile,  viz.  a  :b=  1  :  1*777.  Anatase 
crystallizes  as  a  rule  in  the  fundamental  form  P.  The  specific 
gravity  of  anatase  is  3'826.  The  third  form  is  brookite;  it 
crystallizes  in  flat  rhombic  prisms,  and  possesses  a  specific 
gravity  of  4*22. 

THERMAL  AND  OPTICAL  RELATIONS  OF  CRYSTALS. 

499  Crystals  which  belong  to  the  regular  system  being 
equally  developed  in  all  directions  expand  when  heated  equally 
in  every  direction.  On  the  other  hand,  crystals  belonging  to 
the  hexagonal  and  quadratic  systems  expand  differently  in  the 
directions  of  the  primary  and  of  the  secondary  axes.  This  is 
clearly  seen  in  the  following  table,  which  gives  the  coefficients 
of  expansion  of  substances  crystallizing  in  these  two  systems.1 

Primary  axis.         Secondary  axis. 

Quadratic  System.— Tinstone  .    .  0*0004860          0*0004526 
Zircon      .    .  0*0006264          0*0011054 

Hexagonal  System.— Qu&rtz     .    .  0*0008073          0*0015147 
Tourmaline    0*0009369          0*0007732 

Crystals  belonging  to  the  other  systems  possess  coefficients 
of  expansion  which  differ  for  each  of  the  three  directions  of  the 
1  Pfaff,  Pogg.  Ann.  104,  171. 


872  CRYSTALLOGRAPHY 

axes.  Hence  whilst  the  angles  of  substances  crystallizing  in  the 
regular  system  remain  constant  under  change  of  temperature, 
those  of  crystals  belonging  to  the  other  systems  undergo  small 
deviations  with  alteration  of  temperature. 

The  same  relation  is  exhibited  by  crystals  with  regard  to  the 
conduction  of  heat  as  holds  good  in  the  case  of  expansion.  The 
conducting  power  of  crystals  of  the  regular  system  is  the  same 
in  all  directions.  Those  belonging  to  the  quadratic  and  hexa- 
gonal systems  conduct  equally  in  two  directions,  and  unequally 
in  the  third,  whilst  crystals  belonging  to  the  other  systems  con- 
duct differently  in  every  direction.  This  difference  in  the  con- 
ducting power  of  crystals  can  be  well  shown  by  covering  a  face 
of  the  crystals  with  a  thin  coating  of  wax,  allowing  the  wax  to 
solidify  and  then  bringing  the  point  of  a  hot  needle  or  other 
pointed  hot  body  against  the  wax  coating.  If  the  crystal  conduct 
equally,  the  wax  will  melt  in  a  circle  of  which  the  hot  point  is 
the  centre.  If  the  conduction  be  unequal,  the  melted  wax  will 
be  seen  to  assume  an  oval  form. 

Pyro-electric  Action  of  Crystals. — Certain  hemimorphous  or 
tetartohedral  crystalline  forms  when  heated  exhibit  a  peculiar 
development  of  electricity,  one  end  of  the  crystal,  or  fragment  of 
crystal,  becoming  negatively  electrified,  whilst  the  other  end  ex- 
hibits positive  electricity.  Amongst  these  crystals,  tourmaline 
exhibits  this  property  in  a  very  high  degree.  Boracite,  cane- 
sugar,  topaz,  and  silicate  of  zinc  are  crystals  which  exhibit  pyro- 
electrical  reactions. 

500  Optical  Properties  of  Crystals. — The  relations  of  crystals  to. 
light  are  of  great  importance,  as  enabling  us  to  determine  crys- 
talline forms  in  cases  in  which  the  usual  methods  either  give 
uncertain  results  or  fail  entirely.  Transparent  crystals  belonging 
to  the  regular  system  exert  no  peculiar  action  on  a  ray  of  light ; 
they  behave  in  this  respect  like  glass  or  any  other  amorphous 
substance.  The  incident  ray  gives  rise  to  one  refracted  ray,  and 
crystal  possesses  one  refractive  index.1 

Crystals  belonging  to  the  hexagonal  and  quadratic  systems 
are  doubly  refractive.  Each  incident  ray  separates  into  two  re- 
fracted rays,  one  being  bent  more  out  of  its  course  than  the  other. 
This  phenomenon  of  double  refraction  is,  however,  only  observed 

1  Cases  have  indeed  been-  observed  by  Brewster  in  which  a  crystal  belonging  to 
the  cubic  system  exhibits  double  refraction,  but  this  is  due  to  unequal  tension  in 
the  mass  of  the  crystal  and  resembles  the  case  of  double  refraction  by  unequally 
heated  or  compressed  glass. 


OPTICAL  PROPERTIES  OF  CRYSTALS  873 

when  the  incident  ray  falls  on  the  crystal  in  such  a  direction  as 
to  make  an  angle  with  the  primary  axis.  If  the  ray  pass  into 
the  crystal  parallel  to  this  axis,  no  double  refraction  is  observed. 
Crystals  belonging  to  the  hexagonal  and  quadratic  systems  are 
accordingly  called  uniaxial  crystals,  whilst  those  of  the  rhombic 
and  the  inclined  systems  are  termed  biaxial  crystals,  because- 
they  possess  two  directions  in  which  the  ray  of  light  may  fall 
without  producing  the  effect  of  double  refraction. 

Polarization  by  Absorption  in  Crystalline  Media. — Doubly  re- 
fracting crystals,  especially  tourmaline,  have  the  power  of  only 
allowing  rays  of  light  to  pass  through  them  when  those  rays  are 
polarized  in  a  given  direction  with  regard  to  their  optic  axes. 
Thus,  if  a  ray  be  allowed  to  pass  through  a  plate  of  tourmaline 
cut  with  its  faces  parallel  to  the  optic  axis  (the  axis  c  of  the 
hexagonal  system),  it  will  refract  doubly,  but  the  ordinary  ray 
will  be  completely  absorbed,  and  only  the  extraordinary  ray  will 


pass  through.  This  ray  is  polarized  in  a  plane  perpendicular  ta 
the  axis  of  the  crystal,  and  so  that  a  second  plate  of  tourmaline 
cut  in  a  similar  way  and  placed  with  its  axis  in  a  direction  at 
right  angles  to  that  of  the  first  will  not  allow  any  light  to  pass, 
as  in  Fig.  374,  though  when  the  two  axes  are  placed  parallel  to 
each  other  the  polarized  ray  will  be  transmitted  by  both  crystals, 
as  in  Fig.  373.  The  first  crystal  used  to  polarize  the  ray  is 
termed  the  polarizer,  whilst  the  second,  used  to  examine  the 
direction  of  the  polarization,  etc.,  is  termed  the  analyzer. 

The  property  of  circular  polarization  is  possessed  not  only  by 
crystals  but  also  by  many  organic  liquids,  such  as  solution  of 
sugar,  tartaric  acid,  and  many  essential  oils.  In  these  substances 
the  plane  of  polarization  is  rotated  sometimes  to  the  right,  some- 
times to  the  left  hand.  The  specific  rotary  power  of  any  sub- 
stance is  the  angle  through  which  a  column  of  the  substance 
of  unit  length  rotates  the  plane  of  polarization  of  a  beam  of 
polarized  light. 


874 


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ADDENDUM. 

P.  .  .  According  to  Colefax  (Journ.  Chem.  Soc.  1892,  1083),. 
the  equation  given  does  not  represent  the  change  which  occurs. 
The  primary  products  of  the  reaction  are  a  sulphate  and  a 
tetrathionate ;  any  trithionate  formed  is  the  product  of  a 
secondary  change  between  sulphite  and  tetrathionate. 


INDEX 


INDEX 


ABSOLUTE  temperatures,  62 

Absorptiometer,  Bunsen's,  286 

Acetylene,  693,  701  ;  metallic  deriva- 
tives of,  703  ;  preparation  of,  702  ; 
properties  of,  702 

Acid-forming  oxides,  234 

Acids,  235 

After-damp,  696 

Agate,  817 

Agricola,  9 

Air,  523  ;  analysis  of,  530  ;  bacteriology 
of,  546  ;  composition  of,  528,  531  ; 
liquefaction  of,  528  ;  mean  tempera- 
ture  of,  527  ;  weight  of,  523,  527 

Albertus'  Magnus,  6 

Alchemy,  5-10 

Alcohol,  694 

Alkaline  waters,  297 

Allophane,  822 

Allotropic  modifications  of  the 
elements,  56 

Amethyst  quartz,  815 

Amidoguanidine,  768 

Amidophosphoric  acid,  610 

Amidosulphonic  acid,  520 

Ammonia,   452  ;  aqueous   solution  of, 

462  ;  atmospheric,  542 ;  combination 
with    acids,    469  ;    composition     of, 

463  ;    detection    and    estimation  of, 
469  :  liquefaction  of,  457  ;  occurrence 
of,  452  ;  percentage  in  aqueous  solu- 
tions   of    different   specific   gravity, 
463  ;  preparation  of,  455  ;  properties 
of,    456;   solulility   in   water,   460; 
sulphonic   acids   of,   519  ;    synthesis 
of,  454 

Ainmoniacal  liquor,  454,  770 

Ammonium  carbamate,  738 

Ammonium  salts,  470 

Analcime,  822 

Analysis,  definition  of,  86  ;  qualitative, 

87  ;  quantitative,  87 
Andalusite,  821 
Animal  charcoal,  678 
Anthracite,  683 


Antozone,  243 

Apatite,  867 

Aqua  regia,  497 

Aqueous  solutions,  properties  of  dilute, 
116  ;  vapour,  tension  of,  276 

Aquinas,  Thomas,  6 

Arnold  Villanovanus,  6 

Arsenic,  atomic  weight  of,  616  ;  detec- 
tion of,  in  cases  of  poisoning,  633  ; 
Marsh's  test  for,  637  :  occurrence 
of,  612;  preparation  of,  614;  pro- 
perties of,  615  ;  Reinsch's  test  for, 
640 

Arsenic  acid,  628 

Arsenic  di-iodide,  621 

Arsenic  disulphide,  630 

Arsenic  pentafluoride,   619 

Arsenic  pentasulphide,  632 

Arsenic  pentiodide,  621 

Arsenic  pentoxide,  627 

Arsenic  selenosulphide,  633 

Arsenic  thioselenide,  633 

Arsenic  tribromide,  620 

Arsenic  trichloride,  619 

Arsenic  tri cyanide,  769 

Arsenic  trifluoride,  619 

Arsenic  tri-iodide,  621 

Arsenic  trioxide,  622 

Arsenic  trisulphide,  630 

Arsenic  vapour,  molecular  weight  of, 
615 

Arsenic,  white,  622 

Arsenious  acid.  625 

Arsenious  oxide,  622 

Arseniuretted  hydrogen,  616 

Arsine,  616 

Asmanite,  816 

Asymmetric  system  of  crystallography, 
863 

Atmolysis,  67 

Atmosphere,  the,  523  ;  composition  of, 
528  ;  ozone  in,  240  ;  weight  of,  523, 
527 

Atmospheric  ammonia,  542  ;  carbonic 
acid,  538 ;  moisture,  541  ;  organic 
matter,  544  ;  pressure,  526 

Atomic  theory,  34,  95 


880 


INDEX 


Atomic  weight,  definition  of,  100 

Atomic  weights,  52,  96 

Atomic  weights,  Dalton's  tables  of,  37 

Augite,  821 

Avogadro's  hypothesis,  39,  98 

Azoimide,  472 

Azulmic  acid,  751 


B. 


BACON,  Roger,  6 

Bacteriological  examination  of  water, 
300 

Bacteriology  of  air,  546 

Balance,  the,  56 

Basic  oxides,  234 

Becher,  13 

Benzene,  693 

Bergman,  32 

Berthollet,  33 

Berzelius,  38 

Biaxial  crystals,  873 

Bituminous  coal,  684 

Biuret,  744 

Black,  15 

Bleaching,  317,  369 

Bleaching  powder,  319 

.Boart,  667 

Boghead  coal,  684 

Borates,  655 

Boric  acid,  650  ;  spectrum  of,  654  ; 
sources  of,  650,  653 

Boron,  640  ;  adamantine,  642  ;  amor- 
phous, 641  ;  atomic  weight  of, 
643  ;  crystallized,  642  ;  occurrence  of, 
640  ;  preparation  of,  641 

Boron  ammoniochloride,  649 

Boron  carbide,  769 

Boron  hydride,  645 

Boron  nitride,  657 

Boron  pentasulphide,  656 

Boron  phosphide,  658 

Boron  phosphoiodide,  658 

Boron  tribromide,  649 

Boron  trichloride,  647 

-Boron  trifluoride,  645 

Boron  trifluoride,  hydrate  of,  646 

Boron  tri-iodide,  649 

Boron  trioxide,  650 

Boron  trisulphide,  656 

Boyle,  10 

Boyle's  law,  59  ;  deviations  from,  70 

Brimstone,  340 

Bromic  acid,  331 

Bromide  of  nitrogen,  480 

Bromides,  197 

Bromine,  atomic  weight  of,  193 ; 
detection  and  estimation  of,  198  ; 
occurrence  of,  188  ;  preparation  of, 
189  ;  properties  of,  191 

Bromine  hydrate,  192 

Bromine  monochloride,  198 

Brown  coal,  688 


C. 


CALAMINE,  821 

Calorie,  definition  of,  230 

Cannel  coal,  684 

Carbaiuic  acid,  738 

Carbamide,  739 

Carbaminic  chloride,  739 

Carbon,  658;  allotropic  modifications  of, 
658  ;  amorphous,  673  ;  atomic  weight 
of,  689  ;  halogen  derivatives  of,  705 

Carbon  bisulphide,  732 

Carbon  bo  ride,  769 

Carbon  dioxide,  710  ;  composition  of, 
714  ;  critical  point  of,  722  ;  deter- 
mination of,  728  ;  liquefaction  of, 
716  ;  occurrence  of,  711  ;  preparation 
of,  712  ;  properties  of,  713  ;  proper- 
ties of  liquefied,  720  ;  solidification 
of,  720 

Carbon  monoxide,  705  ;  poisonous 
action  of,  709  ;  preparation  of,  706  ; 
properties  of,  708 

Carbon  oxybromide,  732 

Carbon  oxy chloride,  731 

Carbon  oxy  sulphide,  736 

Carbon  tetrachloride,  705 

Carbon  tetrafluoride,  705 

Carbonado,  660 

Carbonated  waters,  297 

Carbonates,  724 

Carbonic  acid,  725  ;  atmospheric, 
538  ;  and  the  carbonates,  Black's 
researches  on,  15 

Carbonic  acid  gas,  710 

Carbonic  anhydride,  710 

Carbonic  oxide,  705 

Carbonyl  bromide,  732 

Carbonyl  chloride,  731 

Carbonyl-dibiuret,  745 

Carbonyl-diurea,  745 

Carbonyl  sulphide,  736 

Carborundum,  825 

Carburetted  hydrogen,  heavy,  700 ; 
light,  700 

Carburetted  water  gas,  791 

Cavendish,  15—22 

Chalcedony,  817 

Chnlybeate  waters,  297 

Charles's  law,  60 

Chemical  action,  illustrations  of,  42 

Chemical  combination,  laws  of,  86 

Chemical  decomposition,  42 

Chemical  equations,  102 

Chemical  formulae,  98 

Chemical  nomenclature,  118 

Chemical  symbols,  100 

Chemistry,  derivation  of  the  term,  4  ; 
general  principles  of,  41 

Charcoal,  674  ;  absorption  of  gases 
by,  680  ;  animal,  678  ;  employment 
in  decolourising  and  deodourising, 
678  ;  manufacture  of,  675  ;  proper- 
ties of,  676  ;  wood,  674 


INDEX 


881 


Chlorates,  323 

Chlorhydric  acid,  168 

Chloric  acid,  322 

Chloride  of  nitrogen,  475 

Chlorides,  185 

Chlorine,  153  ;  allotropic  modification 
of,  163 ;  atomic  weight  of,  188  ; 
bleaching  power  of,  164  ;  detection 
and  estimation  of,  186  ;  explosion 
with  hydrogen,  169  ;  occurrence  of, 
153  ;  preparation  of,  154  ;  properties 
of,  159  ;  solubility  of,  168 

Chlorine  hydrate,  165 

Chlorine  monoxide,  315 

Chlorine  peroxide,  320 

Chlorites,  319 

Chloroform,  694 

Chloroforiuamide,  739 

Chlorosulphonic  acid,  404 

Chlorous  acid,  319 

Circular  polarisation,  873 

Coal,  682  ;  analysis  of,  685-687  ; 
bituminous,  684  ;  Boghead,  684 ; 
brown,  688  ;  cannel,  684 ;  parrot, 
684  ;  yield  of,  in  Great  Britain,  688 

Coal  gas,  analysis  of,  ?86  ;  composition 
of,  783  ;  condensing  apparatus 
employed  in  the  manufacture  of, 
774  ;  determination  of  illuminating 
power  of,  780  ;  history  of,  769  ; 
manufacture  of,  770  ;  purification  of, 
776  ;  'scrubbers,  775  ;  retorts  and 
retort  settings  for  the  manufacture 
of,  770  ;  washers,  775 

Coal-mines,  explosions  in,  695 

Coal  tar,  770 

Coke,  682 

Colloids,  819 

Combination  by  volume,  97  ;  by  weight, 
87  ;  chemical,  42  ;  heat  of,  230 

Combination  in  definite  and  unalter- 
able quantities,  discussion  of,  by 
Berthollet  and  Proust,  33 

Combination  of  gases  by  volume, 
Gay-Lussac's  observations  of,  38 

Combining  weights,  93 

Combustion,  Lavoisier's  views  of,  27 

Compound,  definition  of  the  term,  51 

Compound  radical,  470 

Compound  radical,  definition  of,  748 

Constitutional  formulae,  128 

Critical  point  of  liquefaction  of  gases, 
73 

Critical  pressure  of  gases,  73 

Cryohydrates,  281 

Crystalline  form,  determination  of, 
837 

Crystallization,  279  ;  water  of,  103, 
279 

Crystallographic  axes,  832 

Crystallographic  symbols,  832 

Crystallography,  827  ;  asymmetric 
system  of,  863  ;  cubic  system  of, 
840  ;  early  ideas  of,  827  ;  hexagonal 

67 


system  of,  847  ;  isometric  system  of, 
840 ;  monoclinic  system  of,  860  ; 
monosym metric  system  of,  860  ; 
prismatic  system  of,  853  ;  quadratic 
system  of,  853  ;  regular  system  of, 
840  ;  rhombic  system  of,  857  ; 
tetragonal  system  of,  853  ;  triclinic 
system  of,  863 

Crystalloids,  819 

Crystals,  angles  of,  830  ;  artificial 
growth  of,  837  ;  axes  of,  832  ;  biaxial, 
873  ;  classification  of,  829  ;  double 
refraction  of,  872  ;  edges  of,  830  ; 
faces  of,  830  ;  general  characteristics 
of,  828  ;  hemihedral,  833  ;  hemi- 
morphous,  833 ;  hemitropic,  835  ; 
holohedral,  833  ;  optical  relations  of, 

872  ;  polarization  of  light  by,  873  ; 
predominant    faces  of,    831  ;    pyro- 
electric  action  of,  872  ;  secondary  faces 
of,  831  ;  simple  and  complex  forms 
of,  830  ;  specific  rotatory  power  of, 

873  ;  symmetry  of,   328  ;   tetartohe- 
dral,  833  ;  thermal  relations  of,  871  ; 
uniaxial,  873 

Cubic  system  of  crystallography,  840 
Cyamelide,  762 
Cyanamide,  764 
Cyanic  acid,  761 
Cyanides,  756 

Cyanogen  compounds,  history  of,  747 
Cyanogen  gas,  749 
Cyanogen  bromide,  760 
Cyanogen  chloride,  7.'>1.' 
Cyanogen  iodide,  761 
Cyanogen  selenide,  764 
•Cyanogen  sulphide,  764 
Cyanuric  derivatives,  766 


D. 


DALTON,  34-38 

Dalton's  law  of  gases,  60  ;  deviations 

from,  70 

Dalton's  law  of  partial  pressures,  284 
Davy  lamp,  228 
Davy,  Sir  Humphry,  38 
Deliquescence,  280 
Density  of  gases,  103 
Dialysis,  819 
Diumide,  470 

Diamidophosphoric  acid,  610 
Diamond,  658  ;  artificial  production  of, 

661  ;  sources  of,  660  ' 
Dicyanamide,  766 
Dicyanogen,  749 
Diffusion  of  gases,  63  ;   illustrations  of, 

67 

Dihydroxylaminesulphonic  acid,  521 
Di  imidodiphosphaminic  acid,  611 
Di-imidodiphosphoric  acid,  611 
Dilute  solutions,  properties  of,  111 
Dimetaphosphates,  597 


INDEX 


Dimorphism,  870 
Dioptase,  821 
Dipeiiodates,  336 
Di phosphoric  acid,  601 
Dissociation,  109 
Dissociation,  electrolytic,  116 
Distilled  water,  290  " 
Distorted  crystals,  835 
Disulphuryl  chloride,  406 
Dithionic  acid,  414 
Dithiophosphoric  acid,  607 
Double  cyanides,  757 
Double  refraction,  872 
Dowson  gas,  791 
Drammond  light,  268 
Dulong  and  Petit's    law    of    specific 
heats,  39 


hydrogen,  267;  Smithells'  researches 
on,  799  ;  structure  of,  792 

Flame  of  Bunsen  burner,  797 

Flame  of  burning  hydrocarbons,  793 

Flint,  817 

Fluoboric  acid,  646 

Fluorides,  152 

Fluorine,  atomic  weight  of,  147  ;  de- 
tection of,  151  ;  isolation  of,  143  : 
occurrence  of,  142 ;  properties  of, 
146 

Formulae,  chemical,  100 ;  constitu- 
tional, 128 

Freezing  mixtures,  282 

Freezing  point,  depression  of,  by  dis- 
solved substances,  314 

Fuming  sulphuric  acid,  402 


E. 


EARTH'S  crust,  composition  of,  55 

"Eau  de  Javelles,"  315 

Efflorescence,  280 

Effusion  of  gases,  70 

Electrolytes,  116 

Electrolytic  dissociation,  116 

Elements,  allotropic  modifications  of, 
56  ;  Aristotelian  doctrine  of,  3  ; 
atomic  weights  of,  52  ;  Boyle's  view 
of,  11  ;  classification  of,  53  ;  com- 
bining weights  of,  93  ;  definition  of, 
51  ;  distribution  of,  54  ;  equivalents 
of,  92  ;  Geber's  views  of,  7  ;  list  of, 
52 

Elixir  vitse,  8 

Emerald.  821 

Enstatite,  821 

Equations,  chemical,  102 

Equivalents,  92 

Ethane,  6S8 

Ethene,  699 

Ethine,  701 

Ethylene,  693,  699  ;  preparation  of, 
700  ;  properties  of,  701 

Ethyl  hydride,  698 

Euchlorine,  322 

Explosions  in  coal  mines,  695 

Explosion  in  gases,  phenomena  of, 
265 


F. 


FELSPAR,  821 

Ferricyanic  acid,  757 

Ferrocyanic  acid,  757 

Fire-damp,  695 

Flame,  Davy's  researches  on,  793,  794  ; 
dissection  of,  799  ;  Frankland's  re- 
searches on,  794  ;  Lewes'  researches 
on,  795,  797  ;  luminous,  rendered 
non-luminous  by  addition  of  other 
gases,  797 ;  nature  of,  792  ;  oxy- 


G. 


GARNET,  821 

Gas-carbon,  674 

Gas-liquor,  454,  770 

Gaseous  and  liquid  states,  continuity 
of,  72 

Gases,  absorption  of  by  water,  284  ; 
critical  point  of,  73  ;  critical  pres- 
sure of,  73  ;  density  of,  103  ;  deter- 
mination of  molecular  weight,  105  ; 
diffusion  of,  63  ;  effusion  of,  70  ;  ex- 
plosion of,  265  ;  kinetic  theory  of,  60 ; 
liquefaction  of,  74  ;  molecular  weight 
of,  99  ;  properties  of,  59  ;  relation  of 
volume  to  pressure,  59  ;  relation  of 
volume  to  temperature,  60  ;  transpi- 
ration of,  66  ;  van  Helmont's  inves- 
tigations of,  9 

Gay-Lussac,  38 

Gay-Lussac's  law,  60 

Gay-Lussac  tower,  employment  in  sul- 
phuric acid  manufacture,  389 

Geber,  researches  of,  5,  7 

Generator  gas,  790 

Glauber,  10 

Glover's  tower,  employment  in  sulphuric 
acid  manufacture,  385 

Goniometer,  837 

Graphite,  667  ;  artificial  production  of, 
668  ;  properties  of,  668,  670  ;  uses 
of,  671 

Graphitic  acid,  729 

Graphitite,  670 

Graphon,  730 

Guanidine,  766 


H. 

HALLOYSITE,  822 
Halogens,  the,  142 
Hard  water,  298 
Hartshorn,  spirits  of,  453 
Heat  of  combination,  230 


INDEX 


883 


Heat  of  liquidity,  273 

Heavy  carburetted  hydrogen,  700 

Hemihedral  crystals/ 833 

Hemimorphous  crystals,  833 

Hemitropic  crystals,  835 

Hexagonal  system  of  crystallography, 

847 

Hexametaphosphates,  598 
Hexathionic  acid,  422 
Higgins,  34 

Holohedral  crystals,  833 
Hooke,  12 
Humboldt,  39 
Hydrated  oxides,  234 
Hydrazine,  470  ' 
Hydrazine  hydrate,  471 
Hydrazoic  acid,  472 
Hydriodic  acid,  206  ;    preparation  of, 

206  ;  properties  of,  208 
Hydrobromic  acid,  193  ;  preparation  of, 

193  ;  properties  of,  196 
Hydrocarbons,  constitution  of,  690 
Hydrochloric   acid,    168  ;    composition 

of,  175  ;    formation  of,   169  ;    manu- 
facture of,  179  ;   occurrence  of,  168  ; 

preparation  of,   173  ;    properties    of, 

174  ;  properties  of  aqueous  solutions 

of,  118 
Hydrocyanic  acid,  752  ;  detection  and 

estimation  of,  756,  758  ;  preparation 

of,  752  ;  properties  of,  754 
Hydrocyanic  acid  hydriodide,  759 
Hydrocyanic  acid  hydrobromide,  759 
Hydrocyanic  acid  hydrochloride,  759 
Hydrofluoric  acid,  147  ;  preparation  of, 

148  ;    properties    of,    150  ;     vapour 

density  of    150 
Hydrofluosilicic  acid,  806 
Hydrogen,     12$ ;      absorption     of,    by 

metals,  136  ;  experiments  with,  139  ; 

explosion  of,  with  chlorine,  169  ;  ex- 

Elosion  of,  with  oxygen,  261  ;  lique- 
iction  of,  135  ;  occurrence   of,   129  ; 

preparation    of,   129  ;  properties    of, 

134  ;  spectrum  of,  139 
Hydrogen  arsenide,  gaseous,  616  ;  solid, 

618 

Hydrogen  bromide,  193 
Hydrogen  chloride,  168 
Hydrogen  dioxide,  308  ;  detection  and 

estimation   of,  312  ;    preparation  of, 

309  ;  properties  of,  309 
Hydrogen  disulphide,  357 
Hydrogen  fluoride,  147 
Hydrogen  iodide,  206 
Hydrogen  monosulphide,  349 
Hydrogen  monoxide,  244 
Hydrogen  peroxide,  308 
Hydrogen  persulphide,  355 
Hydrogen    phosphide,    gaseous     566  ; 

liquid,  570  ;    solid,  573 
Hydrogen  selenide,  *27 
Hydrogen  sulphate,  377 
Hydrogen  tellnride,  439 


Hydrogenium,  138 
Hydrographitic  acid,  730 
Hydrosulphurous  acid,  408 
Hydrothiosulphoprussic  acid,  745 
Hydroxides,  234 
Hydroxycarbamide,  743 
Hydroxylamine,  475 
Hydroxylaminedisulphonic  acid,  520 
Hydroxylaminesulphonic  acid,  521 
Hygrometry,  542 
Hypobromous  acid,  329 
Hypochlorous  acid,  317 
Hypochlorous  anhydride,  315 
Hypoiodous  acid,  332 
Hyponitrous  acid,  504 
Hypophosphites,  582 
Hypophosphoric  acid,  586 
Hypo  phosphorous  acid,  581 
Hyposulphites,  408 
Hyposulph'urous  acid,  408 


I. 


ICE,  artificial  production  of,  458  ;  crys- 
talline form  of,  274  ;  melting  point 
of,  273 

Ignition,  temperature  of,  228 

Imidodiphosphoric  acid,  611 

Imidodisulphonates,  520 

Imidosulphurylamide,  522 

Indestructibility  of  matter,  47 

Induction,  photochemical,  172 

Introduction,  historical,  3 

lodates,  333 

lodic  acid,  332 

Iodide  of  nitrogen,  480 

Iodides,  209 

Iodine,  199  ;  atomic  weight  of,  206  ; 
detection  and  estimation  of,  211  ; 
occurrence  of,  199  :  preparation  of, 
199  ;  properties  of,  203 

Iodine  bromide,  214 

Iodine  fluoride,  212 

Iodine  monochloride,  213 

Iodine  pentoxide,  332 

Iodine  trichloride,  213 

Isometric  system  of  crystallography, 
840 

Isomorphism,  39,  865 

Isuretine,  743 


J. 


JET,  688 


K. 


KlESELGUHR,   814 

Kirwan,  32,  34 


884 


INDEX 


L. 


LAMP-BLACK,  673 

Laughing-gas,  503 

Lavoisier,  24-42 

Law  of  partial  pressures,  284 

Laws  of  chemical  combination,  86 

Lefebre,  10 

Lemery,  10 

Leucite,  821 

Libavius,  9 

Light  carburetted  hydrogen,  700 

Lignite,  688 

Lime  light,  268 

Liquefaction  of  gases,  74 

Liquefied  gases,  tension  of,  74 

Liquid  and  gaseous  states,  continuity 

of,  72 

Liquidity,  heat  of,  273 
Liquids,  boiling  points  of,  85 
Liquor  ammonise,  462 
Lucifer  matches,  564 
Lully,  Raymond,  6 


M. 


MACLES,  834 

Marggraf,  15 

Marsh-gas,  694 

Marsh's  test  for  arsenic,  637 

Matches,  565 

Matter,  different  states  of,  41 

Matter,  indestructibility  of,  47^ 

Mayow,  12 

Mesoperiodates,  336 

Metaboric  acid,  654 

Metals,  transmutation  of,  5 

Metaperiodates,  336 

Metaphosphoric  acid,  591,  596 

Metarsenates,  629 

Metasilicic  acid,  820 

Metathioarsenates,  632 

Methane,  694 ;  preparation  of,  697  ; 
properties  of,  698 

Methenylamine,  759 

Methyl  hydride,  694 

Metrical  system,  comparison  of,  with 
English  measures.  874,  875 

Microbes  in  the  atmosphere,  546 

Milk  of  sulphur,  345 

Milk  quartz,  815 

Mimetisite,  867 

Mineral  waters,  296 

Mitscherlich,  39 

Moisture,  atmospheric,  541 

Molecular  motion,  61 

Molecular  weight,  determination  of,  in 
solution,  111 

Molecular  weight,  experimental  deter- 
mination of,  104 

Molecular  weight  of  gases,  99 

Molecule,  definition  of  the  term,  98 

Molecules,  size  of,  97;  velocity  of,  61, 63 


Monoclinic  system  of  crystallography, 
860 

Monometaphosphates,  597 

Monosy  in  metric  system  of  crystallo- 
graphy, 860 

Monothiophosphoric  acid,  607 

Muriatic  acid,  168 

Muscovite,  821 


N. 


NATROLITE,  822 

Natural  waters,  289 

Nitric  acid,  482  ;  action  of  metals  on, 
494 ;  detection  and  estimation  of. 
495  ;  formation  of,  482  ;  hydrates  of, 
488  ;  manufacture  of,  488  ;  prepara- 
tion of,  483  ;  properties  of,  486  r 
properties  of  aqueous  solutions  of, 
486 

Nitric  anhydride,  497 

Nitric  oxide,  505 

Nitrilosulphonates,  519 

Nitrilotrimetaphosphoric  acid,  612 

Nitrites,  interaction  with  sulphites,. 
522 

Nitrogen,  446  ;  assimilation  of,  by 
plants,  451  ;  combustion  of,  451  ; 
discovery  of,  446  ;  liquefaction  of, 
450  ;  preparation  of,  447  ;  properties 
of,  450  ;  spectrum  of,  451 

Nitrogen  bromide,  480 

Nitrogen  chloride,  477 

Nitrogen  dioxitte,  505 

Nitrogen  iodide,  480 

Nitrogen  mjmoxide,  499 

Nitrogen  pentoxide,  497 

Nitrogen  peroxide,  512 

Nitrogen  selenide,  516 

Nitrogen  sulphide,  516 

Nitrogen  tetroxide,  512 

^Nitrogen  trioxide,  508 

Nitroguanidine,  768 

Nitrometer,  Lunge's,  496 

Nitrosohydroxylaminesulphonic  acid, 
521 

Nitrosulphonic  acid,  516 

Nitrosulphonic  anhydride,  499,  518 

Nitrosulphonic  chloride,  5 1 7 

Nitrosyl  bromide,  511 

Nitrosyl  chloride,  510 

Nitrosyl  tri bromide,  511 

Nitrous  acid,  509 

Nitrous  anhydride,  508 

Nitrous  oxide,  499 

Nitroxypyrosulphuric  acid,  518 

Nomenclature,  chemical,  116 


0. 


OCCLUSION,  137 
Oil  gas,  788 


INDEX 


885 


Olefiant  gas,  699 

defines,  693 

Oliviue,  821 

Opal,  816 

Optical  relations  of  crystals,  872 

Organic  constituents  of  water,  300 

Organic  matter,  atmospheric,  544 

Organic  substances,  first  artificial  pre- 
paration of,  40 

Orpiment,  630 

Ortho-arsenates,  628 

Orthoboric  acid,  650 

Orthoclase,   821 

Orthophosphoric  acid,  591,  592 

Orthosilicic  acid,  819 

Orthothioarsenates,  632 

Osmotic  pressure,  112 

Oxides,  234 

Oxy-acids  of  chlorine,  constitution  of, 
328 

Oxygen,  214  ;  atomic  weight  of,  255  ; 
discovery  of,  15  ;  explosion  of,  with 
hydrogen,  261  ;  occurrence  of,  214  ; 
preparation  of,  215  ;  properties  of, 
222;  separation  of, from  nitrogen,  289 

Oxy-hydrogen  flame,  267 

Oxynitrosulphonic  anhydride,  518 

Oxythiocarbamic  acid,  745 

Ozone,  235  ;  atmospheric,  240  ;  mole- 
cular formula  of,  239  ;  production 
of,  235  ;  properties  of,  242 


P. 


PARACELSUS,  9 
Paracyanogcn,  752 
Paraffins,  692 
Paraperiodates,  336 
Parasulphatammon,  520 
Parrot  coal,  684 
Peat,  688 

Pentathionic  acid,  419 
Perbromic  acid,  331 
Perchlorates,  327 

Perchloric  acid,  324  ;  hydrates  of,   327 
Periodates,  335 

Periodic  acid,  335  ;  hydrates  of,  335 
Peroxides,  234 
Persulphuric  acid,  410 
Petalite,  821 
Pharaoh's  serpents,  763 
Phenakite,  820 
Phlogistic  theory,  13,  23,  31 
Phosgene  gas,  731 
Phospham,  610 
Phosphamide,  610 
Phosphamidic  acid,  610 
Phosphine,   566 
Phosphites,  585 
Phosphoniuin'bromide,  570 
Phosphonium  iodide,  570 
Phosphoric  acid,    590 ;   determination 
of,  601 


Phosphoric  acids,  constitution  of,  591  ; 

poisonous  action  of,  592 
Phosphorous  acid,  584 

Phosphorous  anhydride,  583 

Phosphorous  diamide,  609 

Phosphorous  oxide,  583 

Phosphorus,  549  ;  allotropic  modifica- 
tions of,  554 ;  amorphous,  561  ; 
black,  564  ;  detection  of,  559  ;  dis- 
covery of,  549  ;  luminosity  of,  556  ; 
manufacture  of,  551  ;  metallic,  563  ; 
molecular  weight  of,  555  ;  occurrence 
of,  550  ;  octohedral,  554  ;  proj  evties 
of,  554  ;  poisonous  action  of,  560  ; 
red,  561 ;  rhombohedral,  563 

Phosphorus  bromonitride,  612 

Phosphorus  chlorobromide,  579 

Phosphorus  chloronitride,  612 

Phosphorus  di-iodide,  580 

Phosphorus  oxybromide,  604 

Phosphorus  oxybromochloride,  605 

Phosphorus  oxychloride,  602 

Phosphorus  oxyfluoride,  602 

Phosphorus  pentabromide,  579 

Phosphorus  pentachloride,  575 

Phosphorus  pentafluoride,  573 

Phosphorus  pentasulphide,  606 

Phosphorus  pentiodide,  580 

Phosphorus  pentoxide,  587 

Phosphorus  suboxide,  581 

Phosphorus  sulphides,  606 

Phosphorus  sulphoxide,  607 

Phosphorus  tetroxide,  586 

Phosphorus  tribromide,  578 

Phosphorus  trichloride,  574 

Phosphorus  tricyanide,  768 

Phosphorus  trifluoride,  573 

Phosphorus  trifluorodibrouiide,  579 

Phosphorus  trifluorodichloride,  578 

Phosphorus  tri-iodide,  580 

Phosphoryl  bromide,  604 

Phosphoryl  bromochloride,  605 

Phosphoryl  chloride,  602 

Phosphoryl  fluoride,  602 

Phosphoryl  nitride,  611 

Phosphoryl  triamide,  612 

Phosphuretted  hydrogen,  567 

Photochemical  induction,  172 

Photometer,   781 

Plastic  sulphur,  345 

Plumbago,  667 

Polarization,  circular,  873  ;  in  crystal- 
line media,  873 

Portable  gas,  788 

Potash  mica,  821 

Potassium  hydroxylaminedisulphonate, 
520 

Potassium  imidodisulphonate,  520 

Potassium  nitrilosulphonate,  519 

Potassium  sulphaziuate,  521 

Pott,  15 

Priestley,  15-22 

Prismatic  system  of  crystallography, 
853 


88(5 


INDEX 


Producer  gas,  790 
Proust,  33 
Prussic  acid,  752 
Pyro-arsenates,  629 
Pyroboric  acid,  654 
Pyro-electric  action  of  crystals,  872 
Pyrographitic  acid,  730 
Pyrographitic  anhydride,  730 
Pyromorphite,  867 
Pyrophosphaminic  acids,  611 
Pyrophosphoric  acid,  591,  599 
Pyrophosphoryl  chloride,  604 
Pyrophosphoryl  thiobromide,  609 
Pyrothioarsenates,  632 


QUADRATIC  system  of  crystallography, 

853 
Quartz,  814  ;  coloured  varieties  of,  815 


R. 


RADICALS,  compound,  470 
Rain-water,  295 

Raoult's  method  of  determining  mole- 
cular weights,  114 
Realgar,  630 
Red  phosphorus,  561 
Regelation,  274 

Regular  system  of  crystallography,  840 
Reinsch's  test  for  arsenic,  640 
Rhombic  system  of  crystallography,  857 
Richter,  32 
River  water,  305 
Rose  quartz,  816 
Ruby  sulphur,  630 


SAFETY  LAMP,  228 

Sal  ammoniac,  453 

Saline  waters,  297 

Salts,  235 

Sand,  815 

Sceptical  Chymist  (Boyle),  10 

Scheele,  22 

Sea-water,  306 

Selenic  acid,  433 

Selenious  acid,  431 

Selenium,  423  ;  action  of  light  on,  426  ; 

atomic  weight  of,   427  ;    preparation 

of,  424  ;  properties  of,  425 
Selenium  chlorobromide,  430 
Selenium  dioxide,  431 
Selenium  fluoride,  431 
Selenium  moniodide,  430 
Selenium  moriobromide,  429 
Selenium  monochloride,  428 
Selenium  oxychloride,  433 
Selenium  tetrabromide,  430 


Selenium  tetrachloride,  429 

Selenium  tetriodide,  430 

Seleniuretted  hydrogen,  427 

Selenosulphur  trioxide,  435 

Selenosulphuric  acid,  435 

Selenotrithionic  acid,  436 

Selenourea,  746 

Selenyl  bromide,  433 

Selenyl  chloride,  433 

Serpentine,  821 

Silica,  814 

Silica,  amorphous,  816 

Silicates,  820  ;  artificial  production  of, 

822  ;  classification  of,  821 
Silicic  acid,  819 
Silicious  waters,  297 
Silico-bromoform,  812 
Silico- chloroform,  810 
Silicofluoric  acid,  806 
Silicofluorides,  808 
Silicotbrmic  anhydride,  811 
Silico -iodoform,  813 
Silicomethane,  804 
Silicon,  800  ;    amorphous,  801  ;  atomic 

weight   of,    803  ;    crystallized,    802  ; 

mixed  halogen    derivatives  of,   813  ; 

occurrence  of,   800  ;    preparation  of, 

801  ;  properties  of,  801 
Silicon  carbide,  825 
Silicon  carbonitride,  824 
Silicon  carboxide,  825 
Silicon  chlorohydrosulphide,  824 
Silicon  dioxide,  814 
Silicon  disulphide,  823 
Silicon  hydride,  804 
Silicon  nitride,  824 
Silicon  oxychloride,  822 
Silicon  oxysulphide,  823 
Silicon  subsulphide,  823 
Silicon  tetrabromide,  811 
Silicon  tetrachloride,  808 
Silicon  tetrafluoride,  806 
Silicon  tetra-iodide,  812 
Silicon  tribromide,   812 
Silicon  trichloride,  810 
Silicon  tri-iodide,  812 
Silico-oxalic  acid,  813 
Smoky  quartz,  815 
Soap  test  for  hardness  of  water,  299 
Soda-bleach,  313 
Soft  water,  298 
Solutions,   electrolytic    dissociation  of, 

283  ;  properties  of,  282 
Specific  heats,  Dulong  and  Petit's  law 

of,  39 

Specific  rotatory  power,  873 
Spirits  of  hartshorn,  453 
Springs,  thermal,  296 
Spring-water,  296 
S  ahl,  13 
Steam,  latent  heat  of,  275* ;  volumetric 

composition  of,  253 
Suffioni,  651 
Sulphamide,  523 


INDEX 


887 


Sulphatammon,  520 

Sulphates,  400 

Sulphazinates,  521 

Sulphazotinatt  s,  522 

Sulphimide,  525 

Sulphites,  372 

Sulphites,    interaction    with     nitrites, 

522 

Sulphites,  isomerism  of,  373 
Sulpho-arsenates,  632 
Sulpho-arsenites,  631 
Sulphocyanic  acid,  762 
Sulphoselenoxytetrachloride,  436 
Sulphur,  336 
a-Sulphur,  342 
)8-Sulphur,  343 
7-Sulphur,  345 
S-Sulphur,  347 

Sulphur,  allotropic  modifications  of, 
342  ;  atomic  weight  of,  349  ;  colloi- 
dal, 346  ;  crystalline  forms  of,  342  ; 
detection  and  determination  of,  348  ; 
flowers  of,  340,  346  ;  manufacture  of, 
338  ;  milk  of,  345  ;  occurrence  of, 
336  ;  properties  of,  342  ;  plastic, 
345  ;  recovery  of,  from  waste  pro- 
ducts, 341  ;  spectrum  of,  348 
Sulphur,  vapour,  molecular  weight  of, 

347 

Sulphur  dichloride,  360 
Sulphur  dioxide,  364  ;  bleaching  action 
of,  369  ;  detection  and  estimation  of, 
370  ;  liquefaction  of,   368  ;    prepara- 
tion of,  366  ;  properties  of,  367 
Sulphur  diphosphide,  605 
Sulphur  fluoride,  363 
Sulphur  heptoxide,  410 
Sulphur  hexiodide,  363 
Sulphur  moniodide^-362 
Sulphur  monobromide,  362 
Sulphur  monochloride,  359 
Sulphur  oxytetrachloride,  406 
Sulphur  sesquioxide,  407 
Sulphur  subiodide,  363 
Sulphur  tetrabromide,  362 
Sulphur  tetrachloride,  360 
Sulphur  tetraphosphide,  605 
Sulphur  trioxide,  374 
Sulphuretted  hydrogen,  349  ;  action  of 
on  metals,  355  ;  composition  of,  353  ; 
preparation  of,    350 ;    properties   of, 
352 

Sulphuretted  waters,  297 
Sulphuric    acid,    377  ;    action    of,    on 
metals,  399  ;  action  of  water  on,  398  ; 
fuming,  402' ;   history  of,   377  ;   hy- 
drates of,  399  ;  manufacture  of,  381  ; 
properties   of,   397  ;   purification    of, 
396  ;    rectification  of,   391  ;    specific 
gravity  of  aqueous  solutions  of,  401  ; 
theory  of  the  formation  of,  379 
Sulphuric  anhydride,  374 
Sulphurous  acid,  371 
Sulphurous  anhydride,  364 


Sulphuryl  bromide,  407 
Sulphuryl  chloride,  405 
Sulphuryl  hydroxychloride,  404 
Symbols,  chemical,  100 
Synthesis,  definition  of,  86 


T. 


TALC,  821 

Tellurates,  445 

Telltiretted  hydrogen,  439 

Tellurium,  436  ;  atomic  weight  of.  438  ; 

preparatiop  of,  437  ;   properties  of, 

438 

Tellurium  dibromide,  441 
Tellurium  dichloride,  440 
Tellurium  di-iodide,  441 
Tellurium  dioxide,  442 
Tellurium  disulphide,  445 
Tellurium  hydride,  439 
Tellurium  monoxide,  442 
Tellurium  oxybromide,  445 
Tellurium  oxychloride,  445 
Tellurium  sulphate,  442 
Tellurium  sulphoxide,  446 
Tellurium  tetrabromide,  441 
Tellurium  tetrachloride,  440 
Tellurium  tetrafluoride,  442 
Tellurium  tetriodide,  441 
Tellurium  trioxide,  443 
Tellurous  acid,  443 
Temperature,  absolute  zero  of,  62 
Tension  of  aqueous  vapour,  276 
Tetartohedral  crystals,  833 
Tetragonal  system  of  crystallography, 

853 

Tetrametaphosphates,  598 
Tetraphosphoric  acid,  601 
Tetraphosphorus  trisulphide,  606 
Tetrathionic  acid,  418 
Thermal  relations  of  crystals,  871 
Thermal  springs,  296 
Thermo-chemistry,  233 
Thioarsenates,  632 
Thioarsenites,  631 
Thiocarbamic  acid,  745 
Thiocarbamide,  746 
Thiocarbonic  acid,  735 
Thiocarbouyl  chloride,  736 
Thiocyanic  acid,  762 
Thiocyanic  anhydride,  764 
Thionyl  bromide,  374 
Thionyl  chloride,  373 
Thiophosphoric  acid,  607 
Thiophosphoryl  bromide,  608 
Thiophosphoryl  chloride,  608 
Thiophosphoryl  fluoride,  608 
Thiosulphuric  acid,  412 
Thiourea,  746 
Thomson,  38 
Torricellian  vacuum,  524 
Transmutation  of  metals,  5 
Transpiration  of  gases,  66 


888 


INDEX 


Tricarbon  disulphide,  735 

Trichlorosilicomethane,  810 

Triclinic  system  of  crystallography,  863 

Tridymite,  815 

T»imetaphosphates,   598 

Trimorphism,  870 

Triphosphorus  hexasulphide,  606 

Trithionic  acid,  416 

Trithiophosphoric  acid,  607 

Turf,  688 

Twin  crystals,  834 


U. 


UN  i AXIAL  crystals,  873 

Urea,  739  ;  detection  and  estimation  of, 

742  ;  preparation  of,  740  ;  properties 

of,  740  ;  synthesis  of,  739 
Urea  hydrochloride,  741 
Urea  nitrate,  741 
Urea  oxalate,  741 


V. 


VACUUM,  Torricellian,  524 
Valency,  127 
Valentine,  Basil,  8 
Van  Helmont,  9 
Vanadinite,  867 

Vapour  density,  determination  of,  106 
Vapour  pressure,  diminution  of,  by  dis- 
solved substances,  115 
Ventilation,  545 
Volume,  combination  by,  97 


284  ;  alkaline,  297  ;  analysis  of,  300  ; 
bacteriological  examination  of,  300  ; 
carbonated,  297  ;  Cavendish's  investi- 
gations of,  20  ;  chalybeate,  297  ; 
composition  of,  by  volume,  250  ;  com- 
position of,  by  weight,  255  ;  dis- 
covery of  the  composition  of,  20,  244  ; 
dissolved  gases  in,  291  ;  distillation 
of,  290  ;  electrolytic  analysis  of,  251  ; 
eudiometric  synthesis  of,  246  ;  ex- 
pansion and  contraction  of,  270  ; 
freezing  point  of,  273  ;  hard,  2C8  ; 
latent  heat  of,  272  ;  mineral,  i96  ; 
natural,  289  ;  organic  constituents  of, 
300  ;  permanent  hardness  of,  299  ; 
point  of  maximum  density  of,  271  ; 
properties  of,  270  ;  purification  of, 
290  ;  rain,  295  ;  river,  305  ;  saline, 
297  ;  sea,  306  ;  silicious,  297  ;  soft, 
289  ;  solvent  action  of,  278  ;  s]>ecih'e 
gravity  of,  at  different  temperatures, 
272  ;  spring,  296  ;  sulphuretted,  297 ; 
synthesis  of,  by  volume,  250  ;  tem- 
porary hardness  of,  299 

Water-gas,  790  ;  carburetted,  791 

Water  of  crystallization,  103,  279  ; 
volume  occupied  by,  281 

Weight,  combination  by,  87 

Weights  of  elements  in  compounds, 
relations  of,  87-95 

White  arsenic,  622 

Willis,  10 

Wrollaston,  38 

Wollastonite,  821 

Wood-charcoal,  674 

Wood-gas,  787 

Wood-tar,  788 


W. 

WACKENRODER'S  solution.  420,  422 
Water,    244  ;  absorption  of  gases  by, 


ZEOLITES,  822 
Zosimus  of  Panopolis,  4 


(V) 


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